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The Residual Effects of Some “Defeated” Powdery Mildew Resistance Genes in Isolines of Winter Wheat. H. A. Nass, Graduate student, Department of Plant Pathology, the Pennsylvania State University, University Park 16802, Current address of senior author: Fitopatología CIARCO, Apto. Postal 102, Acarigua, Portuguesa, Venezuela; W. L. Pedersen(2), D. R. MacKenzie(3), and R. R. Nelson(4). (2)(3)(4)Senior project associate, associate professor, and Evan Pugh professor, respectively, Department of Plant Pathology, the Pennsylvania State University, University Park 16802. Phytopathology 71:1315-1318. Accepted for publication 26 March 1981. Copyright The American Phytopathological Society. DOI: 10.1094/Phyto-71-1315.

Six near-isogenic winter wheat lines (isolines), each possessing a different powdery mildew resistance gene, were evaluated for their potential residual effects against an isolate of Erysiphe graminis f. sp. tritici possessing all the virulence genes needed to overcome the six resistance genes. The infection efficiency, disease efficiency, and sporulation capacity of the isolate on each of the isolines were assessed relative to the susceptible recurrent parent, Chancellor winter wheat. The isolines with resistance genes Pm3c, Pm4, and a gene known as Michigan Amber (MA) demonstrated significant residual effects (relative to Chancellor). Mean number of sporulating colonies was dramatically less on the Pm3c, Pm4, and MA isolines than on Chancellor (32, 40, and 65% fewer spores, respectively). No statistically significant residual effects were obtained for resistance genes Pm2, Pm2+, and Pm5. Prevailing resistance theory assumes that “defeated” single, major resistance genes are of no value when confronted with a pathogen genotype possessing the matching virulence genes. The present study demonstrates that some defeated major resistance genes have measurable residual ability to restrict disease increase and disease severity.

Additional keywords: disease resistance.