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Partial Resistance of Near-Isogenic Wheat Lines Compatible with Erysiphe graminis f. sp. tritici. M. H. Royer, Graduate student, Department of Plant Pathology, Pennsylvania State University, University Park 16802, Current address: U.S. Department of Agriculture, Agricultural Research Service, Plant Disease Research Laboratory, Bldg. 1301, Ft. Detrick, Frederick, MD 21701; R. R. Nelson(2), D. R. MacKenzie(3), and D. A. Diehle(4). (2)(3)(4)Evan Pugh professor, professor, and laboratory technician, respectively, Department of Plant Pathology, Pennsylvania State University, University Park 16802. Phytopathology 74:1001-1006. Accepted for publication 10 February 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1001.

Six near-isogenic lines of Chancellor winter wheat, differing in known powdery mildew resistance genes (Pmx), were evaluated relative to Chancellor for partial resistance to compatible isolates of Erysiphe graminis f. sp. tritici collected in central Pennsylvania. The variability due to the host always exceeded that due to the isolates for the parasitic fitness attributes that were investigated: latent period (LP), cumulative sporulation (CSP) per colony, and corrected infection efficiency (CIE). Fitness genes other than virulence genes, or multiple alleles of the virulence genes, caused significant differences in certain fitness attributes for different isolates of the same race when compared on the same host. In certain instances, lines carrying a particular Pmx gene were more susceptible than Chancellor. Significant isolate host interactions and significant differences in isolate ranking were demonstrated for several near-isogenic lines, indicating the potential for erosion of partial resistance over time. Pooled data over all isolates indicated partial resistance for several near-isogenic lines carrying known Pmx genes relative to Chancellor: C114122 with Pm3c (LP); CI14123 with Pm4 (CSP); and CI14122 with Pm3c, CI14123 with Pm4, and CI14033 with Pmx gene from cultivar Michigan Amber (CIE).

Additional keywords: aggressiveness, epidemiology, horizontal resistance, stabilizing selection, Triticum aestivum, virulence.