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Pathotype Diversity of Pyrenophora teres f. teres on Barley. Brian J. Steffenson, Former graduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Present address: Department of Plant Pathology, North Dakota State University, P.O. Box 5012, Fargo 58105; R. K. Webster, professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 82:170-177. Accepted for publication 5 July 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-170.

From 91 isolates collected in California, 13 pathotypes of Pyrenophora teres f. teres, the causal organism of net blotch of barley, were identified on 22 differential barley genotypes. Pathotype 3-10-15-19-21 (numbers correspond to the differential barley hosts that exhibit a high infection response [HIR] to this pathotype) was most prevalent, comprising 28.6% of the isolates. The next most common pathotypes included 15, 15-20, and 3-10-15-19-20-21 with 19.8, 15.4, and 8.8% of the isolates, respectively. Pathotype 3-10-15-19-20-21 was virulent on the greatest number of host genotypes (six), whereas pathotype 0 was not virulent on any of the hosts. Prato, Kombar, and Atlas were the individual host differentials susceptible (exhibited HIRs) to the greatest percentages of isolates: 82.4, 56.0, and 50.5%, respectively; however, 10 of the 22 differential genotypes were resistant (exhibited low infection responses) to all of the pathotypes identified in California. Pathotypes from Minnesota, Mexico, and England were distinctly different from each other. Mexican pathotypes were similar to the prevalent California pathotypes in virulence on specific differential genotypes. Pathotypes from Minnesota (1-6-13-16-18) and England (22) differed from two rare California pathotypes (6-13-16-18 and 11-22) for virulence on one differential genotype. The structure and diversity of isolates from two populations were compared. One population consisted of 25 isolates sampled randomly at a field station (Armstrong population) and the other of 59 isolates collected from random fields around the state (off-station population). The most complex pathotypes, 3-10-15-19-21 and 3-10-15-19-20-21, were present at a higher frequency in the Armstrong population (72.0%) than in the off-station population (23.7%); however, the latter population was genetically more diverse (Shannon index = 1.96) than the former (1.30). The set of barley differentials used in this study was effective for typing the virulence phenotypes of a wide collection of isolates of P. t. f. teres and should be useful to other workers investigating this host/parasite system.