14 breeding for resistance to ear rots caused by

Plant Breeding 131, 1—19 (2012) Ó 2011 Blackwell Verlag GmbH doi:10.1111/j.1439-0523.2011.01936.x Review Breeding for resistance to ear rots caused by Fusarium spp in maize – a review ´ K O S M E S T E R H A´ Z Y , M A R C L E M M E N S and L A N A M R E I D A Cereal Research Non-profit Company, PO Box 391, H-6701 Szeged, Hungary, E-mail: akos.mesterhazy@gabonakutato.hu; Department for Agrobiotechnology, University for Natural Resources and Life Sciences, A-3430 Tulln, Konrad Lorenz Street 20, Vienna, Austria; Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, ON K1A0C6, Canada With tables Received February 17, 2011/Accepted November 7, 2011 Communicated by T Miedaner Abstract Ear rots caused by different Fusarium spp are one of the most dangerous food and feed safety challenges in maize production At present, the majority of the inbreds and hybrids are susceptible Gibberella and Fusarium ear rots (caused by Fusarium graminearum and Fusarium verticillioides, respectively) are the two main diseases, but more than 10 further Fusarium spp cause ear rots Natural infection is initiated by a mixture of the local Fusarium spp., but usually one species predominates Many maize breeders rely on natural infection to create sufficient levels of disease severity for selection-resistant genotypes; however, there are few locations where the natural infection is sufficiently uniform to make this selection efficient and successful Thus, an artificial inoculation method normally performed with one fungal species is now used by more breeders Most published papers on breeding for ear rot resistance are focused on either F graminearum or F verticillioides, and reports involving both or more Fusarium spp are rare Several reports support the hypothesis that resistance to multiple species especially F graminearum, F culmorum and F verticillioides may be common Significant differences in genotypic resistance after inoculation exist Resistance to the two major modes of fungal entry into the ear, via the silk or through kernel wounds, is not correlated in all genotypes The reason is not clear When silk channel resistance was assessed, the data from natural and artificial inoculation trials correlated well Analogous data relating to kernel resistance have not been published Both native and exotic sources of resistance are important, but surprisingly little information is available Few papers report on the use of artificial inoculation during inbred development Most of the publications on inoculation are concerned with testing at later stages when combining ability is tested Inbreds differ in general and specific combining ability for ear rot resistance The expression of resistance to disease severity and resistance to toxins is often used as synonyms, but in fact they are not Higher resistance to visual disease severities mostly results in lower toxin contamination, and the resistance level seems to be the most important factor regulating the toxin content The mode of inheritance of resistance appears to differ: additive, possibly non-additive effects, digenic (dominant) and polygenic patterns have been identified Improved phenotyping methods that take into account the influence of stalk rot and the use of several independent isolates are available The QTLs mostly exhibit small effects and some are validated; however, marker-assisted selection in breeding cannot yet be foreseen As the severity of natural infections tends to correlate with the artificial inoculation results, the incorporation of artificial inoculation methods in breeding programmes is now the most important task As genotypic resistance differences between hybrids are high, the registration of hybrids should consider the use of the inoculation tests to choose most resistant hybrids for commercial production This is the most rapid way to increase feed safety Key words: Fusarium graminearum — Fusarium verticillioides — Fusarium — inoculation methods — resistance — inheritance of resistance — mycotoxins — breeding — corn — maize — Zea mays — ear rot Ear rot diseases of maize, or corn, caused by Fusarium spp have long been known One of the first scientific reports was that of Bisby and Bailey (1923) in Canada In contrast to Fusarium stalk rots that often result in direct yield losses, ear rots rarely so; however, occasional high yield losses have been reported (Vigier et al 2001) As a consequence, only sporadic breeding efforts have been undertaken to increase resistance to ear rots It was not until the discovery of Fusarium mycotoxins that the full impact of the indirect economic loss from an ear rot outbreak became known New regulations for the allowable mycotoxin limits in food and feed have been put in place in most countries Today, more and more maize breeding programmes at both public and private institutions are initiating and expanding breeding programmes to develop resistant inbreds and hybrids for both human and animal consumption In the 1920s and 1930s, hybrid breeding of maize was developed in the USA By the late 1950s, hybrid varieties of maize dominated the maize acreage (Sprague 1977) This resulted in two significant changes in maize breeding: (i) the use of open pollinated landraces and varieties with their broader genetic diversity decreased and (ii) inbreeding for line and hybrid development was carried out in the same location, and owing to a lack of continuous epidemics, selection for ear rot resistance was not possible and susceptibility levels increased As a result, sporadic resistance breeding programmes were initiated in North America and elsewhere Initial studies were mostly on Gibberella ear rot (GER) caused by Fusarium graminearum Schwabe [teleomorph = Gibberella zeae (Schwein.) Petch] Gibberella ear rot is not to be confused with Fusarium ear rot (FER) that is caused by a different Fusarium species, Fusarium moniliforme, which was recently reclassified by Seifert et al (2004) as Fusarium verticillioides [=F moniliforme J Sheld (sexual stage: G moniliformis Wineland)] Early on, it was reported that resistance to both diseases is inherited in a quantitative manner (Boling and Grogan 1965, Ullstrup 1977) Interest in increasing ear rot resistance spiked with the detection of Fusarium mycotoxins in the grain Zearalenone wileyonlinelibrary.com ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A Table Fusarium spp found in naturally infected maize grains in several maize-producing countries Country Canada Canada China Europe France Germany Germany Germany Hungary Mexico Switzerland USA USA USA USA USA Zambia Fac Fant Fav Fc Fcer Fcr Feq Fg Fm Fo Fp Fpro Fsg Fsp Ftr Fv Fven Other x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Author Sutton and Baliko (1981), Sutton (1982) Martens et al (1984) Tanaka et al (1988) Logrieco et al (2002) Folcher et al (2009) Focke (1966), Focke and Kuăhnel (1964) Sass et al (2007) Goertz (2009), Goertz et al (2008, 2010) Mesterha´zy and Vojtovics (1977) Garcia-Aguirre and Martinez-Flores (2010) Dorn et al (2009) Koehler (1959) Koehler (1950) Ullstrup (1953) Jugenheimer (1976) Munkvold (2003b) Schjoth et al (2009) Fac, F acuminatum, Fant, F anthophilum, Fc, F culmorum, Feq, F equiseti, Fg, F graminearum, Fm, F moniliforme, Fo, F oxysporum, Fp, F poae, Fpro, F proliferatum, Fsg, F subglutinans, Fsp, F sporotrichioides, Ftr, F tricinctum, Fv, F verticillioides, Fven, F venenatum (ZEA) was the first toxin to be identified followed by deoxynivalenol (DON) (Mirocha 1974, Vesonder et al 1979), both produced by F graminearum The F verticillioides mycotoxin, fumonisin B1, was discovered in 1988 (Gelderblom et al 1988) The main goal of breeding efforts was to develop either toxin resistance or disease resistance; however, the relationship between these two types of resistance was not clearly defined and they were often used as synonyms (Clements and White 2004, Menkir et al 2006, Williams 2006) Knowledge relating to mycotoxins grew rapidly from many different laboratories The metabolic synthesis and physiological effects on humans and livestock are now known for most of the toxins This resulted in the establishment of legally allowable limits for many toxins in all important grain-producing regions of the world Unfortunately, it did not result in the development of commercial hybrids with improved resistance as most are susceptible or very susceptible (Munkvold 2003b, Reinprecht et al 2008) As an alternative to or in addition to resistance breeding, breeding transgenic maize for increased insect resistance and better agronomic practices are suggested for ear rot control Insect wounds, especially to the ear, from pests such as Ostrinia nubilalis, Diatraea grandiosella, Diabrotica virgifera virgifera, Heliocoverpa zeae and Frankliniella spp., increase the levels of Fusarium infection by creating new points of entry for the fungus to enter the plant (Archer et al 2001, Parsons and Munkvold 2010a) Numerous recent articles have indicated that transgenic plants modified by Bt genes (genes from Bacillus thuringiensis with insecticidal properties) have significantly decreased DON and other toxins levels (Munkvold et al 1997a, Dowd 2001, Munkvold 2003a, de la Campa et al 2005, Papst et al 2005, Wu 2006) No fungicide control has yet to be released until just recently in Europe (Loăer et al 2010a) Folcher et al (2009) reported a 90% reduction in Fusarium mycoflora with some fungicides; however, further analysis is required Resistance to ear rots is important for both grain and silage maize Stalk rot resistance in silage maize is a crucial trait, as mycotoxins can be found in stalks, but a detailed evaluation of stalk resistance is beyond the scope of this paper For more information, the reader is referred to Christensen and Wilcoxson (1966) who published an excellent review of the early literature More recent partial reviews can be found in Afolabi et al (2008), Barrie`re et al (1997), Nagy and Cabulea (1996), and Pappelis (1984); however, a comprehensive review of the past 50 yearsÕ research has yet to be written The more recent binding EU regulations on toxin contamination for human consumption and recommendations for animal feeding (Anonymous 2005, 2006, 2007) have forced a renewed interest in breeding efforts for ear rot resistance as the preferred method of control As the growing knowledge on resistance has not been reviewed in detail in the last few decades and numerous papers on resistance have been published since the two partial reviews by Munkvold (2003b) and Clements and White (2004), we decided to summarize the novel results and concepts developed and discuss several important conclusions for the future The Pathogens and Associated Mycotoxins The pathogens One of the first reports on ear rots caused by Fusarium spp originated from Canada by Bisby and Bailey (1923) The ear rot caused by F graminearum was later called GER and is widespread in Canada, the USA and many other countries, Table (Koehler 1957, 1959, Sutton and Baliko 1981, Reid et al 1996a) Fusarium verticilliodes (referred to as F moniliforme in papers published prior to the reclassification by Seifert et al (2004) as F verticillioides) was identified as a weak ear rot pathogen in the earlier reports (Ullstrup 1953, cited by Jugenheimer 1976) but has since been shown to be more of an endophyte fungi that tends to have lower visible symptoms of infection in the kernels and can be systemic in the maize plant (Munkvold et al 1997a,b) This pathogen is the causal agent of FER Other members of the Liseola Section like Fusarium proliferatum are also understood to be pathogens responsible for FER Many different species occur in maize grain besides these three (Table 1), and in many cases, they play an important role in causing GER and FER Folcher et al (2009) identified 12 Fusarium spp in France In the southern regions, mainly F verticillioides and F proliferatum were present, and in the northern part, mainly F graminearum and Fusarium culmorum Hungary is similar; Mesterha´zy and Vojtovics (1977) found that in dry years the distribution of the Fusarium spp is closer to the distribution of Fusarium spp in Breeding for resistance to ear rots Table 2: Mycotoxigenic Fusarium species associated with cereal crops and their mycotoxins (Logrieco et al 2002) Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium Fusarium species1 acuminatum anthophilum avenaceum cerealis chlamydosporum culmorum equiseti graminearum heterosporum nygamai oxysporum poae proliferatum sambucinum semitectum sporotrichioides subglutinans tricinctum verticillioides Mycotoxins2 T2, MON, HT2, DAS, MAS, NEO, BEA BEA MON, BEA NIV, FUS, ZEN, ZOH MON DON, ZEN, NIV, FUS, ZOH, AcDON ZEN, ZOH, MAS, DAS, NIV, DAcNIV, FUS, FUC, BEA DON, ZEN, NIV, FUS, AcDON, DAcDON, DAcNIV ZEN, ZOH BEA, FB1, FB2 MON, BEA DAS, NIV, FUS, MAS, T2, HT2, NEO, BEA FB1, BEA, MON, FUP, FB2, DAS, T2, NEO, ZEN, MAS, BEA ZEN, BEA T2, HT2, NEO, MAS, DAS BEA, MON, FUP MON, BEA FB1, FB2, FB3 Fusarium nomenclature according to Nelson et al (1983) Bold letters indicate the important mycotoxins AcDON, monoacetyldeoxynivalenols (3-AcDON, 15-AcDON); AcNIV, monoacetylnivalenol (15-AcNIV); BEA, Beauvericin; DiAcDON, Diacetyldeoxynivalenol (3,15-AcDON); DAcNIV, diacetylnivalenol (4,15-AcNIV); DAS, diacetoxyscirpenol; DON, deoxynivalenol (vomitoxin); FB1, fumonisin B1; FB2, fumonisin B2; FB3, fumonisin B3; FUP, fusaproliferin; FUS, fusarenone-X (4-acetyl-NIV); FUC, fusarochromanone; HT2, HT-2 toxin; MAS, monoacetoxyscirpenol; MON, moniliformin; NEO, neosolaniol; NIV, nivalenol; T2, T-2 toxin; ZEN, zearalenone; ZOH, zearalenols (a and b isomers) the south, and in wetter years the Fusarium spp of the northern regions predominated Other Hungarian results (Be´ke´si and Hinfner 1970, Bı´ ro´ne´ 1975) support this complexity of the pathogen population Similar species distributions are found in North America with GER predominantly found in the northern regions and FER in the southern regions or in dryer years in a northern area (Reid et al 1999) A single ear or grain can occasionally be infected by different Fusarium spp depending on the conditions of a growing season in a region (Logrieco et al 2002) Identification of Fusarium spp is easier than it used to be The most important species have speciesspecific markers and can be quantified by PCR analysis (Reid et al 1999, Pauls et al 2001, Waalwijk et al 2004, Xu et al 2008, Nicolaisen et al 2009) Pathogenicity between Fusarium spp and aggressiveness within a species is quite variable Isolates of F graminearum and F culmorum are commonly highly aggressive with a considerable proportion of the isolates considered very aggressive Fusarium verticillioides and the other Fusarium spp tend to display lower aggressiveness Careful screening of isolates is necessary to select the more aggressive ones for artificial inoculation in a breeding programme (Mesterha´zy 1978, Reid et al 2002, Iglesias et al 2010, Miedaner et al 2010) For F graminearum and F culmorum from wheat, Mesterha´zy (1983, 1995, 2002) and Mesterha´zy et al (1999) found large variation in aggressiveness and DON production between isolates and years Considerable differences in aggressiveness and toxin production were observed also when inocula from the same test tube were tested, and this did not correlate with the conidium concentrations measured Similar data were obtained for maize (A´ Mesterha´zy, unpublished) Garcia et al (2009) concluded that it would be difficult to predict mycotoxin levels because these levels are highly associated with the contaminating fungal strain not just the environmental conditions Marı´ n et al (2008) also reported high variability in mycotoxin production between different fungal strains and on different substrates Therefore, complicating prediction models based on visible symptoms and mycotoxin formation may be difficult to create From the same isolate, inocula with different aggressiveness can be produced (Kova´cs et al 1994) For this reason, a change of inoculum, even though originating from one and the same isolate, within an experiment is not recommended as the two levels of aggressiveness of the different inocula can vary and influence infection levels Therefore, each experiment is so planned that a change of inoculum should not occur Further research on aggressiveness, fungal mass and conidium concentration is necessary Fusarium graminearum possesses different pathogenesisrelated (PR) genes (Dufresne et al 2008), but these are not necessarily virulence genes and they cannot be used in a breeding programme It is certain that Fusarium spp have virulence factors, but it is not known whether the different Fusarium spp possess common and/or different virulence factors Mesterha´zy et al (1999) reported that DON production is a virulence factor of F graminearum in wheat Harris et al (2005) also proved this for F graminearum in maize; its role, however, is not known Another risk factor may be the change in pathogenic population structure when more resistant hybrids are grown No reliable information is available on this topic, but it will presumably significantly influence breeding programmes Although no data exist for maize, in wheat there is no evidence of race-specific specialization within Fusarium spp (Snijders and van Eeuwijk 1991, Mesterha´zy 1995) The same QTLs gave protection to all Fusarium spp tested (Mesterha´zy et al 2007) Research in this field for maize should be undertaken For resistance breeding, the appreciable number of Fusarium spp creates a serious challenge Mycotoxins Fusarium spp produce a large number of chemically very different mycotoxins (Table 2) (Logrieco et al 2002) The high diversity of these toxins and those detected recently (Barto´k et al 2006, 2010) excludes the possibility of selecting for general toxin resistance The two toxins of most importance to GER are DON and ZEA If contaminated grain is fed to livestock, especially swine, the trichothecene DON results in vomiting, feed refusal, decreased weight gain and reproductive problems (Vesonder et al 1981, Prelusky et al 1994) Storage workers might also be exposed to toxins, mainly through powder of infected grains in the air through inhalation, but this problem was not mentioned in the monograph of Christensen (1982) In the authorized Hungarian regulation, there is no word on this problem We think that this research branch will be important in the future to see clear and make the necessary regulations This toxin is also an immunosuppressant and thus predisposes animals to other diseases and masks underlying toxicoses (Pestka and Bondy 1994) Zearalenone causes reproductive problems including reduced litter size, swine oestrogenic syndrome and male infertility (Prelusky et al 1994) Human relations are also important, immunsuppression of trichethecenes was proved (Berek et al 2001), and ZEA influences also children hormone household causing telarche (Szuăts et al 1997) Therefore toxin contaminated grains may cause human health problems FER results in the contamination of the grain with the polyketide fumonisin mycotoxins such as FB1, which causes equine leukoencephalomalacia (Kellerman et al 1990), porcine pulmonary oedema (Harrison et al 1990), liver cancer in rats (Gelderblom et al 1988) and neural tube defects in mice (Voss et al 2006) Fumonisins have also been associated with human oesophageal cancer (IARC 1993) Besides causing direct and indirect economic losses, these fungi can also affect the health of grain handlers and processors Throughout the literature, resistance to these toxins and resistance to disease symptoms have been treated as two different types of resistance Some Fusarium spp with apparent lower aggressiveness, as measured by visible disease symptoms, pose an additional challenge as these pathogens may be excellent toxin producers like F verticillioides with its many fumonisin toxins and Fusarium sporotrichioides with its T-2 toxin Resistance to these pathogens should therefore not be neglected Resistance to one species may not be correlated to resistance to other species (Reid et al 2002, 2009) Also, selection for resistance to a pure isolate of one species may not result in resistance to a multi-Fusarium population in commercial maize fields Infection Pathways, Disease Symptoms and Evaluation of Resistance There are three main modes of fungal entry or infection pathways, by which Fusarium spp enter maize ears: (i) by fungal spores landing on the silks, germinating and then the fungal mycelia grow down the silks to infect the kernels and cob (rachis) (Koehler 1942); (ii) wounds created by insects feeding on the ear or from bird or hail damage offer a point of entry for fungi (Sutton 1982); and (iii) some Fusarium spp are systemic, such as F verticilliodes, and can enter the ear from infected stalks (Foley 1959, Munkvold et al 1997b) Which infection pathway is more important depends on the Fusarium spp that is predominant and insect pressures in a given geographical location In some locations, ear rot outbreaks are mainly associated with infection through the silk while in other locations where maize boring insects are a problem and are not controlled by other measures, infection through the kernels is predominant ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A Gibberella ear rot The symptoms of GER, caused mainly by F graminearum, are characterized by a pinkish coloured mould (White 1999) Similar symptoms are found with F culmorum infections; this species is a pathogen also associated with GER, but it lacks a Gibberella teleomorph Infection of the ear commonly begins as white mycelium moving down from the ear tip This mycelium later turns reddish-pink on infected kernels In some cases, pinkish fungal growth can be found on the exterior husk leaves, and in severe infections, it is impossible to separate the husks from the kernels as the entire ear becomes a tightly bound mass of fungal and plant tissue that appears ÔmummifiedÕ If infection occurred through kernel wounds, similar fungal growth is seen but it starts from the initial wound site and tends to spread to the tip of the ear faster than to the butt of the ear The latter occurs because silks from the butt kernels emerge from the ear earlier than kernels from the tip of the ear and are hence pollinated earlier and dry earlier (Reid and Sinha 1998) Once the kernels reach 22– 23% moisture, it is difficult for the fungus to further infect (Christensen and Kaufmann 1969); however cob (rachis) moisture can be 15–25% higher than kernel moisture, so the infection may spread in the cobs and can enter younger kernels via the pedicel In some cases, only the cob is infected; the ear may appear to be symptomless but when squeezed by hand, it will be quite spongy feeling and the cob will be wet and often pink/red in colour How fast symptoms develop in a given year is highly dependent on the environment that influences not only ear development and subsequent kernel drydown but also fungal growth The optimum temperature for GER development is 26– 28°C, while FER rot has broader range extending to higher temperatures (Reid et al 1999) GER also requires a much longer period of precipitation after infection usually around the time of plant (Reid et al 1996a) Infection through the silks cannot proceed once the silks have dried out (Reid et al 1992a, Reid and Sinha 1998) Recently, Xiang et al (2010a) reported that there is a direct relationship between kernel drydown rates of a given maize genotype and the extent of ear rot severity symptoms Husk tightness (Koehler 1959), ear declination and physiological resistance mechanisms all influence the spread of infection Stalk rot and ear rot are strongly interrelated (Mesterha´zy 1983, Mesterha´zy and Kova´cs 1988, Mesterha´zy et al 2000) as stalk rot interrupts the water supply to the ears and speeds up development and drying of the ear This can reduce ear rot by 50% or more Thus, it is important that in ear rot breeding nurseries, like all disease nurseries, the plants be as healthy as possible prior to ear rot inoculation; stalk rot should be controlled and only ear rot data on healthy stalks should be accepted to estimate resistance levels (Mesterha´zy 1979, Kova´cs et al 1994) Fusarium ear rot In contrast to GER, symptoms of FER from F verticillioides infection occur mainly on individual kernels or on limited areas of the ear (White 1999) In some ears, many independent infection sites may develop Infected kernels develop a cottony growth or may develop white streaks on the pericarp and fungal growth on the cob Ears infested by earworms are usually infected with F verticillioides Eller et al (2008a) state that the disease is prevalent in warm, dry conditions, like those common to the southern United States, and F verticillioides is found in grain or crop residue of virtually all mature maize Breeding for resistance to ear rots fields in the United States However, F proliferatum and F subglutinans are also minor causal agents of FER, as are probably other members of the Liseola Section of Fusaria (Iglesias et al 2010) Reid et al (1996a) adds that rainy weather or irrigation at silking thereafter significantly increases disease severity for FER and especially for GER Duncan and Howard (2010) studied the initial phases of the infection process by F verticillioides and described in detail how the hyphae spread and colonize host cells and how the macroscopic symptoms develop In many cases, the extent of toxin contamination is proportional to the visual severity of infection; however, asymptomatic kernels may also be infected and may contain toxins, usually trace amounts (Reid et al 2009) Bacon and Hinton (1996) reported on the endophytic colonization of maize plants by F moniliforme (F verticillioides) and presumed that this can contribute to toxin contamination of the plants Murillo-Williams and Munkvold (2008) reported that systemic infections from the stalk to the ear leading to more asymptomatic infections and toxin accumulation may be a problem predominantly in hot regions but not in the cooler areas As global warming seems to be durable, this problem may become more serious in the currently cooler regions like central Europe and Canada Other ear rots For the remaining Fusarium spp., a clear demonstration of the significance of a given species, its symptoms and epidemiological conditions is poorly documented White (1999) does not even mention them Their toxins, however, play a significant role; thus, their neglect is by no means justified For example, T-2 toxin is produced by Fusarium sporotrichioides (Table 1) and others As we have only FER and GER to describe these ear rots, a new term should be found for ear rots caused by other Fusarium spp Perhaps, they can be grouped under the term ÔOther Fusarium Ear RotÕ (OFER) It is unpractical to assign a new term to each of the species unless it is found that the incidence of a given species increases because of environmental or agronomic changes Reid et al (1996a, 1999) stress that the two main Fusarium spp may produce mixed infections Some species will dominate in a field (Table 1) because of environmental conditions, but natural infection is almost always mixed This makes breeding for resistance complicated as minor species should also be considered Visual identification of a pathogen is often not possible: with mixed infections, atypical symptoms can occur In inoculated trials, the breeder is determining which species is the predominant one; in trials where multiple species are inoculated at the same time, the environmental conditions after inoculation determine which species will predominate (Reid et al 2001a,b) When natural infections are evaluated, general ear rot values based on visual symptoms are given without species identification Most breeders follow this procedure, and Variety Offices like that in Hungary similarly report disease severity (percentage or rating of diseased area on the ear) and incidence (percentage of the number of visually infected ears) because of Fusarium spp without species identification (Gergely et al 2010) Seedling blight A greenhouse method was evaluated to check seedling stage resistance (Mesterha´zy 1982) Commercial garden perlite (a very light-heated volcanic stone) is used, and this makes possible the washing of the whole plant and checking of plant damage and root rot more precisely The method is used also for testing seed dresser against Fusarium seedling blight (Mesterha´zy 1982) Evaluation of disease Disease incidence is measured as the number or percentage of visually infected ears Disease severity is often rated visually on scales ranging from to 5, to and to or as percentage of the ear surface with symptomatic kernels For breeding purposes, the rating scales are commonly used, but for scientific studies the percentage of infected kernels may give more precise data In addition to disease incidence and severity, it is important to assess other natural infections (e.g Aspergillus) as well as the occurrence of insect, bird, wind or hail damage on the ear because all of these factors will influence infection levels As wounds are often points of fungal entry, extensive damage may influence resistance measurements In silk resistance evaluations, only ears without insect or other wound-mediated infection should be considered for evaluation and toxin analyses Reid and Sinha (1998) found that visual symptoms of GER stabilized and reached a maximum 6–8 weeks after inoculation of the silk channels The rate of fungal spread after inoculation is quite variable and highly related to genotypic resistance Mesterha´zy et al (2000) measured a fungal spread of 0.1–0.3% of the ear surface per day in highly resistant genotypes compared to 2–3% per day for the most susceptible genotypes measured after inoculation and until kernel moisture reached 28% Similar to GER, working with FER Bush et al (2004) found that fumonisin content is at a maximum at 20% kernel moisture and cannot be detected earlier than 35–40% moisture content All of these studies indicate that for both GER and FER, symptoms reach a maximum weeks after pollination and that toxin levels reach a maximum at weeks The later is therefore an optimum harvest time for ear rot evaluations; however, it should be noted that a wet fall and/or wet conditions in the grain after harvest can lead to more toxin development Considerable debate exists whether or not toxin evaluations should also be used to evaluate disease resistance Without a doubt, the level of mycotoxins is an extremely important trait to record; however, mycotoxin analyses can be cost-prohibitive in some breeding programmes Many breeders rely on visual ratings of disease severity for most of the inbred development and use toxin analysis for parent selection, testcross evaluation and the final developmental stages of inbreds and hybrids It must be recognized that although there are published reports on high correlations between disease symptoms and toxin levels (Reid et al 1996b) for F graminearum, it seems to be necessary for F verticillioides where significant differences in amount of toxins can be detected at the same visual severity rating (Butron et al 2006) In their review relating to the prediction of mycotoxins in food, Garcia et al (2009) state that not all fungal growth results in mycotoxin formation and the detection of mycotoxic fungi may or may not imply the presence of mycotoxins Strains of mycotoxigenic species are able to synthesize mycotoxins in different amounts, and conditions conducive to fungal growth may not be conducive to mycotoxin production For this reason, special attention must be paid to the relationship between visual symptoms and the level of toxin contamination, the NIRS may provide a cheap and efficient way to check toxin content Resistance Components and Artificial Inoculation As the severity of natural infection is not consistent from year to year, maize researchers must use artificial inoculation methods to inoculate the plant material with fungal spores (Schaafsma et al 1997) Currently, the only way to screen for resistance to GER is in the field Satisfactory levels of infection and reliable genotypic differentiation have not been achieved under greenhouse conditions, and there is no laboratory technique or seedling test that can be used to screen for resistance that is exhibited in a fully grown plant (Reid et al 1996a) At present, the literature highlights the silk channel method of inoculation rather than the kernel wound inoculation by colonized toothpicks or injection of a fungal spore suspension Both techniques result in the spread of infection from infected kernels to neighbouring kernels; however, with the silk channel inoculation, the infection must first proceed down the silks to the kernels Whichever technique is used, the research is cautioned to first determine the major mode of fungal entry for the Fusarium spp of interest in a given geographical area The methods used for artificial inoculation and the evaluation of resistance are similar for both GER and FER as well as other Fusarium spp Researchers working with two or more Fusarium spp used the same methodology for all (Reid et al 2002, 2009, Loăer et al 2010a) Methods of inoculation A number of artificial inoculation methods and their variants have been developed They were first assessed by Ullstrup (1970) The oldest is the toothpick method of Young (1943) Working with this method, several improvements were made (Mesterha´zy 1982, 1983) The toothpicks are boiled in deionized water three times to wash out tannins and other fungal growth–inhibiting compounds from the wood Thereafter, they are air-dried and submerged into a suitable liquid medium (e.g Czapek-Dox) for h in an Erlenmeyer flask Most of the medium is then removed leaving only 5–10 mm depth of media in the flask to ensure high humidity; the flask is then autoclaved for h at 120°C A small amount of mycelium is transferred to the sterilized flask, and after weeks the fungus grows through the toothpicks that are now ready to use (Mesterha´zy 1982, 1983) The toothpicks are generally used for the inoculation of ears in two ways: insertion into the centre of the ear or into the silk channel representing kernel resistance or silk channel (silk) resistance, respectively (Reid et al 1996a, Plienegger and Lemmens 2002) After 7–9 weeks, disease severity is estimated by the percentage of visually infected kernels directly or by using a rating scale as discussed earlier (Mesterha´zy 1978, Enerson and Hunter 1980, Reid et al 1996a) One of the disadvantages of this technique for silk channel resistance evaluation is with maize genotypes where the cob outgrows the husk leaves, causing the toothpick to fall out thus reducing infection severity Another criticism of the toothpick technique is that infection levels can be too high because of the fact that the toothpick itself is a substrate for fungal growth and kernel inoculations result in severe wounding of several kernels and the cob (rachis) ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A It is important to choose an inoculation technique that results in a sufficient level of infection to differentiate genotypic differences in resistance but not so severe of an inoculation that these differences are hard to observe It is rare that natural inoculations are as severe as artificial inoculations As a consequence, several other methods of inoculation have been developed since the toothpick technique; however, it should be noted that for some researchers, the toothpick technique is necessary to achieve the best genotypic differentiation Many researchers produce Fusarium suspensions in liquid media with or without aeration with sterile air, or the Fusarium conidia are washed from solid media poured into Petri dishes The production methods are very variable (Mesterha´zy 1978, Reid et al 1996a, Plienegger and Lemmens 2002) Mesterha´zy (1983) used the bubble breeding method that allowed the facile production of large quantities If a researcher desires to inoculate with a mix of isolates or species, to avoid competition in the media, the isolates/species are grown separately and then just prior to inoculation mixed (Reid et al 1996a, Presello et al 2006) Loăer et al (2010a) used this method, but the isolates were not mixed to avoid isolate x isolate interactions Conidia have been shown to be as good infection materials as mycelium (Takegami and Sasai 1970) and in fact, in natural infections, conidia are often the source of infection The developed conidial suspension can be used in different ways (Papst et al 2007): the silks can be sprayed with the suspension, or the cob tip can be immersed into the suspension The resistance type observed after this type of inoculation is termed silk resistance After spraying, the silks can be covered with polyethylene bags to achieve higher humidity and consequently higher disease severity However, the toothpick method gave more than double level of GER and FER severity than these methods (A´ Mesterha´zy and E Toldi, unpublished) and covering the ear with a polyethylene bag also serves to increase the development of other fungi and bacteria, which may complicate resistance evaluations (L M Reid, unpublished) A more common use of the suspension is to inject it into the silk channel (silk channel resistance) or the centre of the ear (kernel resistance) using syringes or vaccinators (Reid et al 1996a) The amount of inoculum injected can vary from several ll to ml, with the larger volumes used for silk or silk channel resistance Reid et al (1996a, 2009) and Presello et al (2008), for example, used ml Loăer et al (2010a) utilized only ml The conidium concentration varies: Presello et al (2008) and Loăer et al (2010a) used à 106 conidia/ml for FER, while Loăer et al (2010a) used à 105 for GER Clements et al (2003) compared several inoculation methods for FER: injection of inoculum through the ear husk leaves at the R2 developmental stage; spray inoculation with different variants (coverage of silks with shoot bags, and re-inoculation after week), and insertion of six Fusarium-colonized toothpicks into the silk channel Only the injection through the husk leaves significantly increased fumonisin concentration and infection severity Eller et al (2008a) developed a methodology to identify resistance against F verticillioides and identified superior genotypes They preferred silk inoculation, which appears to be more important for the entrance of F verticillioides Furthermore, they compared four inoculation methods and found that the highest infection severity and largest genotypic differentiation were found when inoculum was inserted through the husks Bush et al (2004) compared five inoculation methods and concluded the most useful to be Breeding for resistance to ear rots penetrating husks with pin bars and injecting inoculum down the silk channel Comparatively few data are available on the relationship between kernel and silk channel resistance Lemmens (1999, 2010) found low correlation (r = 0.12), whereas Loăer et al (2010a) reported a much closer relationship (rP = 0.66) for a wider genetic stock Chungu et al (1996b) also found a correlation between the two traits (r = 0.77–0.89) Few genotypes exist with good resistance to both modes of fungal entry (Reid et al 2003) The relationship between silk vs kernel resistance is very important as two parallel methodologies would be cost-prohibitive in a breeding programme Loăer et al 2010b has reported signicant genotypic variances for kernel and silk channel resistance The correlations between silk and kernel resistance were moderate (r = 0.66), but there were genotypes with very different resistance level with both methods Therefore, authors suggest the use of both inoculation methods There are arguments on the use of both inoculation techniques and on the number of isolates to be used, including whether to use a pure or mixed conidial suspension The breeder is cautioned to carefully determine which mode of fungal entry is predominant at their research station and how heterogeneous is the Fusarium population at the station before making decisions on inoculation methods In addition, the genotypic resistance that the breeder initially has to work with will influence this decision as sources of resistance to one mode of fungal entry may not be available in adapted material For example, Presello et al (2005) found kernel resistance for FER useful, but the sources for this resistance are very limited compared to sources for silk resistance On the other hand, the breeders need the assistance of the research to answer better the questions they have Timing of inoculation In a summary report, Jugenheimer (1976) cited Andrew (1954), who initiated GER by placing infected barley grains under the husk leaves 5–10 days after silking, that is, a very early form of silk channel inoculation, and further applied by spraying silks at silking to promote FER In both cases, symptoms appeared after month This study can possibly be considered the first of many silk resistance tests Since that time, many other researchers have performed silk and kernel inoculations at various stages of ear development For GER, Papst et al (2007) sprayed the silks 4–7 days after mid-silking and applied inoculum with wounding (toothpicks) 10–15 days after midsilking For kernel resistance, later inoculation times result in a general decrease in disease severity for all species; though, it was not as strong as that reported with silk inoculations (Reid and Hamilton 1996, Reid et al 2002) The best differentiation of genotypes was achieved when the kernel inoculation was performed 15 days after mid-silking (Reid and Hamilton 1996, Reid et al 2002) Silva et al (2007) used 14 days in regular tests; later inoculations (19, 21 and 23 days after mid-silking) resulted in significantly less disease severity Reid et al (1996a) used ml of suspension inoculated 4–6 days after mid-silking for both GER and FER; earlier inoculations (2–3 days after mid-silking) led to very high infection severities, and later inoculations (10 days or more) resulted in little or no infection (Reid et al 1992a, 2002) Schaafsma et al (1997) concluded that the beginning of silk browning is the ideal time for the silk channel inoculation They found irrigation useful for the initial stages of infection for both inoculation methods; this supports the data of Reid et al (1996a) Silva et al (2007) compared different inoculation methods Robertson et al (2006) and Eller et al (2008a) used silk and direct ear inoculations at 10 days after mid-silking and days later, respectively Both methods gave replicable results for several superior lines Disease levels between cultivars ranged from 20 to 50% Interestingly, inoculation of the husk leaves resulted in significantly more severe infection of the grains Miller et al (2007) found that silk-mediated F graminearum infection reached the kernels in 7–9 days in susceptible genotypes, and 12–15 days in more resistant genotypes These differences are of great importance in our goal of understanding disease severity and consequently resistance differences This explains the roughly week difference in timing of inoculation between the two inoculation methods Adding week to the 5–7 days postsilking of the silk channel inoculations, we achieve the optimum time for inoculating the kernels at the same stage in which the fungus growing down the silk would reach the kernels It should be mentioned that there are differences in the optimum timing of inoculation depending on the level of resistance For early-maturing genotypes, the kernel development is more rapid than it is in later-maturing genotypes Thus, a 10-day delay after mid-silking for kernel inoculations may be too late for some early genotypes In large-scale breeding or screening programmes, the researcher should not rely just on chronological time for determining inoculation time; genotype maturity and growth stage must be taken into consideration as well as the effect of the environment in influencing growth and development of the ear Control genotypes with known resistance levels in different maturity groups can be used to monitor this in large screenings such as a mapping population with very diverse silking and maturity rates Natural vs Artificial Inoculation of Maize Ears Disease severity after natural infection varies strongly from year to year, from location to location and with the length of the vegetation period; earlier hybrids normally show less ear rot infection and less toxin contamination (Papst et al 2007) Late-sown or later hybrids tend to be more infected by F graminearum and contaminated by DON than early ones (Manninger 1978, Blandino et al 2009a) Good agronomy practices can also decrease the risk of fumonisin contamination (Blandino et al 2009b) Weather conditions affect different Fusarium spp in different ways (Reid et al 1999), and the types and levels of physiological resistance strongly influence the severity observed The many Fusarium spp that are present in the fields cannot be controlled, and this complicates the comparison of natural and artificial inoculation results (Loăer et al 2010b) Ear tips that are fully covered by the husk leaves tend to be subject to more infection than those with early cob outgrowth as the humid period is longer in the former case (Butron et al 2006) The optimum time for natural infection to occur is similar to that for artificial inoculation, about 10– 14 days after silking after which the infection severity sharply decreases Natural infection is moderate to low in most years, and it is absent in some years and does not allow an efficient selection of genotypes with high resistance These restrictions also explain the high ratio of susceptible inbreds and commercial hybrids bred under natural infection pressure In 2010, an unusual strong epidemic caused by different FERs ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A was observed in Hungary Disease incidence vas very high, and severity was on average 20% No genotype could be rated as resistant; rather, differences in susceptibility could be observed In the most severely infected location, the incidence was between 28 and 87%, and the severity was between 8.5 and 29.4% Most of the hybrids were susceptible or very susceptible, indicating the need for a more successful breeding against ear rots (Anonymous 2010) Artificial inoculation methods have several advantages over relying on natural infection to create an outbreak of ear rot The Fusarium spp isolate, inoculation times and mode of entrance/infection pathway are known The disease severity is normally much higher and results in more uniform ratings that make genotypic comparisons easier and reproducible especially when phenotyping for QTLs Genotype differences are normally much larger for artificial inoculation regimes than for natural ones (Oberforster and Felder 2010) Silva et al (2007) compared natural and artificial inoculation results and concluded that when only the husks were wounded, the sterile water control resulted in higher disease severity than the natural control As the ranking order of the cultivars after wounding only and after inoculation did not seem to differ from that of the natural control, it was suggested to wounding only and to allow natural infection to proceed through the wounds in areas with high inoculum pressures as that found in the Andes where the study was conducted Geographical areas such as these with reliable levels of natural infection are rare, especially in most maize-growing regions of the world In spite of all the concerns, both artificial and natural infections are of great significance The genotypes selected under artificial infection pressure must demonstrate their superior resistance under natural infection pressures For this reason, research that clarifies the relationships between artificial and natural infection results is of high priority Lemmens (2010) observed a close correlation between the results of the silk channel method and natural infection (r = 0.75–0.96) In 2004, 2005 and 2006, Palaversˇ ic´ et al (2010) found medium to close (r = 0.66, 0.61, 0.84) correlations between the silk channel and natural infection severity data The data support the view that artificial and natural infection data tend to be closely correlated This could mean that some form of complex resistance to different Fusarium spp exists The data suggest that this hypothesis may hold true Some maize breeders oppose the use of artificial inoculation methods The reasons listed include the opinions that the natural infection severity is sufficient for efficient selection and that wounding at inoculation is far from the natural mode of infection and introduces instability in the system However, in tests on commercial hybrids, these views cannot be fully justified as many hybrids from these programmes display considerable susceptibility The effect of wounding seems to be overemphasized In susceptible genotypes, the infection spreads rapidly to the unwounded, healthy kernels, whereas in resistant genotypes, the spread is very limited or does not occur at all Some companies in many countries facing the food and feed safety problems have started intensive selection work with artificial inoculation methods Inheritance, Genotypic Differences and Sources of Resistance Boling and Grogan (1965) estimated several additive, dominant and additive x dominant digenic epistatic gene effects They estimated an average dominance of approximately 0.5, and the number of participating genes was estimated at 1.47, a relatively low number Hart et al (1984) reported that GER resistance is governed by genetic factors Symptomatic and asymptomatic kernel infections have been studied in sweet corn hybrids between inbreds with different susceptibility levels (Nankam and Pataky 1996); a broad range of heritabilities for the two symptom groups were recorded, and resistance was determined to be controlled by several genes In two maize populations, Robertson et al (2006) found genotypic and phenotypic correlations between fumonisin and FER data of 0.96 and 0.40, and 0.86 and 0.64, respectively; heritability estimates for fumonisin were 0.75 and 0.86, and for ear rot resistance, 0.88 and 0.47, respectively These high genetic correlations suggested that it is highly possible to reduce fumonisin contamination indirectly by increasing FER resistance levels Using a silk channel inoculation method, Headrick and Pataky (1991) observed a significant maternal effect on hybrid performance Eller et al (2008a) concluded that the US hybrid maize crop was based on crosses between proprietary inbred lines, and many of them were developed from older, publicly developed inbreds representing a rather narrow gene pool As the resistance level is seemingly unsatisfactory, they suggested a search for germplasms with higher resistance not closely related to this group Natural ear rot There are many surveys for natural infection; in Europe, several Variety Offices regularly use the data for decisionmaking (Hertelendy et al 2010, Oberforster and Felder 2010, Palaversˇ ic´ et al 2010, Pastircˇa´k et al 2010) In most years, the infection severities are low In Hungary in 2010, the strongest epidemic was recorded for at least 20 years: the severity of maize ear rot across eight sites (the best-yielding hybrids were tested from many leading companies) was 35% with hybrid reactions ranging between 27 and 48%, the severity (coverage) by ear rot was 10.2% (range, 7–14%) The most infected site had an average disease incidence of 63% (range, 28–87%) and mean severity of 20% (range, 8–29%) This underlines two facts: (i) we cannot be satisfied with the results of breeding under natural infection regimes and (ii) a significant improvement in hybrid resistance is necessary Similar outbreaks occur in other countries, and when this happens the result is high levels of localized disease incidence and severity Gibberella ear rot The inheritance of resistance to Fusarium spp is complex Reid et al (1992b) carried out a complete diallel analysis with 12 inbreds representing highly resistant to highly susceptible selected from a screening of 37 inbreds after silk channel inoculation Both general (GCA) and specific combining ability (SCA) were significant The GCA values were correlated to disease severity data; however, the performance of the hybrids could not be predicted based on the GCA of the parents Four inbreds exhibited significant GCA for resistance to F graminearum Chungu et al (1996a) tested the inheritance of kernel resistance by injecting a small amount of liquid inoculum into the centre of the ears Generation mean analysis indicated that resistance to F graminearum was under both simple (additive and dominance) and digenic (dominance x dominance) effects Estimates of the number of factors Breeding for resistance to ear rots affecting kernel resistance ranged from 4.6 to 13.7 Lemmens (1999) described a similar phenomenon – maize hybrids seem to possess different resistance levels as regards kernel and silk channel resistance In some hybrids, Kova´cs et al (1994) found maternal effects, and in others, a paternal effect; kernel resistance to F graminearum and F culmorum in hybrids could be predicted only when both parents were solidly resistant Inbreds A632 and WP9 and their relatives exhibited GER resistance that was far above the average (Kova´cs et al 1994) Reid et al (2000) observed resistance differences in sweet maize F graminearum after silk channel inoculation, but the differences were relatively small and overall all genotypes were quite susceptible In this study, DON levels increased rapidly such that by weeks after silking concentrations were above mg/kg Reid et al (2001a,b, 2003) have released eight maize inbreds (CO387, CO388, CO389, CO430, CO431, CO432, CO433, CO441) specifically bred for increased resistance to GER using both silk channel and kernel inoculation techniques The latest release, CO441, has the highest published inbred resistance and possesses both silk and kernel resistance as well as acceptable grain yields when tested in combining ability trials (Reid et al 2003) This it is feasible to develop resistant inbreds and high-yielding hybrids, thus breaking the Ôresistance-yieldÕ barrier so often found when breeding for disease resistance Inbreds developed with selection for GER also exhibited high levels of resistance to FER and common smut (Ustilago zeae) in inoculated trials (Reid et al 2009), indicating that it may be possible to develop hybrids with resistance to multiple Fusarium spp Schaafsma et al (1997) tested 61 commercial hybrids for GER by silk channel and kernel inoculation methods and concluded that only two ranked highly resistant with both inoculation methods de Oliveira et al (2009) found some resistant sources among landraces with higher grain hardiness Mesterha´zy (1978, 1982, 1983) and Mesterha´zy et al (2000) tested maize hybrids against four isolates (two F graminearum and two F culmorum) with the toothpick method The severity data on F graminearum and F culmorum correlated very closely, but the correlations with the less-pathogenic F verticillioides and F avenaceum were not convincing Research was initiated to find an indirect way for selecting to ear rot resistance However, resistance to ear rot, stalk rot and seedling blight did not correlate, indicating that a preliminary selection at the seedling stage will not automatically result in higher ear or stalk rot resistance (Mesterha´zy 1982) The data clearly showed (Mesterha´zy and Kova´cs 1988) that although the traits not interrelate genetically, they may influence each other physiologically In most maize-growing regions, with the exception of developing countries, commercial varieties are hybrids; thus, it is critical that inbreds developed for improved GER carry this resistance into the hybrid Kova´cs et al (1994) tested 18 hybrids with their respective inbred parents for GER The mean for the maternal lines on scale 0–10 was 1.23, and that for the paternal lines was 2.40 Their calculated mean was 1.82, but the actual value was 1.08 This means that the hybrids had 41% less ear rot than the calculated mean In six cases (33%), the hybrid was more resistant than the more resistant parental line These data indicate that selection for resistant inbreds will result in resistant hybrids Correlation between mother and hybrid performance was r = 0.65 (P = 0.01), and between father and hybrid performance, r = 0.85 (P = 0.001), indicating a stronger father influence Recently, researchers in Germany conducted more formal studies to compare the relationship between inbred line resistance and toxin contamination (Bolduan et al 2010, Loăer et al 2010b) It was concluded that eective line selection is possible, and toxin contamination follows closely the severity (rg = 0.88 between GER and FER, and rg = 0.77 for DON and FUM), indicating that the resistance to the different pathogens seems to be closely related Toxin contamination is proportional to symptom severity when inoculation is performed with the same isolate It seems that both proper line and testcross evaluations for ear rot resistance are equally important Some researchers have sought relations between seedling blight, ear rot and stalk rot (Mesterha´zy and Kova´cs 1988, Reid et al 1996a), but failed to demonstrate any Direct measurement of the resistance in the given organ is therefore necessary It is not clear what the mechanism of resistance to GER is in the resistant maize genotypes Changes in silk flavone content and resistance to GER have been reported (Reid et al 1992a,b) Assabgui et al (1993) found a correlation of r = 0.70 between the (E)-ferulic acid content and resistance to F graminearum Bily et al (2003) identified dehydrodimers of ferulic acid as a resistance component to F graminearum ear rot 4-Acetylbenzoxazolin-2-one (4-ABOA) was also reported to lead to a higher resistance level to GER (Miller et al (1997) Recently, Cao et al (2011) researched the role of hydoxycinnamic acids in resistance to GER and concluded that several changes in cell wall-bound compounds of silk tissues were observed after inoculation Further studies are required in this research area Fusarium ear rot Pascale et al (2002) tested the resistance of 29 hybrids against F verticillioides and F proliferatum The hybrid ÔMonaÕ was the most resistant For fumonisin B1 + B2, Mona contained an average of 1.9 mg/kg for the years, whereas a more susceptible hybrid Milpa had levels that reached 108 mg/kg Fusarium proliferatum was more pathogenic, resulting in higher disease severities and smaller differences between the hybrids Mona had a total fumonisin content of about 70 mg/ kg, while that of Milpa was about 50% less Visibly infected grains had a high toxin concentration, indicating that separation of these grains from less-infected grain could significantly decrease the level of contamination Presello et al (2008) detected significant hybrid differences using silk channel inoculations with one isolate of F verticillioides; the more susceptible hybrids also had higher natural infection levels However, in symptomless grains, fumonisin could be measured in quantities exceeding the recommended limits Diseased grains may be smaller, resulting yield losses up to 58% (Jovicevic and Sultan 1979, Warfield and Davis 1996) although F verticillioides is thought to have no or only minimal impact on the yield Sweet corn is generally considered to be a susceptible crop Du Toit and Pataky (1999) observed highly significant resistance differences, but none of the hybrids was highly resistant The silk channel injection method led to higher variation and was more laborious as compared with the toothpick method Schjoth et al (2008) found very good differences in resistance when medium disease pressure was applied, but these differences were less apparent when high pressure was applied Clements et al (2004) screened 1589 10 inbreds and a B73-type inbred for resistance to FER On the basis of the fumonisin concentration, only 11 inbreds (A188, A682, B8, B66, C127, CK31, CM5, CQ201, H117, M14 and ND211) were superior and stable in the two trials conducted Pericarp thickness has been considered to play a role in FER resistance The results of Ivic et al (2008) clearly showed that no correlation between pericarp thickness and resistance exists in Croatian genotypes, so that breeding for this trait would not increase resistance In contrast, Sampietro et al (2009) identified various properties of the pericarp and its wax layer as resistance factors to F verticillioides These traits were consistent over years under very differing ecological conditions When the wax was removed, infection severity increased significantly Waxy hybrids exhibit a higher average contamination of fumonisins (+440% in 2000 and +234% in 2001) than normal hybrids (Blandino and Reyneri 2007) Hoenisch and Davis (1994) observed a correlation between higher pericarp thickness and resistance They considered that the thicker pericarp inhibits the fungus and may also act as a barrier to insect feeding These studies may explain why sweet corn is so susceptible to both GER and FER; sweet corn varieties, a food crop, are intentionally bred to have thinner pericarps to improve texture upon eating Long-chain alkanes on the surface of maize silks have also been implicated in resistance to GER (Miller et al 2003) The An2 gene encodes an ent-copalyl synthase gene that has a role in gibberellin synthesis This gene was strongly upregulated after Fusarium infection of the silk (Harris et al 2005) It was postulated that the gene might play a role in silk resistance Haptoglobin-related protein (HRP) (Harris et al 2005) genes have also been reported to play a potential role in resistance Several flavonoids have been identified that have a possible role in resistance (Sekhon et al 2006) Choi and Xu (2010) reported the cAMP signalizing pathway in F verticillioides, which is important for conidiation and infection, may play a role in the infection process Farrar and Davis (1991) and Parsons and Munkvold (2010b) detected different thrips species on ears that increased the severity of FER infection Husk looseness correlated with FER at the brown silk stage and also with the size of the thrips population It was concluded that husk tightness plays an important part in epidemiology and disease development Eller et al (2008b) reported that the kernel moisture content influences the degree of ear rot This concurs with studies on stalk rot causing pathological drydown and thus influencing GER (Mesterha´zy 1983) as well as FER Eller et al (2008b) did not find a relation between ear rot resistance and yield performance This would support the hypothesis that high resistance and high yield are not mutually exclusive for FER as well as GER Relationships between different ear rots Many researchers work with more than one Fusarium spp in their tests Presello et al (2004, 2006) found highly significant genotypic differences to different Fusarium spp Resistance tests in both Canada and Argentina demonstrated correlations between F graminearum and F verticillioides resistance Six populations (mostly being Andean landraces ARZM 01107, ARZM 07138, ARZM 10041, ARZM 13031, ARZM 16002 and Pora INTA) were identified that had very high levels of resistance to both pathogens and could be used as sources of high and stable resistance Czembor and Ochodzki (2009) ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A found higher ear rot resistance in flint genotypes than in dent ones Loăer et al (2010a) reported the opposite experience As the tested sets of genotypes were not the same in both studies, the data are not necessarily contradictory Loăer et al (2010a) observed good phenotypic and genotypic correlations in the flint and dent groups between F graminearum and F verticillioides Negative correlations emerged between the silking date and ear rot severity for both Fusarium spp (F graminearum, rP = )0.28; F verticillioides, rP = )0.26) So the lower severities for the later genotypes were confirmed In the cited literature, there are several indications that resistance to F graminearum and F culmorum may be closely related (Mesterha´zy 1982, 1983) As both cause GER, this is not surprising For other Fusarium spp., however, the picture is not clear In wheat, resistance protects all Fusarium spp tested (Mesterha´zy 2002, Mesterha´zy et al 2005) In maize, this remains to be clarified There is no clear evidence of resistance to specific toxins produced by the different Fusarium spp.; however, many studies (Reid et al 1996a, Pascale et al 1997, Perkowski et al 1997, Reid and Sinha 1998) clearly indicate that severity of infection is highly correlated to toxin contamination, thus indicating a role of resistance in toxin regulation Bolduan et al (2009) found a high correlation between toxin contamination and the severity of the disease in response to F graminearum with r = 0.94 Visual scoring can therefore be sufficient in selection work for GER resistance For F verticillioides, the infection severities are often significantly less and not as highly correlated to toxin levels (Miedaner et al 2008); however, it was concluded that toxin analysis for both GER and FER are not necessary at all stages of breeding Loăer et al (2011) found that the heritabilities for mycotoxin values were similar or higher than those found for ear rot data (both F graminearum and F verticillioides) However, as the close correlation was not characteristic for all genotypes, they recommended separate testing of F graminearum and F verticillioides and corresponding mycotoxins Henry et al (2009) identified genotypes with good resistance to both F verticillioides and Aspergillus flavus Correlations between ear rot severities of the two pathogens (r = 0.72) and between aflatoxin and fumonisin concentrations (r = 0.61) led to the conclusion that good resistance to both species in the same genotype is attainable Robertson-Hoyt et al (2007b) came to the same conclusion It is interesting that the available literature does not concentrate on sources of resistance, the use of alien species for this purpose and the selection of inbred lines Reid et al (2009) identified inbreds with differing silk and kernel resistance types In their study, silk channel resistance was investigated using one F verticillioides and one F graminearum isolate It appeared that both additive and non-additive effects contributed to the resistance, and similarities in reaction to these pathogens were found In tests in Szeged in Hungary, the correlation between the mean ear rot severity and DON contamination was r = 0.67, and for fumonisin B1-4, a total r = 0.68 was calculated, both significant at P = 0.1% (Toldi et al 2008 and not published data) Presello et al (2011) described resistance of inbreds selected from F2 populations and their hybrids inoculated with F proliferatum The populations were also inoculated by F verticillioides and F graminearum Selection was similarly effective against F graminearum and F verticillioides Breeding for resistance to ear rots Henry et al (2009) tested 20 inbreds for F verticillioides and A flavus resistance The ear rot values correlated for the two pathogens (r = 0.72, at P = 0.0002), as did the aflatoxin and fumonisin concentrations (r = 0.61, P = 0.0004), indicating that inbreds with aflatoxin resistance may be good sources for breeding for fumonisin resistance Williams and Windham (2009) analysed fumonisin accumulation in a diallel analysis using A flavus-resistant and A flavus-susceptible inbreds inoculated with F verticillioides and A flavus The inbreds Mp715 and MP 717 revealed high aflatoxin and fumonisin resistance; however, inbred Mp313E revealed resistance only against fumonisins, not aflatoxin Farrar and Davis (1991) concluded that maize genotypes behaved very similarly to A flavus and F moniliforme (syn F verticillioides) Their data clearly show that resistance can be effective to different Fusarium spp., but also against other pathogens from different genera, for example A flavus There also appears to be a connection with common smut caused by Ustilago maydis (Reid et al 2009) Eight GER-resistant inbreds were also resistant to Ustilago maydis It should be further investigated whether this is caused by linkage or pleiotropy The lesson is clear The literature indicates that toxin and ear rot severity data are correlated more often than not for FER and GER Artificial inoculation followed by selection on the basis of disease severity is sufficient during the inbred selection process Toxin evaluations should be made with initial parental selection for new populations, in the final stages of inbred development and in the second and third year of the official registration tests (in countries where this is applicable) to demonstrate the low toxin-producing capacity of the given hybrid The close similarities in resistance to Fusarium, Gibberella and Aspergillus ear rots that have been described indicate with high probability that the resistance to these diseases may be common However, exceptions have also been found It is not known, however, whether the same resistance genes are responsible for this in a given maize genotype, or what is the interaction between the environment and different pathogen-specific genes This will be an important research field in the future Molecular Genetics and QTL Mapping QTL mapping for GER resistance In an F5 RIL population, Ali et al (2005) found 11 QTLs for ear rot following silk inoculation and 18 QTLs after kernel inoculation (explaining 6–35% of the phenotypic variation) However, only two QTLs could be detected that were active across environments for silk resistance and only one for kernel resistance, indicating a strong influence of the environment The majority of the favourable alleles came from the resistant parent CO387 Reinprecht et al (2008) set out to identify the genes behind QTLs About 100 genes were identified, among them chitinase and protein kinase were similar to previous gene-based markers that cosegregated with Fusarium resistance QTLs Recently, Martin et al (2011a,b) identified co-localized QTL for both GER resistance and reduced levels of DON in different mapping populations; they suggested that it may now be possible to conduct marker-assisted selection to improve GER resistance in the off-season but that classical phenotypic selection with field inoculations continue to be used during the cropping season 11 QTL mapping for Fusarium ear rot resistance Eller et al (2008b) established that resistance to FER is determined by polygenes Robertson-Hoyt et al (2006c) tested two populations for resistance to F verticillioides In the FER population, seven QTLs were identified, explaining 47% of the phenotypic variation for FER, and nine were found for fumonisin content, explaining 67% of the variation In the NCB population, five QTLs explained 31% of the FER variation and six QTLs with three epistatic interactions explained 81% of the phenotypic variation Of the QTLs in the two populations, three QTLs for FER and two for fumonisin were mapped in similar positions Two QTLs, localized on chromosome and 5, appeared to be consistent in both populations Ding et al (2008) tested a RIL population of 187 genotypes for F verticillioides resistance Phenotyping was performed in four environments (location–year combinations) Two QTLs on chromosome were identified with stability across environments The major QTL explained 13– 22% of the phenotypic variation for FER Perez-Brito et al (2001) identified nine and seven QTLs in two populations, three of which were co-located Kozhukhova et al (2007) found a codominant marker RGA11 on the short arm of chromosome for FER at 18.3 cM to the resistance locus in an F2 mapping population For the SSR locus, the phi001 polymorphic amplicon 180 bp was identified R2 or heritability values were not given Relationship of QTL to Fusarium ear rot resistance to other ear rots and agronomic traits Robertson-Hoyt et al (2007a) found that QTLs for F verticillioides resistance were also effective against A flavus The genotypic correlations between ear rot data of the two pathogens (rG = 0.99) were very close On chromosome 5, a large effect QTL was identified The resistance QTLs against A flavus and F verticillioides were occasionally clustered on the same chromosomes However, it was considered that a fine-scale genetic mapping will be necessary to distinguish linked QTLs, such as those in a resistance cluster from pleiotropic QTLs that influence resistance This supports the view of common resistance to different Fusarium spp (Robertson-Hoyt et al 2007b) Robertson-Hoyt et al (2007b) mapped two fumonisin QTLs to similar positions as that for grain yield, but the two QTLs were mapped to distinct genomic positions Generally, close relations were not found between resistance and agronomic traits, and selection for higher resistance should therefore not unduly affect agronomic performance A new attempt is the meta-analysis of QTLs associated with ear rot resistance (Xiang et al 2010a,b) The data of 14 studies representing F graminearum, F verticillioides and A flavus QTL studies were analysed; resistance QTLs against the three fungi were clustered on the same chromosomes These data seem to support the idea of common resistance on QTL level Of the 87 individual QTLs, 29 metaQTLs were identified with 2–6 individual QTLs within a cluster One resistance source can contribute to different clusters, for example CO387 influenced 18 of the 29 metaQTLs At present, it is not clear whether the QTLs in a cluster are individually effective to all three fungal pathogens or whether they are specialized to different fungal species and the cluster effect secures the broad sense resistance We think 12 based on wheat studies; the former is more likely (Mesterha´zy et al 2007) Molecular genetics of Gibberella ear rot and mycotoxin accumulation Yuan et al (2008) found a guanylyl cyclase-like gene (Zmgc1) that ensures resistance to G zeae; it is nearly identical to one resistance gene of the G zeae-resistant line CO387 Jenczmionka and Schaefer (2005) described Gpmk1 MAP kinase disruption mutants and concluded that the infection process depends on the secretion of cell wall-degrading enzymes, especially during the early infection stages Igawa et al (2007) tested a ZEA-detoxifying enzyme in transgenic plants It was expressed in the vegetation period and was also active up to 16 weeks during storage The problem is that the disease was not or only moderately inhibited and other toxins may contaminate maize Boutigny et al (2008) surveyed naturally occurring mechanisms to the reduction of trichothecene toxins They identified Class mechanisms for detoxifying these toxins Class comprises mechanisms that result in reduced mycotoxin accumulation through inhibition of their biosynthesis Some might work in practice, but their cost will certainly be considerable Molecular genetics of Fusarium ear rot and mycotoxin accumulation Alexander et al (2009) compared the biosyntheses of trichothecenes and fumonisins and concluded that the genes participating in these processes could possibly be used to enhance resistance to disease and reduce toxin contamination Lanubile et al (2010) found that in a resistant line, the assayed defencerelated genes (b-tubulin and FUM21 genes of F verticillioides) were transcribed at high levels before infection and provided basic defence against the fungus In the susceptible line, the same genes are qualitatively induced from a basal level and respond specifically to pathogen infection Zhang et al (2011) identified the FvMK1 mitogen-activated protein kinase gene in F verticillioides, which regulates conidiation, pathogenesis and also lowers the activity of the FUM1 and FUM8 genes Use of QTL mapping in practical breeding The existence of meta-QTLs does not change the fact that most of the QTLs found are not validated and have only small effects (Robertson-Hoyt et al 2006c, 2007a, Eller et al 2008a,b) The 19 meta-QTLs (Xiang et al 2010b) explain much of the resistance, but the individual QTLs are of small effect Only one or two QTLs can be considered to have medium to large effects (Robertson-Hoyt et al 2007a) Their additive (in some cases epistatic) effect seems to be proved Therefore, their use for marker-assisted selection is limited at this time Further complexity arises from the fact that in some hybrids, a maternal or paternal effect was dominant The hybrid effect can be explained to some extent, but the resistance level in hybrids cannot be predicted with utmost certainty This is similar to the situation in wheat (Buerstmayr et al 2009) The results of Wilde et al (2007) on wheat indicated that marker-assisted selection resulted in a twofold higher susceptibility in the progeny than in the phenotypically selected variant We consider that a strong selection for ´ M e s t e r h a´ z y , M L e m m e n s and L M R e i d A increased resistance may give novel material for the development of new mapping populations that will allow the determination of new QTLs with higher effects, or identification of QTLs that ensure transgressive segregation Other attempts with new genes may be of importance especially when the given enzymes can be identified in the QTL regions (Reinprecht et al 2008) Breeding Aspects The development of genetic resistance to F graminearum, F verticillioides and other Fusarium spp in maize should be a high priority in light of the toxins these species contaminate maize grain with (Reid et al 1996a, Munkvold 2003b) Duvick (2001) suggested three theoretical approaches to decrease fumonisin contamination Resistance is mentioned first, but resources may be limited Molecular markers can also be applied to identify QTLs, but validated and effective markers are rare A possibility in the future is to transfer resistance genes into maize and ensure higher resistance in this way, but at present no resistance genes are available Presello et al (2005) suggested pedigree selection to improve F graminearum resistance Both silk channel and kernel resistance were investigated: the selection for kernel resistance was more effective, and the resistance was more stable Reid et al (2001a,b, 2003) used modified pedigree selection to develop eight inbreds with improved GER resistance, some with high levels of both silk and kernel resistance Researchers in Germany are using double haploid technology to develop GER-resistant inbreds (Martin et al 2011b) Robertson-Hoyt et al (2007a) did not find a close correlation between FER resistance and other traits such as yield; therefore, they hope that a strong selection for resistance will not result in lower yield and other unwanted consequences This would question the yield penalty we mentioned regarding the high resistance in this paper Reid et al (2003) did develop an inbred, CO441, with high resistance to GER and excellent combining ability for yield Mesterha´zy et al (2000) reported that resistant hybrids will be bred when both parents have good or excellent resistance; otherwise, the amount of resistance in the hybrid cannot be predicted with certainty Ali et al (2005) and Robertson et al (2006) found transgressive segregation in maize; this could be utilized in maize breeding as was the case in wheat where the best F graminearum-resistant source (Sumai-3) was bred from two medium-resistant lines Breeding for ear rot resistance involves two important steps Munkvold (2003a) stresses the identification and use of native resistant sources Breeding programmes could be based on these sources, because they are adapted and already available in the breeding nurseries Nearly every paper cited reported significant differences in ear rot resistance; breeders need only start with the more resistant genotypes in their programme de Oliveira et al (2009) added that valuable breeding material can be identified among landraces; however, breeding with landraces can be time-consuming as many unwanted traits may have to be bred out of the populations first Breeding can be started from hybrids made from crossing involving one or more lines with good resistance or from existing hybrids with proven superior resistance, providing no proprietary issues are involved that may restrict the use of the hybrids in a breeding programme There is not much in the literature about when resistance selection should be performed in segregating generations In most inbred breeding programmes, testing of the Breeding for resistance to ear rots combining ability occurs in the S3, S4, or later generations; thus, the first resistance evaluation of testcrosses can be made at this time However, as Loăer et al (2010a) has reported, many inbreds are susceptible as the infection pressure during inbreeding was not strong enough Reid (1999) collected germplasms from around the world (adapted and unadapted) with moderate to high resistance to various ear pathogens, but no mention was made of how the inbreds were produced However, as indicated in Reid et al (2001a,b, 2003), inbreds were inoculated every generation in their development with the exception of few generation advances that were performed in off-season winter nurseries Some of these inbreds resulted from crosses between a resistant inbred and an inbred with good agronomic performance, and other inbreds were selfed out of reciprocal recurrent selection populations that were subject to intense selection with artificial inoculations and toxin evaluations Eller et al (2010) also reported on inoculation during inbreeding; they concluded that the backcross method, that is normally used to transfer single major genes, was also successful in improving FER resistance This might work for major QTLs, but for the normally polygenic trait it is less suitable as QTLs might be lost In this case, the reciprocal recurrent selection suggested by Boling and Grogan (1965) might work better Bolduan et al (2010) suggested that the focus should be more on testing of hybrids and less on inbreds We think both are important, as inbreds having superior resistance we can bred with to produce more resistant hybrids after several years It is more important to start breeding of new resistant inbreds from materials with good or superior resistance These sources with superior resistance should also be adapted to the target environment, otherwise commercially competitive inbreds are hard to achieve If possible, both inbreds of a single cross should have good or excellent resistance to allow the breeding of commercial hybrids with maximum resistance Artificial inoculation methods must be extensively used in the testing of the new hybrids and their parents Currently, most breeders use conidial suspensions, but toothpicks are also used by some The interest in artificial inoculation is growing It is interesting that fumonisin content has proven to be a more reliable trait than visual scores (Eller et al 2008a) However, field evaluation of symptoms is rapid and can extend to thousands of genotypes, whereas any toxin analysis demands more time and is money-consuming For both ear rots, the correlations between visual symptoms and toxin contamination are generally close (with a few exceptions) Accordingly, visual symptoms are suggested for screening To verify the low toxin response of the new hybrids, at the end of the breeding process toxins should be evaluated (Bolduan et al 2009) Many breeders screen either for GER or for FER only, and few screen for both Loăer et al (2010a) and Miedaner et al (2008) stress the testing of inbreds and hybrids in several locations with a single isolate or a mixture of isolates As no specialization is known, theoretically no difference exists between single isolates and mixtures of isolates of the same Fusarium spp Therefore, the isolates may be used separately, and so (in the case of four isolates), the amount of resistance can be evaluated more precisely as with one single epidemic situation Even so, resistance evaluation is made 2–3 years (Mesterha´zy 1983, Kova´cs et al 1988) Some prefer isolate mixtures to avoid the possibility of a single isolate being less aggressive in a given field season, resulting in levels of disease too low to make accurate evaluations of resistance in 2–3 years 13 (Reid et al 1993) A mixture of isolates will decrease the environmental interaction The resistance level will be the mean of those of several isolates used, so the extent of resistance can therefore be estimated better A preselection of the isolates for aggressiveness is very important (Miedaner et al 2010) As the aggressiveness varies (Mesterha´zy 1985), the use of more isolates increases the reliability of the evaluations As there are different ideas in these respects and solid scientific material is scare, further research is needed to give better suggestions for breeders Many studies have been published on the differing resistance mechanisms that may play a role in GER and FER resistance, and QTLs and genes have been found responsible for higher resistance It is probable that some of these results will be used in breeding programmes However, a well-working, methodical, phenotyping system must be used; otherwise, the gene expression cannot be checked at a practical level Regarding the two types of resistance, resistance to kernel infection is more stable and can be reproduced better than silk or silk channel resistance as the latter may be under more environmental influence and the window for fungal invasion is smaller as the fungus must grow down the silk channel before the silk dries out (Reid et al 1992a, Reid and Hamilton 1996, Reid and Sinha 1998) Breeders must choose for themselves which mode of fungal entry is more important in their growing region Some breeders separate their nurseries into silk channel and kernel resistance sections based on the sources of resistance used Evaluation for both modes should be made during parental selection for new inbred development programmes and during the hybrid testing and evaluation of lines In some cases where the major source of kernel infection is attributed to boring insects but these are controlled by Bt hybrids in commercial fields, it may be more practical to focus breeding efforts on silk channel resistance However, it is important to remember that several reports indicate that the difference between these two modes may be less pronounced than normally anticipated Conclusions and Outlook The maize–Fusarium system is complicated, but several conclusions can be drawn that are useful for resistance breeding programmes and resistance research Breeding is a decisive part of the integrated production process from farm to fork (van der Fels-Klerx and Booij 2010) In all important maizegrowing regions, many Fusarium spp occur at the same time, and their species structure depends on the region and the local weather conditions Fusarium graminearum and F verticillioides, the two most important species, and their close relatives have very diverging ecosystem requirements for their successful infection of a host plant The available data indicate that common resistance may exist to both species in maize, consequently simplifying the breeding process It is highly important that several authors have found correlations between the resistances to Aspergillus, Fusarium and Gibberella ear rots This can lead to the hypothesis that resistance may be much wider than previously believed This point is very important as hybrids are developed for global needs, and their resistance to all local Fusarium and other fungal populations should therefore be ensured whenever possible Research on this complex resistance is needed to determine the cause of the high correlation between the natural and artificial inoculation results 14 The data are clear in the respect that visual symptom severity and toxin contamination correlate closely for most species Highly resistant genotypes demonstrate low disease severity and low toxin contamination Specific toxin resistance is at present problematic For example, in GER, DON may play virulence-increasing role, and glycosyl transferases (Lemmens et al 2005) can also decrease the DON content in maize, but the significance of this for breeding is not yet known The registration process of new maize hybrids (in countries where this is applicable) should contain results from an artificial inoculation survey of the variety candidates In this way, the susceptible and very susceptible hybrids could be banned from registration This is the most effective way to improve food and feed safety worldwide Unfortunately, as very few resistant hybrids currently exist, such a regulation would mean few if any hybrids get registered in some countries With new regulations on allowable mycotoxin limits in several countries, breeding programmes to develop ear rot resistance are being initiated or strengthened with more resources As a consequence, future hybrids may possess more resistance Until this time comes, it is still paramount that farmers have access to ear rot rating data on hybrids that they may grow This will allow farmers to choose hybrids with more resistance if ear rot is a significant threat on their farm Countries need to organize an unbiased evaluation of all registered hybrids each year and make this information available to farmers until the time that enough resistant sources exist for resistant hybrids to be available The important fundamental steps of breeding for higher resistance have already been taken These steps allow the breeding of inbreds and hybrids with a higher level of resistance than it was previously possible However, many important questions are not yet answered Additionally, more effective selection methods should be developed and existing methodology should be improved and standardized The relationships between natural and artificial infection should also be clarified Food and feed safety requires more healthy grain (also silage); the investment in this sector of breeding and science may be expected to increase We hope that this paper will contribute to the initiation of this process Acknowledgements The authors express their thanks to the following projects: Hungarian Grant Agency NKTH: GAK (OMFB-01286/2004, OMFB 00313/ 2006), NAP-2-2007-0001 and the FP7 MYCORED KBBE-2007-2-5-05 and Deak Zrt Szeged, Hungary, Ontario Corn ProducersÕ Association, Ontario Pork, Agriculture and Agri-Food Canada and the Canadian Field Crop Research Alliance References Afolabi, C G., P S Ojiambo, E J A Ekpo, A Menkir, and R Bandyopadhyay, 2008: Novel sources of resistance to fusarium stalk rot of maize in tropical Africa Plant Dis., 92, 772—780 Alexander, N J., R 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Responses to selection of S5 inbreds for broad-based resistance to ear rots and grain mycotoxin contamination caused by Fusarium spp in maize Euphytica, 178, 23—29 Reid, L M 1999: Breeding for resistance. .. of factors Breeding for resistance to ear rots affecting kernel resistance ranged from 4.6 to 13.7 Lemmens (1999) described a similar phenomenon – maize hybrids seem to possess different resistance. .. for the future The Pathogens and Associated Mycotoxins The pathogens One of the first reports on ear rots caused by Fusarium spp originated from Canada by Bisby and Bailey (1923) The ear rot caused