3.4.8
RESISTANCE TO PSEUDOCERCOSPORELLA HERPOTRICHOIDES, CAUSE OF EYESPOT IN WHEAT AND ITS RELATIVES

TD MURRAY1, SS JONES2, MM CADLE3, A YILDIRIM4 and A SHI2

1Dept of Plant Pathology and 2Dept of Crop & Soil Sciences, Washington State University, Pullman, WA, USA; 3Dept of Biometry and Plant Breeding, Cornell University, Ithaca, NY, USA; 4Gaziosmanpasa University, Ziraat Faculty TBB, 60110 Tokat, Turkey

Background and objectives
Eyespot, which is caused by the necrotrophic fungus Pseudocercosporella herpotrichoides, is a significant yield-limiting disease of wheat in many temperate regions of the world. Control of eyespot has relied upon foliar fungicides and disease-resistant varieties. The widespread occurrence of benzimidazole-resistant strains of the pathogen has limited the usefulness of these fungicides and increased the importance of disease resistance as a control option. Two genes, Pch1 and Pch2, present in VPM-1 and Cappelle Desprez, respectively, have been used in variety development programs around the world. Pch1 is more effective in limiting disease development than Pch2 and, consequently, has been used more widely. Unfortunately, eyespot can be severe enough in varieties containing Pch1 to result in significant yield losses in some years. Previous studies indicated that several wild relatives of wheat possess genes for resistance to eyespot. The purpose of the studies described here was to identify potentially more effective new genes conferring resistance to eyespot for use alone or for pyramiding with other eyespot resistance genes.

Materials and methods
Seedlings of the wheat relatives Dasypyrum villosum, Triticum tauschii and T. monococcum were grown in a growth chamber at 13C with a 12 h light period. Resistance to P. herpotrichoides was determined by inoculating 2-week-old seedlings with a transformed strain of the pathogen that constitutively expresses -glucuronidase (GUS) and measuring disease development 6 weeks after inoculation by performing a GUS enzyme assay [1]. Resistance was inferred by comparison of GUS scores of individual accessions with genotypes of known resistance. Since resistance is associated with reduced colonization by the pathogen, resistant genotypes have lower GUS scores than susceptible lines that do not restrict pathogen growth. Genetic control of resistance was determined by hybridizing resistant with susceptible accessions of a species and determining segregation patterns in the F2.

Results and conclusions
All 219 accessions of D. villosum tested were resistant to eyespot. Analysis of progeny from a cross between a resistant addition line and a susceptible substitution line resulted in a 3:1 F2 segregation ratio for resistance and susceptibility, which indicated that a single dominant gene controlled resistance. Out of 279 accessions of T. tauschii tested, 115 (45%) were resistant. Two different resistant accessions of T. tauschii were hybridized with the same susceptible accession and in both crosses the F2 segregation ratio for resistance and susceptibility was 3:1, indicating the presence of a single dominant gene controlling resistance. In a core collection consisting of 118 T. monococcum accessions, 52 (44%) were significantly more resistant than Cappelle Desprez. Two different resistant accessions of T. monococcum were hybridized with the same susceptible accession and each other. Segregation of the F2 for resistance was continuous in all three populations and discrete groupings of resistant and susceptible individuals were not apparent. These data indicate that the parents in these crosses either were not sufficiently different in their disease reaction to differentiate resistant from susceptible progeny using these techniques, that eyespot resistance in T. monococcum is controlled by multiple genes or, that the gene conferring eyespot resistance in T. monococcum is incompletely dominant. Additional crosses were made with different accessions of T. monococcum and analysis is in progress to determine the genetic control of eyespot resistance in this species. Introgression of these new resistance genes into bread wheat is in progress and, when completed, will broaden the genetic base of eyespot resistance genes available to breeding programs around the world.

References
1. de la Pea, RC, Murray, TD, 1994. Phytopathology 84, 972-979.