Ebook Plant pathology: Concepts and laboratory exercises (Second edition) - Part 2

Continued part 1, part 2 of ebook Plant pathology: Concepts and laboratory exercises (Second edition) provide readers with content about: parasitic seed plants, protozoa, algae, and mosses; abiotic diseases; molecular tools for studying plant pathogens; molecular tools for studying plant pathogens; molecular techniques used for studying systematics and phylogeny of plant pathogens; plant–pathogen interactions; plant–fungal interactions at the molecular level; testing blad, a potent antifungal polypeptide;...

Seed Plants, Protozoa, 24 Parasitic Algae, and Mosses Mark T Windham and Alan S Windham Chapter 24 Concepts • Parasitic seed plants can be chlorophyllous or achlorophyllous and can cause significant economic crop loss • Parasitic seed plants may range in size from a few centimeters tall, such as Indian pipe, to several meters tall, such as buffalo nut • Dodder has a large host range, is entirely aerial, and may destroy entire ornamental plantings of annuals and perennials • Dwarf mistletoe is extremely destructive on black spruce in western North America • Witchweed is an extremely destructive pest of grass crops such as corn, millet, sorghum, and sugarcane in Africa and Asia The pest is established in North and South Carolina in the United States • Flagellated protozoa can cause serious diseases of coffee and oil palm, cassava, and coffee • Algae can cause leaf spots on numerous plants, including southern magnolia and cultivated azaleas in coastal areas of the southern United States Although most of the important plant pathogen groups have been covered in previous chapters, a few remaining plant pathogens or organisms that have been associated with disease symptoms are parasitic seed plants, protozoa, algae, and mosses The most important pathogens among these groups are the parasitic seed plants Parasitic seed plants have flowers and produce seed, but may be deficient in other typical characters associated with plants such as roots or chlorophyll Some parasitic seed plants, such as buffalo nut (Pyrularia pubera), are shrubs that have green leaves and a root system, whereas others, such as squawroot (Conopholis americana), Indian pipe (Monotropa uniflora), pine-sap (M hypopiths), witch weed (Striga lutea), dodder (Cuscuta species), and dwarf mistletoe (Arceuthobium species) and leafy mistletoe (Phoradendron species), have a very small thallus, which does not include roots and/or green leaves In this chapter, several examples of parasitic seed plants; tropical protozoa that have been associated with several diseases; an alga whose colonies are associated with leaf spotting of magnolia and azalea; and ball moss that causes death of shrubs and trees in isolated areas of Louisiana, Texas, and Florida will be considered Parasitic seed plants Dodder Dodder, also known by the names strangle weed and love vine, is a small plant with a thin yellow-to-orange stem Infestations of dodder look like yellow-orange straw spread along roadsides The stems of dodder are leafless but contain abundant whitish flowers that form in early summer and are produced until frost (Figure 24.1) Seeds are tiny, brown to gray to black, and mature in a few weeks Dodder has a very wide host range that includes alfalfa, potatoes, numerous herbaceous annuals and perennials, and young shrubs or seedlings of trees in nurseries Dodder infections can kill young plants Severe infestations can destroy entire plantings of wild flowers along a roadway, or annuals and perennials in a flowerbed Dodder can also be used as a bridge (vector) to transmit a number of plant viruses (Chapter 4), but the economic importance of virus transmission by dodder is unknown Symptoms and Signs of Dodder Infection Symptoms of infected plants include stunting, loss of vigor, poor reproduction (flowering), and death Signs of 253 46691.indb 253 10/15/07 8:29:48 AM 254 Mark T Windham and Alan S Windham Figure 24.1  Close-up of dodder on stem of lespedeza Note knobby protuberances on dodder stems that are haustoria primordia dodder include yellow-orange stems of dodder entwined around plant stems, petioles, and foliage Dodder does not have roots and is entirely aerial Patches of dodder enlarge by growing from plant to plant and by new dodder plants emerging from seed produced during the growing season In container nurseries, dodder can spread rapidly throughout the canopy of plants that are tightly packed together Disease Cycle Dodder seed may remain dormant in soil for many years or may be introduced into a field or flower bed at planting Only a stem is produced when seeds germinate (Figure 24.2), and the young seedlings rotate until they come in contact with a host plant If a host plant is not available, the seedling eventually dies However, if a suitable host is found, the stem of the seedling wraps around the host stem and produces haustoria that penetrate the host stem After successful infections have taken place, the dodder stem begins to grow from host plant to host plant if the host plants are close enough, and produce many small white flowers Seeds mature in approximately three weeks and may contribute to the current epidemic, or lie dormant until the next growing season Seeds are spread by water, animals, tillage equipment and/or in mixtures with host seed 46691.indb 254 Control Dodder infestations can be very difficult to control once the pathogen has become established in a field or flower bed Equipment should be cleaned before moving from an infested area to a noninfested area Livestock in infested areas should be kept there and not moved to areas that are thought to be free of dodder Once dodder infestations are established in a field, it is usually controlled with contact herbicides that destroy both the dodder and host plants before the parasite has a chance to flower Cultivation or fire can be used to destroy dodder if done before flowering Fumigation is possible in flower beds, but is impractical in fields due to the patchiness of dodder infestations Dwarf Mistletoes Dwarf mistletoes are the single most economically important parasitic seed plant in North America and are the most important disease problem in conifer production in the western United States In Oregon and Washington, about 13% of the total wood production is lost to dwarf mistletoes annually In Minnesota, infestations are primary in black spruce (Picea mariana), and about 11% is lost due to the disease Losses in tree production are due to mortality, poor growth and wood quality, reduction in seed production, wind breakage, and predisposition of infected trees to other diseases and 10/15/07 8:29:49 AM 255 Parasitic Seed Plants, Protozoa, Algae, and Mosses Seeds can be scattered with host seed or lay dormant in soil for yeras Germinating seed Dodder seedling rotates to find host Dodder stem wraps around plant Dodder stem produces haustoria that penetrate stem Dodder can mover plant to plant Flowers are produced on dodder stems from early summer till frost Figure 24.2  Generalized life cycle of dodder (Crucuta species) insects There are nearly 40 species of dwarf mistletoe in the genus Arceuthobium Economically important Arceuthobium species and their hosts are listed in Table 24.1 Symptom and Signs Infected portions of the tree become swollen, and excessive branching at the infection site leads to a witches’ broom (Figure 24.3); multiple witches’ brooms can occur on the same tree Although the growth rate of infected tree parts increases, growth in the rest of the tree is retarded, and growing points on the tree often die Signs of dwarf mistletoe include small yellow-toorange plants with sessile leaves and white flowers (Figure 24.4A) Developing berries are white, and darken with age (Figure 24.4B) Disease Cycle The dwarf mistletoe is dioecious, and flowers are pollinated by insects The berries contain one seed and are surrounded by viscin, a sticky mucilaginous pulp The seeds are forcibly discharged for a distance up to 15 m (Figure 24.5) The discharged seeds land directly on foliage of neighboring trees Long-distance spread is by seed that become stuck to the feet of birds During periods of rainy weather, the seeds slide down the needle (or off the feet of birds) and land on twigs Seed usually start to germinate within a few weeks, but germination may be delayed until spring Once the seeds germinate and the radicle comes in contact with a rough area in the bark surface, a primary haustorium penetrates the limb After the primary haustorium is successfully formed, the dwarf mistletoe plant develops an extensive absorption system that eventually includes the xylem of the host tree After three or more years, an aerial shoot of the parasite will emerge from the bark of the infected twig and flower production begins the following year Dwarf mistletoe plants contain chloroplasts, but fix little carbon and obtain practically all their nourishment from the host The dwarf mistletoe is most severe in open stands along ridge tops (higher elevations) and seldom is a problem in bottom lands Table 24.1 Economically Important Species of Arceuthobium (Dwarf Mistletoe) and Principal Hosts Arceuthobium Species Host A abietinum f sp concoloris White and grand firs A americanum Jack, lodgepole, and beach pine A douglasii Douglas fir A laricis Western larch A pusillum Alberta and black spruce A tsugense Mountain and western hemlock A vaginatum subsp vaginatum Apache pine, rough bark Mexican pine, and ponderosa pine Note: Summarized from Tainter and Baker (1996) 46691.indb 255 10/15/07 8:29:50 AM 256 Mark T Windham and Alan S Windham (a) (b) Figure 24.3  (A) Dwarf mistletoe infection of lodgepole pine (B) Close-up of infected limb 46691.indb 256 10/15/07 8:29:51 AM 257 Parasitic Seed Plants, Protozoa, Algae, and Mosses (a) (b) Figure 24.4  (a) Dwarf mistletoe plant growing on conifer (b) Close-up of dwarf mistletoe plant showing sessile leaves and ripening berries Control Because dwarf mistletoes are obligate parasites, clear-cutting infected trees and prescribed burning to remove any ripe berries and fallen limbs on the forest floor can be an effective control tactics In many western areas, eradication of dwarf mistletoe is not practical, and control strategies center around reducing the amount of dwarf mistletoe after the current stand is harvested After timber harvest, heavily infected trees are removed and not used as sources of spruce seed Plantings are accomplished in a manner to take advantage of natural barriers to dwarf mistletoe such as highways, large streams, nonhost species, etc In recreational sites, the parasite may be controlled by pruning infected branches before seeds are produced Resistance to this disease has not been successfully utilized except in the Rouge River valley in Oregon among the ponderosa pine (Pinus ponderosa) This tree has drooping needles, which causes seeds to slide down to the tip of the needles and drop harmlessly to the ground 46691.indb 257 However, seedlings taken from this valley revert to having erect needles Thus, the ability of ponderosa pines in this area to escape the disease may be due to environmental conditions instead of a genetic trait Leafy mistletoes Leaf mistletoes occur throughout the world and usually attack hardwoods in forest and landscape areas Heavily infected trees may begin to decline in vigor, but economic losses are small when compared to losses attributed to dwarf mistletoe infections In North America, infection by leafy mistletoes is usually due to Phoradendron species, but leafy mistletoes of Viscum species, common in Europe, are also found in California Leafy mistletoes are evergreen plants and may be so numerous in a deciduous tree that they may make the tree appear to be an evergreen during the winter months (Figure 24.6) Symptoms of leafy mistletoe infections are similar to those of dwarf mistletoes in that infected branches become swollen and often form a 10/15/07 8:29:53 AM 258 Mark T Windham and Alan S Windham Seed stuck to needle Seed being ejected from fruit Berries are produced at ends of dwarf mistletoe stems After seed washes to base of needle during a rain, seed penetrates twig and the germling established as disease relationship with the host Several years after infection, aerial plant parts are produced Figure 24.5  Generalized life cycle for dwarf mistletoe (Arceuthobium species) Figure 24.6  Hardwood tree with multiple infections by a leafy mistletoe witches’ broom Heavily infected trees may have reduced growth and limb death Leafy mistletoes have white berries that are eaten by birds The seeds are excreted and stick to the branches where the birds perch Control of leafy mistletoes is usually not necessary, but infected branches can be pruned A comparison of dwarf and leaf mistletoes is provided in Table 24.2 Other parasitic seed plants of economic importance Witchweed Witchweed is an economically serious parasite in Africa, Asia, Australia, and in limited areas in the United States (i.e., limited to a few coastal counties in North and South 46691.indb 258 Carolina) Witchweed can parasitize hosts such as corn, millet, sorghum, rice, sugarcane, tobacco, and cowpeas Infected plants are stunted, usually wilted, chlorotic, and often die Infected roots may contain numerous large haustoria from a single witchweed plant, or haustoria from more than one witchweed plant Flowers of witchweed are brightly colored with red or yellow petals that are showy A single plant may produce nearly one-half million tiny seeds Witchweed is difficult to control, and avoiding the introduction of the seed into fields is paramount Quarantines have been effective in limiting spread of the parasite in the Carolinas Eradication of witchweed has reduced the infested area significantly since the disease was discovered in the mid-1950s Witchweed can also be controlled by using trap or catch crops, and by destroying 10/15/07 8:29:54 AM 259 Parasitic Seed Plants, Protozoa, Algae, and Mosses Table 24.2 A Comparison of Dwarf and Leafy Mistletoes Dwarf Mistletoes Found in the southern half of North America Attacks conifers Attacks hardwoods Propagated by forcibly ejected brown-to-gray seeds that stick to needles or to the feet of birds Propagated by white seeds not forcibly discharged and are eaten and excreted by birds Dioecious plants Monoecious plants Economically important in United States Seldom of economic importance in the United States Yellow-orange to brown plants Green plants the host plants and parasite with a herbicide before flowering and seedset Broomrapes Broomrapes (Orobanche minor and O ramosa) attack more than 200 species of plants and occur throughout much of the world In India, broomrape infections in tobacco may destroy one-half of the crop, whereas yield losses due to broomrapes on tobacco in the United States are rare Plants attacked early in the season are stunted, whereas plants attacked later in the growing season suffer few effects from the parasite Broomrape infections are usually clumped in a field Broomrapes are whitish to yellowish to purplish plants that grow at the base of the host plant Often, ten or more broomrape plants can be found attacking the same plant Broomrape can produce more than one million seeds at the base of a single host plant Broomrapes, like witchweed, are hard to control in areas where infestations are severe In India, some control may be obtained by weeding out broomrape plants before seeds are produced Weeding must be done throughout the growing season because more broomrape plants will emerge after the initial weeding Soil fumigation is effective in killing the seed Flagellate Protozoa Some protozoa in the family Trypanosomatidae are known to parasitize plants in tropical areas Although thousands of protozoa can be found in the phloem of diseased plants, formal proof of their pathogenicity has not been achieved because infections with pure cultures of the protozoa have not been accomplished (Koch’s postulates have not been satisfied—see Chapter 39) Diseases associated with flagellate protozoa include sudden wilt of oil palm, heartrot of coconut palm, empty root of cassava, and phloem necrosis of coffee Symptoms of these diseases include chlorosis (yellowing of fronds), stunting, and death These diseases can spread very rapidly For example, heartrot can spread to thousands of trees in one year Protozoa that cause phloem necrosis in coffee and empty root of cassava can 46691.indb 259 Leafy Mistletoes Found in western and northern parts of North America be transmitted by root grafts or grafting Protozoa associated with sudden wilt of oil palm and heartrot of coconut palm are transmitted by pentatomid insects Control is primarily by avoiding infected stock at transplanting Control of potential vectors is of questionable value Algae Although at least 15 species in three families of algae are known to parasitize plants, only three species of Cephaleuros are common worldwide, and C virescens is the only species common in the United States Parasitic algae affect more than 200 species of plants in the United States along coastal areas of North Carolina extending south and westward to Texas Economically important hosts that may suffer noticeable damage include southern magnolia (Magnolia grandiflora) and cultivated azaleas (Rhododendron species hybrids) Algal colonies are only successful if they form in small wounds and develop between the cuticle and epidermis Superficial colonies on the leaf surface wash away during heavy rains Host cells directly beneath the colony die causing necrotic leaf spots Many colonies can be found on a single leaf (Figure 24.7), and colonization is most severe during periods of warm, wet weather Control is usually not recommended, but multiple sprays of copper are effective if needed Mosses Ball moss, Tillandsia recurvata, is a bromeliad that is closely related to Spanish moss It is an epiphyte that is found in the southernmost United States (Florida, Louisiana, Texas, and Arizona) and southward to Argentina Ball moss occurs on many deciduous and evergreen species Large clusters of ball moss may completely encircle smaller branches and prevent buds from developing, which results in branch death Other damage to the trees may be due to the weight of the epiphyte populations causing mechanical damage to host plants Leaf abscission and branch death have also been attributed to abscission substances produced by the epiphyte Control of ball moss is accomplished by using copper or sodium bicarbonate sprays 10/15/07 8:29:54 AM 260 Mark T Windham and Alan S Windham Figure 24.7  Colonies of Cephaleuros virescens on a leaf of southern magnolia Among pathogens covered in this chapter, the most economically important worldwide are dwarf mistletoes and witchweed Yield losses from these two diseases can destroy plantings over a wide area, and both diseases may cause pandemics Other pathogens discussed in this chapter can cause high yield losses in specific locations Flagellate protozoa may destroy plantings of palms, coffee, or cassava in particular locations Dodder-infested flower beds can lead to severe limitations as to what types of annuals or perennials may be grown in those areas Broomrapes may cause severe disease losses in tobacco in India Control of most of these pathogens remains difficult, and little host resistance is known to any of these pathogens Chemical control of any of the pathogens is almost impossible without also destroying the host plants Most of these pathogens have been the subject to lesser amounts of research than many fungal, bacterial, or viral pathogens Until research scientists give pathogens in this group more attention, many questions concerning their life cycles, infection processes, and disease control tactics will remain unanswered 46691.indb 260 Suggested reading Agrios, G.N 2005 Plant Pathology 5th ed Academic Press San Diego 952 p Coyier, D.L and M.K Roane (Eds) 1986 Compendium of Rhododendron and Azalea Diseases APS Press St Paul, MN 65 p Holcomb, G.E 1995 Ball moss: an emerging pest on landscape trees in Baton Rogue Proc Louisiana Acad Sci 58: 11–17 Lucas, G.B 1975 Diseases of Tobacco 3rd ed Biological Consulting Associates Raleigh, NC 621 p Sinclair, W.A., H.H Lyon, and W.T Johnson 1987 Diseases of Trees and Shrubs Comstock Publishing Associates Ithaca, NY 575 p Tainter, F.H and F.A Baker 1996 Principles of Forest Pathology John Wiley New York 805 p 10/15/07 8:29:55 AM 25 Abiotic Diseases Alan S Windham and Mark T Windham Chapter 25 Concepts • Abiotic diseases of plants are often caused by cultural practices or environmental factors • Abiotic diseases may be difficult to diagnose as the causal agent or factor may have dissipated prior to symptom development • Abiotic diseases may predispose plants to infection by plant pathogens • Sun scald occurs when leaves acclimated to low levels of light are exposed to full sun • Drought stress often predisposes woody plants to infection by canker-causing fungi • Bark splitting is often associated with winter injury or a sudden drop in temperature • The most common reasons for pesticide injury are the misuse or misapplication of pesticides, movement from the initial point of application due to vaporization, drift or movement in water, and injurious residue left from a previous crop • Symptoms associated with herbicide injury may be confused with symptoms produced by certain virus diseases Abiotic or noninfectious diseases are caused by cultural practices (Chapter 35) or environmental factors on plants in nature and also on cultivated crops (contrast with Chapter 39 for disease diagnosis) Although not true plant diseases, damage by abiotic extremes, such as light, water, and temperature, can be quite severe under certain circumstances Most plants grow best at optimum levels of environmental conditions If, for example, temperature drops below or exceeds the optimum for growth, damage may occur Also, although water is necessary for normal plant function, excessive amounts of water or insufficient amounts may cause injury Damage caused by abiotic diseases may be difficult to diagnose because symptoms may not appear until well after plants were exposed to suboptimum cultural conditions or environmental extremes Another interesting note is that abiotic diseases may predispose plants of infection by plant pathogens For instance, woody plants exposed to drought stress are more likely to be infected by canker-causing fungi, such as Botryosphaeria species Small grains and turfgrass grown in alkaline soils are at risk of infection by Gaeumannomyces, a soilborne fungus associated with take-all patch diseases (Chapters 22 and 23) In many cases, damage from abiotic diseases is compounded by biotic diseases that follow Light Low light decreases plant vigor, slows growth, elongates internodes, and may reduce flowering and fruit Suboptimum levels of light lead to decreased carbohydrate production and damage to the plant’s photosynthetic system through reduced chlorophyll production It is often difficult to separate the effects of high light levels and high temperature on leaves Plants grown at low light intensities have leaves with little or no wax and cutin Sun scald occurs when plant material is moved from low light conditions to high light An example is moving a shade-loving plant such as rhododendron from a lathe house to a landscape bed in full sun Also, leaves of woody shrubs may develop sun scald after pruning exposes leaves that are acclimated to low light intensities Occasionally, plants grown at high elevations will develop sun scald when exposed to full sun after several days of cloudy weather Low-temperature injury Freeze injury occurs when ice crystals form in intercellular and intracellular spaces If the cell membrane is ruptured, the cell will die Cold-temperature injury to young trees often leads to bark splitting Extremely low winter tem- 261 46691.indb 261 10/15/07 8:29:56 AM 262 Alan S Windham and Mark T Windham Figure 25.2  Drought injury on Kousa dogwood (Cornus kousa) Note marginal leaf scorch Figure 25.1  Winter injury to the foliage of southern magnolia (Magnolia grandiflora) Note marginal leaf desiccation peratures may cause severe injury to woody plants, killing all above-ground plant parts However, woody shrubs may also be damaged by a sudden drop in temperature, which can lead to bark splitting on the lower stem Fungi that cause cankers may cause branch dieback or death Tender foliage or shoots may be injured by freezing temperature (Figure 25.1) Water stress Drought stress and water stress are sometimes used interchangeably However, water stress can also be used when discussing an excessive amount of water, such as flooding in poorly drained soils Drought stress is normally used when discussing a shortage of natural rainfall Drought stress Wilting, chlorosis, shortened internodes, and poor flower and fruit production are all symptoms that have been associated with drought stress Leaf scorch, which is a marginal or interveinal leaf necrosis, is often seen on deciduous plants exposed to drought stress (Figure 25.2) Evergreen plants, such as conifers, may shed needles in response to a shortage of water Remember that any biotic disease that affects the root system, the vascular system, or the trunk and branches of a plant may produce symptoms that could be confused with those associated with drought stress One of the most common causes of losses of landscape plants is the lack of water Annual flowering plants are especially sensitive to drought stress; most herbaceous 46691.indb 262 perennial plants are less sensitive; however this varies by species and even cultivar It is not uncommon to see native trees in forests die after several years of below-normal rainfall Many woody plants are more susceptible to canker-causing fungi, such as Botryosphaeria and Seiridium species, if the plants have been exposed to significant levels of drought stress Sometimes drought stress may be localized Soils with underlying rock or construction debris, sandy soils, or golf greens with hydrophobic areas may have localized areas of plants exhibiting wilting and chlorosis Also, if pine bark media that is used in container nurseries is not stored properly, it may become infested with fungi, such as Paecilomyces, that cause the bark to become hydrophobic Irrigation water forms channels and is not absorbed by the media in affected containers, simulating drought stress This sometimes leads to losses once plants are installed in landscape beds as the bark mix continues to repel water Excessive water or flooding Flooding or prolonged periods of saturated soil conditions can lead to the decline or death of many cultivated plants Chlorosis, wilting, and root necrosis are all symptoms that may be exhibited by plants exposed to seasonal or periodic flooding In waterlogged soils, low oxygen levels lead to root dysfunction and death Water molds such as Pythium and Phytophthora are favored by the conditions found in saturated soils or growth media (Chapter 20) During periods of cloudy weather, uptake of water may remain high, whereas transpiration rates are slow When this occurs, leaf tissue saturated with water ruptures, forming corky growth on the underside of leaves This condition, edema, is fairly common in ornamental flowering plants such as ivy geranium A similar condition, called intumescence, may be observed on sweet potato and ornamental sweet potato The symptoms of intumescence can occur on both the upper and lower leaf surface 10/15/07 8:29:56 AM ... Close-up of infected limb 46691.indb 25 6 10/15/07 8 :29 :51 AM 25 7 Parasitic Seed Plants, Protozoa, Algae, and Mosses (a) (b) Figure? ?24 .4  (a) Dwarf mistletoe plant growing on conifer (b) Close-up... E and G Caetano-Anolles 20 04 Studying the ecology, systematics, and evolution of plant pathogens at the molecular level., pp 20 9? ?21 5 In: Trigiano, R.N., M.T Windham and A.S Windham (Eds.) Plant. .. plants Green plants the host plants and parasite with a herbicide before flowering and seedset Broomrapes Broomrapes (Orobanche minor and O ramosa) attack more than 20 0 species of plants and occur