VIEW ARTICLE

Ecology and Epidemiology

Variation in Virulence Within the Population of Pyrenophora tritici-repentis in New York. A. M. C. Schilder, Graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; G. C. Bergstrom, associate professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853 Phytopathology 80:84-90. Accepted for publication 31 July 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-84.

Seventeen isolates of Pyrenophora tritici-repentis, originating from diverse locations in New York, Maryland, and Ontario (Canada), differed in their virulence on 12 wheat cultivars, as measured by the percentage of total seedling leaf area that was necrotic. Highly significant cultivar, isolate, and cultivar × isolate effects were observed, indicating a differential host-pathogen interaction. Similarity of isolates of P. tritici-repentis was determined by cluster analysis based on their patterns of virulence on the 12 wheat cultivars. Isolates did not differ widely, indicating only a moderate degree of physiologic specialization in the populations of P. tritici-repentis under study. Similarity of wheat cultivars was also determined based on their patterns of resistance to 17 isolates of P. tritici-repentis. Winter wheat cultivars currently grown in New York possess moderate to high resistance, whereas spring wheat cultivars are susceptible to P. tritici-repentis. For wheat breeding purposes, screening wheat genotypes with a few virulent isolates may suffice in selection for a moderate level of resistance to P. tritici-repentis. Cluster analysis may be a useful tool to discern similarities among pathogen isolates and among host genotypes in quantitative host-pathogen systems, but experimental variation may influence cluster composition.