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Effect of Inbreeding on Pathogenicity in Race 8 of Ustilago hordei. W. L. Pedersen, Senior project associate, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; R. L. Kiesling, professor, Department of Plant Pathology, North Dakota State University, Fargo 58105. Phytopathology 69:1207-1212. Accepted for publication 17 May 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1207.

Race 8 of Ustilago hordei, is pathogenic on barley cultivar Odessa, but not on the other seven covered smut differential cultivars. Selfing of an ordered tetrad (isolate K, race 8 of U. hordei) produced a dikaryon (K-1, 1 × 4) pathogenic on the barley cultivars Odessa and Hannchen. The four sporidial lines also were testcrossed with virulent lines and these matings were pathogenic only on Odessa. The sporidial matings from a selfed telio-spore of isolate K-1, 1 × 4 from Hannchen all were pathogenic on cultivars Odessa, Hannchen, Lion, and Trebi. The second generation selfs of isolates K-1, 1 × 4 also were pathogenic on the same four cultivars. By analysis of F1, F2, and testcross data, three single recessive gene pairs were identified in isolate K-2-8 that condition pathogenicity on cultivars Hannchen, Lion, and Trebi. The genes for Lion and Trebi were linked with 13.1% recombination between the loci. A hypothesis involving a regulator gene is proposed for the increase in pathogenicity associated with the inbreeding of race 8 of U. hordei.