Genetic variability for resistance to sorghum downy mildew (Peronosclerospora sorghi) and Rajasthan downy mildew (P. heteropogoni) in the tropicalsub-tropical Asian maize germplasm

Euphytica 138: 23–31, 2004 C 2004 Kluwer Academic Publishers Printed in the Netherlands 23 Genetic variability for resistance to sorghum downy mildew (Peronosclerospora sorghi) and Rajasthan downy mildew (P heteropogoni) in the tropical/sub-tropical Asian maize germplasm Tran Thi Oanh Yen1,4 , B.M Prasanna1,∗ , T.A.S Setty2 & R.S Rathore3 Division of Genetics, Indian Agricultural Research Institute, New Delhi, India; Box 203, Mytho, Tiengiang, Vietnam; Rajasthan College of Agriculture, Udaipur, Rajasthan, India; University of Agricultural Sciences, Regional Research Station, Mandya, Karnataka, India; Present address: Division of Fruit Breeding, Southern Fruit Research Institute; (∗ author for correspondence: e-mail: prasanna@ndf.vsnl.net.in) Received 30 July 2003; accepted May 2004 Key words: downy mildews, genetic variability, inbreds, resistance, Zea mays L Summary Downy mildews cause considerable damage to maize (Zea mays L.) worldwide, particularly in the tropical Asia We have evaluated a set of 42 tropical/sub-tropical maize inbred lines developed in different countries in Asia (India, Thailand and Philippines), and Mexico, for analysing the genetic variability for resistance to sorghum downy mildew [Peronosclerospora sorghi; SDM] and Rajasthan downy mildew [P heteropogoni; RDM] Experiments were carried out in replicated trials under artificial infection in field conditions against SDM and RDM at Mandya in Karnataka, India, and Udaipur in Rajasthan, India, respectively, during 1999 and 2000 The study resulted in identification of five inbred lines offering consistent and strong resistance to both SDM and RDM, while several inbred lines revealed resistance only to RDM It was also revealed that the SDM-resistant inbreds are invariably resistant to RDM, while the RDM-resistant inbreds might show differential responses to the SDM The maize inbred lines identified in this study with broad-spectrum resistance to downy mildews could be potentially useful for basic and applied research work on downy mildews in tropical Asia Introduction Downy mildews in maize, incited by species belonging to the genera Peronosclerospora and Sclerophthora, are some of the most destructive diseases of this crop in the tropical Asian countries The major downy mildew (DM) diseases occurring in Asia include sorghum DM (Peronosclerospora sorghi (Weston & Uppal) C.G.Shaw), Philippine DM (P philippinensis Weston) C.G.Shaw), sugarcane DM (P sacchari (T Miyake) C.G Shaw) and Java DM (P maydis) Any one or combinations of these downy mildew pathogens are prevalent in China, India, Indonesia, Japan, Nepal, Pakistan, Philippines, Thailand and Vietnam (Sharma et al., 1993; Spencer & Dick, 2002) Within the tropical and subtropical regions of the world, there is considerable evidence that intensive maize cultivation favours the development of downy mildew Frederiksen & Renfro (1977) compiled a list of several Latin American countries where the disease has been reported Several inbred lines that are widely used as parents for hybrid seed production in U.S.A were found to be highly susceptible to sorghum downy mildew (SDM), indicating the possibility of serious vulnerability of mid-Western U.S corn hybrids derived from these parental lines to the SDM (Craig et al., 1977) In Egypt, SDM has been considered as a major disease of maize in areas, where sorghum is grown as a forage crop (Nazim et al., 1995) Heavy losses in maize (as high as 100%) have been recorded from one or the other of the DM pathogens in Philippines, Taiwan, Indonesia, Thailand, India, West Africa, Venezuela, Japan, Australia, Europe, North America and other parts of the world (Bonde, 1982; Rifin, 1983) Considering the systemic 24 nature of the disease and the consequent yield losses to the crop, maize downy mildews merit significant attention due to the continuing increase in hybrid maize acreage in various maize-growing countries in tropical Asia In India, various downy mildews such as sorghum DM, Philippine DM, Sugarcane DM and Brown stripe DM (Sclerophthora rayssiae Kenneth, Kaltin, and Wahl var zeae Payak & Renfro), were reported from different agro-ecological regions (Payak, 1975) The sorghum DM (SDM) disease, caused by different strains (maize, sorghum and maize + sorghum strains), is particularly prevalent in the peninsular India, in the states of Karnataka, Tamil Nadu and Andhra Pradesh, reportedly causing losses of 30% and higher (Payak, 1975; Krishnappa et al., 1995) The downy mildew prevalent in the Udaipur district of Rajasthan state in India, with Heteropogon contortus (spear grass) as the collateral host (Siradhana et al., 1980), was recently designated as ‘Rajasthan downy mildew’ (RDM), caused by Peronosclerospora heteropogoni (Smith & Renfro, 1999; Spencer & Dick, 2002) Although the P heteropogoni perpetuates on Heteropogon contortus in the form of oospores, the conidial stage is the sole cause of primary infection of maize in this region With the introduction of some DM-resistant cultivars and with the wide use of metalaxyl seed treatment, the economic impact of the disease subsided in most countries, including Asia (Exconde & Molina, 1978; Lal & Singh, 1984; Odvody & Frederiksen, 1984) However, chemical control is very difficult under conditions of high inoculum or on highly susceptible varieties In USA, efforts to control SDM with chemicals have been largely unsuccessful (Frederiksen et al., 1970) There is also greater awareness in some countries to minimise the application of fungicides in view of the possible environmental hazards Raymundo (2000) discussed the compelling reasons for continued breeding for DM resistance in maize A major emphasis in the breeding programs of several Asian countries, particularly Thailand, Philippines and Indonesia, has been the improvement for resistance to downy mildews prevalent in the respective countries The national programs in Asia and the CIMMYT-Asian Regional Maize Program (ARMP, headquarters located earlier in Thailand) developed several locally adapted populations/open pollinated varieties (OPVs) and more recently, inbred lines for use in hybrid breeding The CIMMYT-ARMP lines were collected or developed by the program for use in resistance breeding in the region (George et al., 2004) International germplasm exchange and testing of downy mildew resistant materials led to the development of several base populations and improved varieties in Asia Although screening for responses of the maize experimental hybrids to downy mildews is routinely being carried out under the All India Coordinated Maize Improvement Project (AICMIP), evaluation of a common set of inbred lines developed in different tropical Asian maizegrowing countries, besides CIMMYT, Mexico, for their responses to the downy mildew diseases in tropical Asia shall provide an opportunity for identification of genotypes with broad-spectrum resistance to the downy mildew pathogens The objective of the present investigation, therefore, was to analyse the genetic variability in a set of tropical/sub-tropical maize inbred lines (developed in Thailand, Philippines, India and Mexico) for resistance to two different downy mildew diseases (sorghum DM and Rajasthan DM) in India Materials and methods Plant materials The materials comprised 42 inbred lines, including: (i) a set of 24 maize inbred lines provided by the CIMMYT-Asian Regional Maize Program (designated as ‘AMB’ lines for convenience in presentation) developed in Thailand and Philippines using diverse germplasm (Table 1); (ii) five CMLs (released ‘CIMMYT Maize Lines’) developed at CIMMYT, Mexico, which had not undergone selection for downy mildew resistance; (iii) seven inbreds generated at the University of Agricultural Sciences-Regional Research Station (RRS), Mandya, Karnataka, India (designated as ‘MAI’ lines); (iv) two inbreds from UAS-Agricultural Research Station (ARS), Nagenahalli in Karnataka, India (designated as ‘NAI’ lines); and (v) three elite inbreds (designated as ‘CM’ (Coordinated Maize’) lines) that are parental lines of popular single-cross hybrids in India, released in public domain under the AICMIP (Table 1) Inoculum preparation and screening of genotypes The inbred lines were evaluated at two ‘hot spots’ for downy mildews at (i) UAS-Regional Research Station, Mandya (12◦ N; 76◦ E; 695 m.s.l elevation; 705 mm/year average rainfall) in Karnataka state for SDM (P sorghi), and (ii) Rajasthan College of 25 Table Germplasm sources and pedigree of the inbred lines used in this study Line Germplasm sourcea /Pedigreeb Typec AMB101 AMB102 AMB103 Philippines: Pi21 [Pop P3228] Philippines: Pi31 [Pop SMC E9] Thailand (DOA): Nei9008 [SW1(s)C9-germplasm/(DA9-1(s)-7-3-1 x SW C9)-S9-177-1] Thailand (DOA): Nei9202 [Pop28(HS)C6/Pop28(HS)C6-S9-129] Thailand (DOA): Nei9203 [Pop28(HS)C6/Pop28(HS)C6-S9-410] Thailand (DOA): Nei9204 [Pop28(HS)C6 x SW(s)C8/NS1(s)C1-S9-251] Thailand (KU): Ki3 [SW1(s)C4/SW1(s)C4-S8-19-5] Thailand (KU): Ki14 [Suwan-1 (S)C4-S8-19-5 (2028)] Asia: AMATLCOHS115-1-2-3-3-1-2-B-B Asia: AMATLCOHS233-1-1-1-1-2-2-B-B-B Asia: P345C3S3B-40-8-1-11-2-2-B Asia: P345C3S3B-46-1-1-1-1-2-B Asia: CA34502 [P345C5S1B-15-4-2-1-2-1-2-B] Asia: AMATLCOHS-9-1-1-1-1-1-2-B Asia: AMATLCOHS245-1-1-1-2-1-1-B-B Asia: P345C4S2B46-2-2-1-2-B-B-B Asia: NS1C1S5-261-7-3-1-2-1-1-B-B Asia: P300C5S2B-17-1-1-1-B Philippines (IPB): IPB9204-1-3-1-2-4-B Asia: CIMMYT [P24 STE-5*24STE-17-BBBB#-#-#-B-1-B-2-B-B-B] Asia: CIMMYT [P24 STE-5*24STE-17-BBBB#-#-#-B-5-B-4-B-B-B] Asia: CIMMYT [SIN.AM.TSR-76-1-1-B-1-BBBB-5-#-#-BBBBBBBBB] Asia: CIMMYT [P24STEC1HC16-1-3-3-1-2-BBB-#-#-#-9BBBBBBB] Asia: CIMMYT [P24STEC2-29-BBBB#-3-BBBBBBB] Asia: CIMMYT [G26C25HS45-3-4-1-6-BBBB] Mexico: CIMMYT Pop 24 [Pob24HC34-2-3-B-###] Mexico: CIMMYT Pop 29 [Pob29STEC1HC17-4-1-1-2-1-BB-f] Mexico: CIMMYT Pop 29 [Pob29STEC1HC1-3-1-1-4-2-BB-f] Mexico: CIMMYT Pop 43 [43*PORILLO8043)-5-1-2-2-BB-F] Mexico: CIMMYT Pop 24 [Pob24STEC1HC23-5-2-1-2-3-BB-f] India (Mandya): Derivative of ICI-770 India (Mandya): Derivative of F-720 India (Mandya): Derivative of EH-46791 India (Mandya): Derivative of BIO-968 (Y) India (Mandya): Derivative of Parbhat India (Mandya): Derivative of EH-46714 India (Mandya): Derivative of JH-1130 India (Nagenahalli): Pool 33 (Sub Trop IY)-90-K-S5-BBBBB India (Nagenahalli): WP-4-91-K-NA-S6-BBB India (AICMIP): Derivative of R-109 Ht (US inbred) India (Ludhiana): H104-21 (LM3) India (AICMIP): A632 #-2-4-⊗-#-f-##-1-2-## Tropical Y F Tropical Y F AMB104 AMB105 AMB106 AMB107 AMB108 AMB109 AMB110 AMB111 AMB112 AMB113 AMB114 AMB115 AMB116 AMB117 AMB118 AMB119 AMB120 AMB121 AMB122 AMB123 AMB124 AMB125 CML20 CML270 CML272 CML281 CML289 MAI101 MAI105 MAI110 MAI113 MAI114 MAI116 MAI117 NAI139 NAI151 CM119 CM124 CM133 a Source: Tropical Y F Tropical Y F Tropical Y F–SD Tropical Y F Tropical Y F Tropical Y F Tropical DMR Tropical DMR Tropical DMR Tropical DMR Tropical DMR Tropical DMR Tropical DMR Tropical DMR Tropical Y F Tropical Y F Tropical DMR Tropical Tropical Tropical Tropical Tropical Tropical Tropical Y D Tropical W SD Tropical W SF Tropical W D Tropical Y F Tropical Y F Tropical Y F Tropical Y F Tropical Y F Sub-tropical Y F Sub-tropical Y F Sub-tropical Y F Sub-tropical Y F Sub-tropical Y F Sub-tropical Y F Sub-tropical Y F Tropical Y F DOA: Dept of Agriculture, Thailand; KU: Kasetsart University, Thailand; IPB: Institute of Plant Breeding, Philippines; Asia: CIMMYT-Asian Regional Maize Program; Mandya: UAS-Regional Research Station, Mandya, India; Nagenahalli: UAS-Agricultural Research Station, Nagenahalli, India; AICMIP: All-India Coordinated Maize Improvement Project; Ludhiana: Punjab Agricultural University, Ludhiana b Pedigree: Pop or P = population; C = Cycle; HS = Half Sibs; B = Selfed and Bulked; -1,-2,-3 = ear to row; # = sibbing; STE = inbreeding tolerant population; AMATL = Asia Mildew Acid Tolerant c Type: Y = Yellow grain; W = White grain; D = Dent; SD = Semi-dent; F = Flint; SF = Semi-flint; DMR = Downy Mildew Resistant 26 agriculture, MPUAT, Udaipur (23◦ 46′ N; 73◦ 09′ E; 577 m.s.l elevation; 633.3 mm/year average rainfall) in Rajasthan for RDM (P heteropogoni) The materials were sown with a plant-to-plant spacing of 20 cm and rowto-row spacing of 70 cm in a randomised block design, with two replications and two rows per replication Test entries were planted in m long rows having 15 plants per row Following are the inoculation methods followed for SDM and RDM SDM disease screening Artificial epiphytotic conditions were created by planting ‘spreader rows’ of a highly susceptible maize genotype, CM500, 30 days prior to planting of the test entries The method employed was as follows: (i) DM infected maize leaves, showing visible conidial growth, were collected from the nursery and spread over a moist gunny bag; (ii) seeds of CM500 were placed over the infected leaves, and over these seeds, another layer of DM infected leaves were spread; and (iv) a moist gunny bag was placed over the infected maize leaves, and the seeds were retained in moist condition in this state for at least 2–3 days This method, locally referred to as ‘sandwich method of inoculation’, facilitates effective entry and establishment of the systemic SDM fungus into the seeds of the susceptible maize line, which were then used for planting as spreader rows on all sides of the experimental block For every two beds of test entries (3 m each), one bed of spreader rows (2 m each) was planted, to ensure good infection pressure on the test material Test entries (two-row plots) were planted 3–4 weeks after planting of spreader rows to facilitate effective conidial spread from the spreader rows To analyse the disease pressure through this method, CM500 was planted after every tenth row of test materials as ‘susceptible check’ Severe susceptibility (99– 100% SDM incidence) of the ‘check’ across the experimental trial in both the years (1999 and 2000), indicated the pathogen pressure through this method, ensuring that there were no ‘escapes’ among the test entries RDM disease screening ‘Whorl inoculation technique’ was carried out on test entries evaluated at the DM nursery in Rajasthan College of Agriculture, Udaipur during 1999 and 2000, as follows: (i) Heteropogon contortus, the collateral host for RDM pathogen, was grown in the nursery, along with susceptible open-pollinated maize cultivars, Kiran and Surya, for collection of conidia; (ii) test entries were planted in mid-August (coinciding with the hot, humid weather in monsoon season) in m beds, with a plant-to-plant spacing of 15 cm and a row-to-row spacing of 70 cm; (iv) RDM-infected plants of Kiran and Surya were covered by wet polyethylene bags in the evening (5:00–6:00 p.m.); (v) conidia were collected from the infected leaves at 2–3 a.m by brushing from the infected leaves into plain water in a petri dish until the water turned turbid; (vi) the conidial suspension was adjusted to a concentration of approximately 40,000– 50,000 conidia/ml, and 2–3 drops of the suspension were carefully placed in the whorl of each seedling of the test entries at 2–3 leaf stage (6–7 days after germination) The inoculation procedure, carried out between 4:00 and 6:00 a.m., was repeated for three consecutive days to ensure no escape of plants from artificial infection In an earlier study, this method of artificial infection with RDM pathogen was found to be distinctly superior to the ‘oospore-infested soil’ and ‘detached leaf pieces’ methods (Siradhana et al., 1976) Disease assessment Data on disease incidence was collected from each replication in the randomised experimental block, and any entry with data from less than 10 plants was not used for statistical analysis Observations on number of DM disease-infected plants were recorded for each test entry at 21 and 35 days after plant emergence (at Mandya for SDM) or after inoculation of test entries (at Udaipur for RDM) Disease incidence at 35 days after plant emergence or inoculation was considered as final rating for further analysis Inoculated plants that did not show systemic symptoms of DM (emergence of characteristic chlorotic leaves) even a month after artificial infection were considered resistant As per the procedure adapted worldwide for rating DM incidence (Lal & Singh, 1984), disease responses of genotypes were analysed as per the ratings given below: 0–10% DM incidence–resistant; >10–30%— moderately resistant; >30–50%—moderately susceptible; and >50%—susceptible Weather conditions, including maximum and minimum temperatures, relative humidity and rainfall at the experimental sites were also recorded (data not presented) and, in general, found to be highly conducive for effective disease establishment and spread at both Mandya and Udaipur during 1999 and 2000 Statistical analysis The data collected from the various test entries with respect to per cent DM incidence at Mandya and 27 Table ANOVA for SDM and RDM incidence at Mandya and Udaipur, respectively, during 1999 and 2000 SDM 1999 Source of variation Replication d.f Genotype 32 Error C.V (%) 32 MSS 0.090 (0.828) 19.875 (0.0001) 1.884 19.84 SDM 2000 d.f 34 34 RDM 1999 MSS d.f 2.126 (0.232) 22.466 (0.0001) 1.435 17.19 30 30 MSS 0.702 (0.470) 12.082 (0.0001) 1.313 25.48 RDM 2000 d.f 38 38 MSS 5.858 (0.080) 10.948 (0.0001) 1.805 26.74 Values in parentheses indicate probability Udaipur during 1999 and 2000, were subjected to statistical analysis in accordance with the experimental design and its associated model Any entry with less than 10 plant data was not considered for analysis The normality of the original data (per cent DM incidence) was analysed by MSTAT-C statistical package using kurtosis and skewness parameters On this basis, transformation of data was carried out using four different options: arc sin, logarithmic, square root and 1/x methods of transformation; data resulting from square-root transformation [sqrt (x + 0.5)] was found to be normal and more efficient than those obtained from all other methods, and was used for further analysis Analysis of variance (ANOVA) associated with linear model was carried out using SAS-6.12, to quantify and evaluate the sources of variation Hartley’s Fmax test was carried out to test the homogeneity of error variances from different experiments; the results showed the possibility of combining data from various experiments for further statistical analysis at two locations or due to poor germination of some entries ANOVA also revealed significant ‘year × genotype’ interaction for SDM and RDM incidence in the inbreds (Table 3) Hartley’s Fmax test was carried out to test the homogeneity of error variance from different experiments The Fmax value for the two experiments at Mandya (1999, 2000) was 1.31, less than the table value (5.48); the Fmax value for the two experiments at Udaipur was 1.37, less than the table value (5.45); the Fmax for the experiments at Mandya and Udaipur during 1999 was 1.43, less than the table value (5.50); and the Fmax value for the experiments at Mandya and Udaipur during 2000 was 1.26, which was also less than the table value (5.45) When all the four experiments were taken into consideration, the Fmax value was 1.43, which was less than the table value (6.92) These results indicated homogeneity of error variance from different Table ANOVA for combined data on SDM and RDM incidence in inbreds across two years (1999 and 2000) SDM Results and discussion Source of variation Evaluation of a specific set of maize inbred lines against both SDM and RDM infection at two downy mildew ‘hot spots’ in India provided an insight into the possible differences in the interaction of the tropical Asian maize genotypes with SDM and RDM pathogens ANOVA revealed significant differences among the genotypes with respect to SDM and RDM disease incidence in 1999 and 2000 (Table 2) However, disease responses of the entire set of 42 inbreds could not be reconfirmed in both the years (1999 and 2000) either due to inadequate seed material for replicated plantings Year Replication (Year) d.f Genotype 41 Year ∗ Genotype 26 Error C.V (%) 66 MSS 0.784 (0.493) 1.107 (0.515) 30.852 (0.0001) 6.066 (0.0001) 1.653 18.52 Values in parentheses indicate probability RDM d.f 40 29 67 MSS 3.148 (0.181) 3.280 (0.157) 15.356 (0.0001) 5.619 (0.0001) 1.723 29.69 28 experiments, facilitating pooling of data from respective locations and years for further analysis The mean per cent disease incidence values in different test entries are presented in Table In general, the genotypes under study revealed good correspondence in disease response ratings across both the years, except for five lines (AMB105, AMB108, AMB115, AMB117 and AMB118) in case of SDM and 10 lines in case of RDM The variation in per cent DM incidence between 1999 and 2000 datasets in these genotypes might be attributed to experimental and climatic variables A summary of the disease response ratings of 41 inbred lines tested against both SDM and RDM is provided in Table For the final disease rating (R, MR, MS and S) of the inbred lines, we considered only the higher per cent DM incidence (1999 vs 2000 data) rather than the mean per cent DM incidence Based on the analysis of the results obtained in two seasons (1999 and 2000), the study revealed five inbreds—AMB103, AMB109, AMB110, AMB112 and AMB119—that showed stable and strong resistance to both SDM and RDM Particularly promising were AMB109, AMB112 and AMB119, which showed consistently high resistance in both the years of evaluation Data for AMB103 and AMB110, which also showed good resistant responses, could be obtained from only one year (2000) Significantly, inbreds showing resistance to SDM were invariably resistant to RDM, while inbreds resistant to RDM showed differential responses to SDM (Table 5) This observation is in agreement with another recent study on the genetic variability in the Indian maize inbred lines with respect to SDM and RDM resistance (Nair et al., 2004) There could be different reasons for such a response, including: (i) involvement of some common major loci controlling resistance to both SDM and RDM, as has been recently revealed by QTL mapping experiments carried out under the Asian Maize Biotechnology Network (AMBIONET) in India, Indonesia, Philippines and Thailand (George et al., 2003); (ii) relatively less number of genetic factors controlling resistance to RDM in comparison with that of SDM; and (iii) lower level of virulence of P heteropogoni as compared to P sorghi Among the five inbreds found resistant to both SDM and RDM in this study, two lines were developed by the national programs in Thailand and Philippines (AMB103 and AMB119, respectively) and three lines were bred from tropical downy mildew resistant materials under CIMMYT-ARMP AMB103 (Nei9008) was derived from Suwan-1, a popular OPV, in Thailand, while AMB119 (IPB9204) owes its origin from Table Per cent SDM and RDM incidence in various maize inbreds evaluated at Mandya and Udaipur, respectively SDM Line 1999 2000 RDM Mean AMB101 57.10 46.15 51.62 AMB102 86.40 91.67 89.03 AMB103 – 0.00 0.00 AMB104 65.40 66.67 AMB105 17.20 61.54 AMB106 57.10 AMB107 30.00 AMB108 AMB109 1999 2000 Mean 0.00 6.90 3.45 5.70 13.80 9.75 – 5.60 5.60 66.03 0.00 0.00 0.00 39.37 0.00 0.00 0.00 75.00 66.05 0.00 15.80 7.90 16.67 23.33 2.22 0.00 1.11 57.70 32.44 45.07 0.00 0.00 0.00 0.00 4.35 2.17 5.40 0.00 2.70 8.57 AMB110 – AMB111 18.20 AMB112 0.00 AMB113 AMB114 8.57 – 3.10 3.10 18.20 0.00 0.00 0.00 2.78 1.39 0.00 0.00 0.00 11.80 14.29 13.04 5.50 7.70 6.60 39.10 – 39.10 0.00 8.30 4.15 – AMB115 0.00 22.22 11.11 0.00 0.00 0.00 AMB116 56.00 67.70 61.85 – – – AMB117 17.60 10.00 13.80 – 0.00 0.00 AMB118 25.00 73.33 49.16 0.00 13.65 AMB119 8.10 AMB120 50.00 AMB121 50.00 AMB122 100.00 AMB123 – 2.00 – 100.00 – 27.30 5.05 0.00 0.00 0.00 50.00 0.00 0.00 0.00 – 2.80 75.00 2.80 100.00 0.00 36.40 77.50 77.50 AMB124 100.00 81.48 90.74 AMB125 100.00 100.00 100.00 CML20 96.00 97.78 CML270 100.00 – CML272 100.00 67.65 CML281 100.00 – CML289 94.10 MAI101 MAI105 MAI110 – 9.10 36.40 5.90 5.90 33.30 23.90 28.60 96.89 27.70 17.90 22.80 100.00 57.10 26.00 41.55 18.00 18.00 83.82 – 18.20 – 100.00 34.30 0.00 17.15 67.75 80.92 16.70 43.60 30.15 92.30 82.00 87.15 – 18.00 18.00 63.80 100.00 81.90 4.50 2.10 3.30 – 60.00 60.00 0.00 0.00 0.00 MAI113 87.20 90.48 88.84 5.40 0.00 2.70 MAI114 45.00 48.84 46.92 25.60 5.90 15.75 MAI116 95.20 – 95.20 0.00 0.00 0.00 MAI117 97.90 100.00 98.95 37.00 0.00 18.50 NAI139 – 100.00 100.00 65.00 59.30 62.15 NAI151 – 64.00 64.00 – 1.90 1.90 CM119 – 95.92 95.92 – 58.80 58.80 CM124 – 100.00 100.00 – 5.40 5.40 CM133 – 100.00 100.00 – 80.90 80.90 29 Table Differential responses of the inbred lines to SDM and RDM infectiona Responses Genotypes R(SDM); R(RDM) AMB103, AMB109, AMB110, AMB112, AMB119 MR(SDM); R(RDM) AMB107, AMB111, AMB113, AMB115, AMB117 MS(SDM); R(RDM) AMB114 MS(SDM); MR(RDM) MAI114 S(SDM); R(RDM) AMB101, AMB104, AMB105, AMB106, AMB108, AMB120, AMB121, AMB122, AMB124, MAI105, MAI110, MAI113, MAI116, NAI151, CM124 S(SDM); MR(RDM) AMB102, AMB118, CML20, CML272, MAI101 S(SDM); MS(RDM) AMB123, AMB125, CML281, CML289, MAI117 S(SDM); S(RDM) CML270, NAI139, CM119, CM133 R-Resistant; MR-Moderately resistant; MS-Moderately susceptible; S-Susceptible a SDM: Sorghum downy mildew; RDM: Rajasthan downy mildew downy mildew resistant germplasm in Philippines It is interesting to note that the resistance sources from Philippines (DMR-1 and DMR-5 whose resistance came from the native cultivar Tiniguib) were crossed in Thailand into a locally developed variety (Thai composite #1, a composite derived using several varieties from Mexico, Central America, and the Caribbean), from which Suwan-1 and four additional improved Suwan varieties were developed (Sriwatanapongse et al., 1993) The Suwan series became extremely popular as breeding material throughout Asia (Sriwatanapongse et al., 1993; Dowswell et al., 1996 ), leading to the development of several downy mildew resistant inbred lines (Table 1) Three tropical lowland maize populations (Mezcla Tropical Blanca, Amarillo Dentado, and Amarillo Cristalino2) developed at CIMMYT, Mexico, and four genetically broad-based populations (early maturing yellow and white and late maturing yellow and white; De Leon et al., 1993) developed at the CIMMYTARMP were also extensively used as sources of resistance in breeding programs in the tropical Asian region None of the MAI, NAI or CM lines evaluated in this study showed resistance to SDM at Mandya In contrast to the paucity of lines resistant to SDM, the study revealed a large number of inbred lines resistant to RDM (Table 5) Many of the AMB lines and four MAI lines (MAI105, MAI110, MAI113 and MAI116) showed resistance to RDM, whereas only CM124 among the three elite CM lines analysed recorded resistance The high vulnerability of the CM lines to the downy mildew diseases was also recorded in another recent study (Nair et al., 2004) Since the CM lines are widely used in the Indian maize breeding programs, the observation highlights the necessity of incorporating DM resistance in such materials Released hybrids based on parental lines that are highly vulnerable to DM infection may incur heavy losses if an epiphytotic DM disease were to occur due to congenial conditions The CMLs (developed at CIMMYT, Mexico) evaluated in this study showed severe susceptibility to SDM, and moderate resistance to susceptibility to RDM (Table 5) In Mexico, representative locations for different maize growing agroecologies assisted in selection of resistance to many important diseases, except downy mildews (Jeffers et al., 2000) This could be the possible reason for the high vulnerability of a majority of CMLs developed at CIMMYT, Mexico, for downy mildews in tropical Asia, unlike those lines developed by CIMMYT-ARMP, where diverse materials from countries such as Philippines and Thailand are utilised for inbred line development (S.K Vasal, personal communication) Genotypes showing resistance to a specific DM pathogen in one country may or may not show a similar response in another country For instance, Mo17Ht was used as a susceptible parent by Singburaudom & Renfro (1982) for P sorghi in Thailand, while it is resistant to P sorghi in Texas, U.S.A (Craig et al., 1977) Such observations were supported by an analysis by Schmidt & Freytag (1977), in which pathogenic differences could be identified between isolates from Texas and Thailand Among the inbreds proven to be highly resistant to both SDM and RDM diseases, AMB103 (Nei9008) is also currently being utilised as a resistant parent for molecular marker mapping and marker-assisted selection for Philippine DM in Philippines and sorghum 30 DM in Thailand under the Asian Maize Biotechnology Network (AMBIONET) program (M.L.C George, personal communication) Such inbred lines with broadspectrum resistance to DM infection in tropical Asian countries could be highly useful for a range of basic and applied research aspects on downy mildews The present investigation, thus, highlights the differences in host-pathogen interaction with respect to SDM and RDM diseases in India Although RDM is currently restricted to isolated pockets in Rajasthan, India, it is important to note that P heteropogoni (spear grass) completes its life cycle through a collateral host, called Heteropogon contortus, which coexists with maize in the tropical and sub-tropical areas of the world, and is most abundant, where the average annual rainfall is 600–1000 mm with marked wet and dry seasons Only a few plant species have such a wide distribution as this wild weedy grass The differential responses of a same set of inbred lines to SDM and RDM evaluated at the ‘hot spots’ using artificial infection, supports the need to treat the two diseases (RDM caused by P heteropogoni versus SDM caused by P sorghi) separately (Smith & Renfro, 1999; Spencer & Dick, 2002) However, the observation in this study that the SDM-resistant lines are invariably resistant to RDM provides a significant opportunity for developing an integrated breeding strategy for development of germplasm/cultivars with resistance to both the diseases Identification of stable sources of resistance to both SDM and RDM diseases through this study also paves the way for undertaking comprehensive genetic and molecular analyses of resistance to these diseases, besides effective utilisation of such genotypes in maize breeding programs in tropical Asia Acknowledgements The study was undertaken as a part of the AMBIONET (Asian Maize Biotechnology Network) program in India, facilitated by CIMMYT, Mexico, and financially aided by the Asian Development Bank (ADB) We thank Drs S.K Vasal (CIMMYT, Mexico) and K.T.P Gowda (UAS-ARS, Nagenahalli, India) for providing the seed material of the CIMMYT-ARMP lines, and the two ‘NAI’ lines, respectively Field experiments were facilitated by Dr N.N Singh (DMR, India) Technical assistance rendered by Dr Rajesh Kumar (IARI, New Delhi) and Mr Govindraju (UAS-RRS, Mandya) is gratefully acknowledged T.T.O.Y was supported by a Junior Research Fellowship from the Indian Council for Cultural Relations (ICCR) and the Vietnamese Government References Bonde, M.R., 1982 Epidemiology of downy mildew disease of maize, sorghum and pearl millet Trop Rest Management 28: 49– 60 Craig, J., A.J Bockholt, R.A Frederiksen & M.S Zuber, 1977 Reaction of important corn inbred lines to Sclerospora sorghi Plant Dis Reporter 61: 563–564 De Leon, C., G Granados, R.N Wedderburn & S Pandey, 1993 Simultaneous improvement of downy mildew resistance and agronomic traits in tropical maize Crop Sci 33: 100–102 Dowswell, C.R., R.L Paliwal & R.P Cantrell, 1996 Maize in the Third World Westview Press, Inc Boulder CO, U.S.A., 268 pp Exconde, O.R & A.B Molina Jr., 1978 Ridomit (Ciba-Geigy2), a 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Compendium of Corn Diseases (3rd edn.), pp 25–32 American Phytopathological Society, U.S.A Spencer, M.A & M.W Dick, 2002 Aspects of Graminicolous Downy Mildew Biology: Perspectives for Tropical Plant pathology and Peronosporomycetes Phylogeny In: R Watling, J.C Frankland, A.M Ainsworth, S Isaac & C.H Robinson (Eds.) Tropical Mycology Vol Micromycetes, pp 63–81 CABI Publishing, Wallingford, U.K Sriwatanapongse, S., S Jinahyon & S.K Vasal, 1993 Suwan-1: Maize from Thailand to the world Mexico, D.F.: CIMMYT  ... Singh, 2004 Genetic variability in the Indian maize germplasm for resistance to sorghum downy mildew (Peronosclerospora sorghi) and Rajasthan downy mildew (P heteropogoni) Field Crops Res [In. .. of maize inbred lines against both SDM and RDM infection at two downy mildew ‘hot spots’ in India provided an insight into the possible differences in the interaction of the tropical Asian maize. .. nature of the disease and the consequent yield losses to the crop, maize downy mildews merit significant attention due to the continuing increase in hybrid maize acreage in various maize- growing countries