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© 2011 Plant Management Network.
Accepted for publication 13 January 2011. Published 23 March 2011.


Screening Exotic Sorghum Germplasm, Hybrids, and Elite Lines for Resistance to a New Virulent Pathotype (P6) of Peronosclerospora sorghi Causing Downy Mildew


Ghada L. Radwan, Graduate Student, Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843; Ramasamy Perumal, Sorghum Breeder, Agricultural Research Center, Kansas State University, Hays, KS 67601; Thomas Isakeit, Professor, and Clint W. Magill, Professor, Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843; Louis K. Prom, Research Plant Pathologist, USDA-ARS, Southern Plains Agricultural Research Center, College Station, TX 77845; and Christopher R. Little, Assistant Professor, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506


Corresponding author: C. R. Little. crlittle@ksu.edu


Radwan, G. L., Perumal, R., Isakeit, T., Magill, C. W., Prom, L. K., and Little, C. R. 2011. Screening exotic sorghum germplasm, hybrids, and elite lines for resistance to a new virulent pathotype (P6) of Peronosclerospora sorghi causing downy mildew. Online. Plant Health Progress doi:10.1094/PHP-2011-0323-01-RS.


Abstract

A recent outbreak of sorghum downy mildew (SDM) in Texas has led to the discovery of both metalaxyl fungicide resistance and a new pathotype, P6, in the causal organism Peronosclerospora sorghi. New and alternate sources of host plant resistance are needed for successful management of SDM. To identify sources of resistance, a total of 333 (242 minicore lines representing diverse germplasm from India, 67 commercial hybrids from Kansas, and 24 elite breeding lines from Texas) were inoculated in the greenhouse. Using an established sandwich inoculation technique, artificial inoculation of test lines with P. sorghi conidia, resulting in < 10% infection, were scored as “resistant.” Fifty-two minicore and 20 accessions from Kansas exhibited ≤ 10% infection and were selected as resistant. Out of 52 resistant minicore accessions, 28 were photo-insensitive. Eleven of 20 commercial hybrids from Kansas showed zero percent infection. Thirteen of 24 elite breeding lines from Texas were also resistant. In this study, resistance sources for the new P6 SDM pathotype were identified. The diversity among these materials is expected to provide different single-gene sources as well as quantitative sources of SDM resistance for use in breeding programs.


Introduction

Sorghum bicolor (L.) Moench is the third and fifth most important cereal crop grown in the United States and in the world, respectively, and is used as an important source of food in many regions in Africa and Asia. In the United States, the major sorghum growing states are Kansas, Texas, Nebraska, Oklahoma, and Missouri, of which 40% of the production is from Kansas. Though sorghum is well adapted to environmental extremes such as drought, the value and productivity can be reduced greatly by diseases (4). Sorghum downy mildew (SDM), caused by Peronosclerospora sorghi (Weston & Uppal) C. G. Shaw, is a disease of sorghum and maize (Zea mays L.) (20). In Africa, reported SDM yield losses range from negligible to as high as 11.7% (1), and 78% in cv. ‘DMS 652’ in India (18).

In Kansas, SDM was first found in Republic and Riley counties in 1967 and 1968 at the same time as an epidemic was occurring in the corn and sorghum production regions of Texas (5,9,14,16). Subsequent economically serious outbreaks occurred in Kansas in 1978 and 1979 and in Nebraska in 1987 (8,16).

Sporadic outbreaks of sorghum downy mildew occur in eastern and south-central Kansas, but rarely cause economic loss. Primary inoculum in Kansas comes from oospores that overwinter within shredded leaf debris. Often, such leaf debris may originate from systemically infected shattercane, sudangrass, or johnsongrass, which presumably initiated the midwest epidemics of 1979 and 1987 (8). Secondary spread of the disease is promoted by cooler than normal night temperatures (18-21°C) and above average rainfall or long periods of dew, which results in the presence of free moisture on leaves.

Thus, control recommendations have relied upon the following: avoiding planting sorghum after sudangrass (which is highly susceptible), use of crop rotation, planting of resistant hybrids, and seed treatments with metalaxyl. As demonstrated by Jensen et al. (8), the pathogen causing the outbreak in 1987 was pathotype 1 (P1). No studies since have characterized P. sorghi populations in the midwest as to their pathotype, although it is considered that P1 and P3 are present because of their ability to infect a wide range of sorghum cultivars, the fact that P2 is relatively rare, and that all of these pathotypes exhibit general sensitivity to metalaxyl. Taken together, this disease in the United States was under control for the last two decades by the regular use of metalaxyl (methyl N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-DL-alaninate) seed treatment along with the deployment of resistant cultivars. The outbreak in several sorghum fields in Wharton and Jackson counties, Texas, during the spring of 2001 and again in 2002, causing significant yield loss poses a serious threat to sorghum productivity. Subsequently, isolates collected from previously resistant hosts revealed the evolution of a new pathogenic race of P. sorghi, pathotype 6 (P6), and demonstrates a need for constant monitoring of the pathogen population (7,12).

As a consequence, it became important to test commercial hybrids and elite breeding lines grown in the major sorghum-producing states, Kansas and Texas, for their reaction to the metalaxyl-resistant, P6 pathotype of P. sorghi. Also, at this crucial juncture, it is imperative to identify new resistance sources from exotic germplasm accessions for the control of downy mildew and for long-term sustainability of genetic resistance. Broadening the genetic base for any trait of interest can be obtained through breeding with exotic germplasm, especially tropical sources. However, the pool of available tropical germplasm is large and diverse, making choices of tropical parents difficult. A minicore sorghum germplasm compilation (242 accessions from 57 countries), which represents most of the diversity in the germplasm collection (11% of 2246 core, < 1% of entire 37,000 original collection) were developed for use as genetic resources at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), India. Due to its greatly reduced size and diversity representation, the sorghum minicore can be economically evaluated for beneficial traits and provides a gateway for enhanced utilization of germplasm for sustainable crop improvement for food, feed, fodder, fuel, and other industrial products like tannins, waxes, protein, starch, and dyes (19). This paper reports on the screening against the new virulent sorghum downy mildew pathotype, P6, to identify resistance status of Texas and Kansas hybrids and elite breeding materials and to identify potential sources of downy mildew resistance within the minicore exotic sorghum collection.


Identifying Sources of Resistance to Pathotype 6

In order to screen hybrids and elite breeding materials from Kansas and Texas, and the minicore exotic sorghum collection, three experiments were conducted in the greenhouse (Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX). The experiments were initiated in June 2009 and completed in May 2010.

In Experiment I, 242 exotic accessions from the minicore collection were screened. The list of 242 minicore accessions, source and country of origin are maintained at ICRISAT. In Experiment II, 67 accessions including commercial hybrids and elite lines commonly grown in Kansas (including those in the 2008 Kansas grain sorghum hybrid performance test) and Texas were screened. In Experiment III, 24 accessions, of which 13 are new hybrids and the remaining 11 are elite breeding lines developed by William R. Rooney (Sorghum Breeder, Texas A&M University, College Station, TX) were screened. The new virulent race, P6, was maintained in the greenhouse on infected susceptible plants (Pioneer hybrid ‘84G62’), which served as the source of inoculum.

In these studies, we followed the sandwich inoculation technique as established by Thakur (17) for downy mildew resistance screening in the greenhouse. Multiplication and collection of conidia/conidiophores and inoculation of germinating seeds are detailed in Fig. 1. The oomycetes that cause cereal downy mildews are obligate parasites that cannot be grown in pure culture. Hence, fresh conidiophores were produced for each experiment from infected leaves. Approximately 50 seeds of a test genotype were placed in a petri dish upon moist filter paper (Whatman, No. 1) and allowed to germinate for 24 to 30 h at 35°C. After germination, a layer of plastic or wire mesh (Fig. 1E) was placed over the germinating seeds and 4 to 6 sporulating leaves collected from the susceptible line were placed on the mesh, lower surface-side down, so that conidia would fall onto developing seedlings. The plates, without tops, were wrapped in moistened paper towels, placed into plastic bags and incubated at 20°C for 24 h to maintain high humidity and sporulation and infection. Each experiment was replicated three times using a randomized complete block design. In each replication, 16-17 inoculated seedlings were transplanted to a half-gallon pot, and allowed to grow in the greenhouse at 25 ± 1°C for 14 days (Fig. 2). Each week, 15 to 20 lines were tested at a time along with the susceptible (Pioneer hybrid ‘84G62’) and resistant (Pioneer hybrid ‘83P67’) checks. Conidia multiplication was found to be very difficult during the winter months due to cooler weather. Therefore, screening studies were not conducted between October 2009 and February 2010. Susceptibility to downy mildew was evaluated two weeks after transplanting. Plants showing systemic and/or local lesions were counted as infected. Disease incidence was determined from the percentage of infected plants in each replication and evaluated for disease symptoms. In this study, as in Prom et al. (13), accessions with 10% or less downy mildew incidence were considered “resistant.” Disease incidence data was subjected to analysis of variance using PROC ANOVA (SAS v. 9.1, SAS Institute Inc., Cary, NC). Mean comparisons among the sorghum accessions were based on Tukey’s Studentized Range test at the 5% probability level.


   
   
   

Fig. 1. Conidia production and inoculation of germinating seedlings. (A, B) Collecting infected leaves in a plastic container from a susceptible cultivar (Pioneer ‘84G62’). (C) Approximately 50 seeds are spread in a petri dish on moist filter paper and allowed to germinate for 24 to 30 h at 35°C. (D) Infected leaf with conidia and conidiophores. (E) Plastic mesh, filter paper, germinating seeds and infected leaves required for inoculation. (F) Four to 6 infected leaves are spread on top on the mesh with sporulating lesions on the lower surface of the leaves. (G, H, I) Plates are covered with moistened paper towels, enclosed in makeshift moisture chambers (ziploc bags) and incubated at 20°C for 24 h for high humidity to induce sporulation.


   

Fig. 2. Screening of sorghum accessions for downy mildew resistance. (A) The inoculated seedlings are planted in three pots with 15 to 20 seeds each. (B) Ten days after planting, susceptible accessions will start showing systemic infection. (C) Scoring for systemic infection is made two weeks after planting.


P6 Resistance in Exotic Sorghum Germplasm, Hybrids, and Elite Lines

Sorghum downy mildew can be effectively managed by a combination of methods, which include cultural, chemical, and host resistance. Cultural practices such as crop rotation, deep plowing, early planting, and the use of trap crops have been shown to significantly reduce downy mildew incidence (3,11,15). Metalaxyl or fosetyl-Al applied as a seed treatment fungicide can provide effective chemical control of SDM (10). However, the recent epiphytotics of sorghum downy mildew observed in Texas occurred in seed-treated fields (7) suggesting that seed treatment may be ineffective under field conditions. In addition, expansion of the population on susceptible varieties being grown in the area has led to the emergence of a new pathotype, able to overcome previously resistant cultivars (7). The sudden outbreak of downy mildew in Texas made apparent the need for continual surveillance and proactive measures. New and alternate sources of host plant resistance are needed for successful management of SDM. Exotic germplasm is increasingly being used to broaden the genetic base and breed cultivars for higher yield, enhanced nutritional quality, and resistance to abiotic and biotic stresses. Under field conditions, a few resistance sources against pathotype 6 of downy mildew have been identified from exotic sorghum germplasm collected from Chad and Uganda (13). Hence, this study was also conducted to identify potential downy mildew resistance sources from the minicore germplasm collection from ICRISAT, India.

Sorghum downy mildew is characterized by two types of symptoms, local lesions, which result from conidial infection of the leaf lamina (Fig. 3B & 3C) and systemic infection (Fig. 3D), which results from oospore or conidial infection of the growing point. Systemic infection occurs when young meristematic tissues of the growing seedlings are infected and symptoms appear within 2 to 3 weeks after planting. Infected plants are stunted and often result in death of seedlings (2,3,11,17). Abaxial local lesion symptoms can occur at any stage of plant development (11). Local lesions usually take 7 days to develop and are characterized by discrete necrotic areas where abundant conidia appear as white, downy growth and are produced during humid conditions. These appear as striped, necrotic lesions on leaf blades. The local lesions become systemic when the conidia from these lesions infect meristematic tissues. The white downy growth of conidiophores on these lesions facilitates dissemination of conidia within the foliage of neighboring plants (2,11). As the plant grows, newly emerging leaves exhibit parallel stripes of green and white tissue; the white interveinal tissue dies and leaf shredding occurs. These shredded tissues contain numerous oospores that develop as a result of compatible mating of intercellular thalli (17). Sorghum biomass is a potential source for biofuels; growth in high-density stands in areas of high humidity would likely favor rapid spread of downy mildew.


     
     
 

Fig. 3. Sorghum downy mildew symptoms and signs. (A) Healthy leaf with panicle. (B) Leaf at vegetative stage with sporulating lesions and conidium/conidiophore at 100× magnification. (C) Bottom of a mature leaf with abundant sporulation. (D) Plants with systemic infection and lacking a panicle.

 

Disease incidence in the susceptible check was 85 to 100% throughout the experimental period in all three experiments indicating the effectiveness of the inoculation method. Results could not be obtained in five [IS5999, IS20635, IS20777, IS21425, and IS33844 (GSS194)] out of 245 accessions. Data from Experiment I showed that downy mildew incidence significantly differed among accessions (F = 6.79; P < 0.0001). Approximately 60% of the accessions showed a disease incidence greater than 30% and mean disease incidence for the 242 accessions was 40%. A total of 38 accessions (16%) [IS995, IS1041, IS1219, IS2389, IS2397, IS2872, IS3158, IS3946, IS5972, IS6351, IS7310, IS10302, IS11473, IS13971, IS14290, IS18039, IS19153, IS19445, IS19450, IS20956, IS21512, IS21891, IS24503, IS28313, IS28614, IS29241 (PL69), IS29582 (PHN160), IS29606, IS5667, IS7679, IS11026, IS13294, IS14779, IS23579 (PAB98), IS23684 (RPM84-2), IS24939, IS25910, and IS29689 (AMM96-2)] showed disease incidence of 80 to 100% (LSD = 12.3). A total of 52 accessions (22%) had a disease incidence of < 10% and are assumed to carry resistance to downy mildew pathotype P6 (Table 1). Out of the 52 resistant accessions, 24 were photosensitive and can be further utilized for breeding of high biomass sorghum lines.


Table 1. Sorghum minicore germplasm showing resistance against downy
mildew (Peronosclerospora sorghi pathotype 6).

IS number Origin Avg. %
infection
Disease
 reaction
x
608 United States 23 S
995 United States 93 S
1041* India 93 S
1212 China  0 R
1219 China 80 S
1233 China 40 S
2205 India 26 S
2382 South Africa 60 S
2389 South Africa 90 S
2397 South Africa 86 S
2413 Iran  6 R
2426 Afghanistan 30 S
2864 South Africa 60 S
2872 Egypt 86 S
2902* Nigeria 60 S
3121* Kenya  2 R
3158 South Africa 93 S
3946 India 100 S
3971 India 20 S
4060* India  0 R
4360* India  0 R
4372 India  0 R
4515 India 50 S
4613 India  0 R
4631 India  0 R
4698 India 40 S
5094 India  0 R
5386 India 60 S
5529* India 53 S
5667 India 86 S
5919* India 53 S
5972* India 93 S
5999* India
6351* India 97 S
6354* India 26 S
6421* India 30 S
7305* Nigeria  6 R
7310* Nigeria 80 S
7679* Nigeria 97 S
7987 Nigeria 33 S
8012 Japan 20 S
8348 Pakistan 23 S
8774 South Africa 53 S
8777 Uganda 63 S
8916* Uganda 23 S
9108* Kenya 76 S
9177* Kenya 70 S
9745* Sudan  0 R
9830 Sudan 70 S
10302* Thailand 83 S
10757* Chad 22 S
10867* Chad 30 S
10969* United States 33 S
11026* Ethiopia 86 S
11374* Ethiopia  6 R
11473 Ethiopia 83 S
11619* Ethiopia 20 S
11919* Ethiopia 23 S
12302 Zimbabwe  3 R
12447 Sudan 34 S
12697 Australia 23 S
12706 United States 30 S
12735 Yemen 60 S
12804 Turkey  0 R
12883 India  0 R
12937 Ethiopia 30 S
12945 Nicaragua 40 S
12965 Cuba  6 R
13264 India  6 R
13294* Venezuela 86 S
13549* Mexico  0 R
13782 South Africa 53 S
13893* South Africa 55 S
13919 South Africa 43 S
13971 South Africa 85 S
14010 South Africa 73 S
14090 Argentina 46 S
14290 Botswana 81 S
14779* Cameroon 86 S
14861* Cameroon 63 S
15170* Cameroon  0 R
15466* Cameroon 23 S
15478* Cameroon  0 R
15744* Cameroon 53 S
15931* Cameroon 33 S
15945* Cameroon  0 R
16382* Cameroon 26 S
16528* Cameroon  0 R
17941 (SAR503) India 40 S
18039* India 80 S
19153* Sudan 80 S
19389 Bangladesh 50 S
19445 Botswana 100 S
19450 Botswana 90 S
19859 (KEP424)* India 43 S
19975* Senegal 33 S
20195 Niger 56 S
20298* Niger 24 S
20625* United States  0 R
20632* United States  0 R
20635 United States
20679* United States 60 S
20697 United States 63 S
20713* United States 33 S
20740* United States 30 S
20747 United States 26 S
20762* United States 66 S
20767* United States 67 S
20771* United States 33 S
20777 United States
20816 United States 73 S
20956* India 86 S
21083* Kenya  0 R
21425* Malawi
21512* Malawi 83 S
21863 Syria 66 S
21891 United States 80 S
21897* United States 40 S
22239 (PMC15) Botswana 50 S
22294 (PMK108) Botswana  0 R
22609 (JM4137)* Sri Lanka 24 S
22616 (JM4217A) Myanmar 73 S
22720 (DRM36)* Somalia  0 R
22799 (DRD106) Somalia 67 S
22986* Sudan 23 S
23216 (ZM137)* Zimbabwe  0 R
23514 (PAB26) Ethiopia 30 S
23521 (PAB34) Ethiopia 33 S
23579 (PAB98)* Ethiopia 83 S
23586 (PAB108)* Ethiopia 30 S
23590 (PAB112)* Ethiopia 66 S
23644 (RC41)* Gambia 73 S
23684 (RPM84-2)* Mozambique 80 S
23891* Yemen 26 S
23992 Yemen  0 R
24139* Tanzania 23 S
24175* Tanzania 76 S
24218* Tanzania 22 S
24348 (VRR563) India 33 S
24365 (VRR726) India 40 S
24453 South Africa  0 R
24462 South Africa  0 R
24463 South Africa  0 R
24492 South Africa 23 S
24503 South Africa 86 S
24939* Zambia 85 S
24953* Zambia 60 S
25089 (DSA147)* Ghana 60 S
25249* Ethiopia 41 S
25301* Ethiopia 60 S
25548 (PS55)* Rwanda 46 S
25732* Mali 30 S
25836* Mali 23 S
25910* Mali 83 S
25981* Mali 40 S
25989* Mali 40 S
26025* Mali 56 S
26046* Mali 93 S
26222* Togo  0 R
26484* Benin  7 R
26617 Madagascar  0 R
26694 South Africa 70 S
26701 South Africa 70 S
26737 South Africa 53 S
26749 South Africa  3 R
27034* Sudan 25 S
27557* Burkina Faso  0 R
27697 (PCI36)* Sierra Leone  6 R
27786 Morocco 28 S
27887 (JM4621)* Yemen 33 S
28141* Yemen 40 S
28313* Yemen 80 S
28389 Yemen 43 S
28449 Yemen 42 S
28451 Yemen 23 S
28614 Yemen 86 S
28747* Yemen 56 S
28849* Yemen 73 S
29091 (PHM104)* Yemen 23 S
29100 (PHM113)* Yemen 26 S
29187 (PL14) Swaziland 46 S
29233 (PL61) Swaziland 46 S
29239 (PL67)* Swaziland  0 R
29241 (PL69) Swaziland 83 S
29269 (PL104) Swaziland 46 S
29304 (PL140) Swaziland 43 S
29314 (PL152) Swaziland  0 R
29326 (PL164) Swaziland 50 S
29335 (PL173) Swaziland 30 S
29358 (PHN2) Lesotho  0 R
29392 (PHM36) Lesotho  0 R
29441 (PC86) Lesotho 60 S
29468 (PC113) Lesotho 50 S
29519 (PC221) Lesotho 26 S
29565 (PHN139) Lesotho 26 S
29568 (PHN142) Lesotho 30 S
29582 (PHN160) Lesotho 80 S
29606 South Africa  0 R
29627 South Africa  7 R
29654 China  6 R
29689 (AMM96-2) Zimbabwe 63 S
29714 (AMM156) Zimbabwe 63 S
29733 (AMM206)* Zimbabwe 43 S
29772 (AMM282)* Zimbabwe 46 S
29914 (AMM589)* Zimbabwe 46 S
29950 (AMM673)* Zimbabwe 36 S
30079 (AMM908)* Zimbabwe 36 S
30092 (AMM938) Zimbabwe  0 R
30231 (AMM1314) Zimbabwe 30 S
30383 China  0 R
30400 China 56 S
30417 China 56 S
30443 China  0 R
30460 China 36 S
30466 China 43 R
30507 Korea 23 S
30508 Korea 20 S
30533 Korea 23 S
30536 Korea 53 S
30562 Korea  0 R
30572 (AD12)* Cameroon  0 R
30838 (AD572)* Cameroon 50 S
30986* Uganda 23 S
31043* Uganda 53 S
31172* Uganda 20 S
31186* Uganda 26 S
31299* Uganda 13 S
31485* Uganda 50 S
31557* Burundi  0 R
31651* South Africa 23 S
31681 Algeria 23 S
31706* Yemen 21 S
31714* Yemen  0 R
32245* Yemen 40 S
32295 India 43 S
32349 (AKG70)* India 40 S
32439 (AKG228)* India 53 S
32482 (AKG313)* India 63 S
32787* Somalia 20 S
33023 (AMF469)* Tanzania 33 S
33090* Honduras 30 S
33353* Kenya 56 S
33844 (GSS194) India
84G62 (Check) Pioneer hybrid 93 S
82P75 (Check) Pioneer hybrid  0 R
Mean 39.9
LSD 12.3

 x S = susceptible, R = resistant. In this study, as in Prom et al. (13), accessions with 10% or less downy mildew incidence were considered “resistant.”

 * Photosensitive accessions (122) in the minicore germplasm collection.


Screening for downy mildew resistance among 67 commercial sorghum hybrids and elite lines grown in Kansas and Texas also showed significant differences (F = 9.76; P < 0.0001) in response to the P6 P. sorghi isolate. Infection ranged from 0 to 66% across hybrids with an average of 37%. Twenty out of 67 accessions (30%) recorded the lowest downy mildew incidence of < 10% infection (LSD = 11.4; Table 2). To confirm the stability of this resistance, these hybrids and elites lines should be further monitored under field conditions in different locations in Kansas and Texas where other pathotypes are prevalent. Five accessions, 766B (Dyna-Grow), SC599 (elite line), O-587 and O-525 (Ohlde), and 670 (Phillips), recorded the highest infection rates (> 60%) indicating their high susceptibility to sorghum downy mildew pathotype 6.


Table 2. Sorghum hybrids and elite lines showing resistance to downy
mildew (P. sorghi pathotype 6).

Source Accession Avg. %
infection
Disease
 reaction
x
Asgrow Pulsar 53 S
Channel 6B10  6 R
Channel 7B11 50 S
Dekalb DKS28-05  0 R
Dekalb DKS29-28 45 S
Dekalb DKS36-06 53 S
Dekalb DKS37-07  0 R
Dekalb DKS44-20  0 R
Dekalb DKS53-67 56 S
Dekalb DKS54-00 46 S
Dekalb DKS54-03 36 S
Drussel DSS B64  0 R
Drussel DSS B6506 45 S
Dyna-Gro 751B 58 S
Dyna-Gro 772B 43 S
Dyna-Gro 764B 46 S
Dyna-Gro 766B 66 S
Dyna-Gro 722B 43 S
Dyna-Gro 778B 56 S
Dyna-Gro 742C  0 R
Dyna-Gro 771B  6 R
Midland MG4748  0 R
Midland MG4772 53 S
Midland MG4665  0 R
Midland MG4790 50 S
Midland MG4765  0 R
Ohlde O-575 50 S
Ohlde O-567 53 S
Ohlde O-587 60 S
Ohlde O-525 66 S
Phillips 670 63 S
Phillips 672 37 S
Phillips 775 43 S
Pioneer 84P74 53 S
Pioneer 85Y40 41 S
Pioneer 85G03 53 S
Pioneer 86G32 46 S
Producers PH246W 28 S
Producers PH256 50 S
Producers PH266 56 S
Sorghum Partners NK5418  6 R
Sorghum Partners NK7655 50 S
Sorghum Partners NK6638 57 S
Sorghum Partners SP6680 56 S
Sorghum Partners X449  0 R
Sorghum Partners KS310  6 R
Sorghum Partners SP3303 58 S
Sorghum Partners X444  3 R
Sorghum Partners NK7633 56 S
Sorghum Partners X698 50 S
Triumph TR452  7 R
Triumph TR458  6 R
Triumph TRX85002  7 R
Triumph TRX85001 45 S
Triumph TRX95003 56 S
Triumph TRX95004 50 S
Triumph TR481  7 R
Triumph TR460 40 S
Triumph TRX95005  6 R
Triumph TRX92016 57 S
Triumph TRX42629 53 S
Triumph TRX85131 53 S
Triumph TRX84732 58 S
Triumph TRX83774  0 R
Triumph TR438 56 S
SC599 Elite line 63 S
Pioneer 84G62 (Check) 66 S
Pioneer 82P75 (Check)  0 R
Mean 36.5
LSD 11.4

 x S = susceptible, R = resistant. In this study, as in Prom et al. (13), accessions with 10% or less downy mildew incidence were considered “resistant.”

 * Photosensitive accessions (122) in the minicore germplasm collection.


Out of 24 accessions screened in Experiment III, 13 were found to be resistant to pathotype 6 (LSD = 10.7) (Table 3). Infection ranged from 0 to 90% with an average of 34%. Three elite lines R01302, R03181, and R03182 and two hybrids ATx2928/R05434 and ATx2928/R03182 were very susceptible, with infection rates between 80 and 90%. Based on this observation, these elite lines and hybrids should not be released for use in areas where downy mildew is prevalent.


Table 3. Disease response of sorghum hybrids and elite lines against downy mildew (P. sorghi pathotype 6).

Source Pedigree Avg. %
infection
Disease
 reaction
z
05CS600/601 ATX2928/R05434 90 S
07CS1538/1537 ATX2928/R01302 46 S
07CS4912/4911 ATX2928/R07331  9 R
07CS4914/4913 ATX2928/R07332  8 R
06CS7100/7099 ATX2928/R01207 52 S
04CS6614/6613 ATX2928/R02105  9 R
05CS2340/2339 ATX2928/R03148 40 S
07CS1238/1239 ATX2928/R03149 47 S
05CS2388/2389 ATX2928/R03150  8 R
05CS2246/2245 ATX2928/R03181  4 R
05CS2270/2269 ATX2928/R03182 86 S
05CS2122/2121 ATX2928/R03214  0 R
05CS2486/2485 ATX2928/R03215  4 R
07CS2199 R.05434  0 R
07CS2002 R.01302 80 S
06CS8002 R.01297  4 R
04CS5105 R.02105  6 R
05CS3054 R.03148  8 R
05CS3055 R.03149  8 R
07CS-DMRL40 R.03150 52 S
05CS3075 R.03181 82 S
05CS3076 R.03182 82 S
05CS3087 R.03214 56 S
05CS3088 R.03215  9 R
Pioneer 84G62 (Check) 76 S
Pioneer 82P75 (Check)  6 R
Mean 33.5
LSD 10.7

 x S = susceptible, R = resistant. In this study, as in Prom et al. (13), accessions with 10% or less downy mildew incidence were considered “resistant.”

 * Photosensitive accessions (122) in the minicore germplasm collection.


The recent outbreak of downy mildew caused by a new pathotype in Texas region as described by Isakeit and Jaster (6), points out the need for continuous monitoring and characterization of host responses to new pathotypes of the pathogen. Although sorghum hybrids grown in Kansas are typically under less disease pressure than those grown in the coastal bend area of Texas, this study revealed that some of the elite lines and commercial hybrids grown in Kansas are highly susceptible to the new virulent pathotype. While losses to downy mildew infection are currently minimal in hybrids grown in Kansas, downy mildew outbreaks have occurred in the past and are likely to do so again when natural fluctuations in the environment lead to favorable conditions. Therefore, it is important that new and potentially useful resistance sources be identified among accessions in the sorghum germplasm collection. In this study, resistance sources for the new virulent P6 downy mildew pathotype were identified among exotic minicore germplasm accessions from highly diverse genetic origins (Table 1). The diversity among these lines is expected to provide different single gene sources as well as quantitative sources of downy mildew resistance for use in breeding programs.


Acknowledgments

The authors extend their sincere thanks to The Global Crop Diversity Trust, Italy, for the funding support. This paper is Contribution No. 11-160-J from the Kansas Agricultural Experiment Station, Manhattan.


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