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Components of Rate-Reducing Resistance in Seedlings of Four Wheat Cultivars and Parasitic Fitness in Six Isolates of Erysiphe graminis f. sp. tritici. D. I. Rouse, Graduate student, Department of Plant Pathology, The Pennsylvania State University, University Park 16802, Current address of senior author: Department of Plant Pathology, University of Wisconsin, Madison 53706; R. R. Nelson(2), D. R. MacKenzie(3), and C. R. Armitage(4). (2)(3)(4)Evan Pugh professor, associate professor, and laboratory technician, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 70:1097-1100. Accepted for publication 19 May 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-1097.

Sporulation capacity and infection efficiency of six isolates of Erysiphe graminis f. sp. tritici collected in central Pennsylvania were measured on winter wheat cultivars Blueboy, Redcoat, and Knox. Only sporulation capacity was measured on a fourth cultivar, Vermillion. Sporulation capacity was measured as the cumulative number of conidia sampled from individual colonies on primary leaves at 24-hr intervals and was estimated by the parameter Xm in the logistic equation by using a nonlinear regression routine. Differential interactions were found between isolates and cultivars where the ranking of isolates changed by cultivar. Infection efficiency was determined by counting the number of conidia out of groups of ten that had formed elongating secondary hyphae 3648 hr after inoculation. Two-way analysis of variance identified significant isolate by cultivar interactions for infection efficiency. The change in ranking and the significant statistical interaction in the analysis of variance of isolates by cultivars with respect to sporulation capacity and infection efficiency demonstrates genetic variability for these traits in wheat and E. graminis and indicates the possibility for erosion of slow mildewing resistance.

Additional keywords: sporulation capacity, infection efficiency, powdery mildew of wheat, Triticum aestivum, horizontal resistance.