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Variability in Isolates of Cercospora zeae-maydis. Wendy Bair, Graduate student, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; J. E. Ayers, professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 76:129-132. Accepted for publication 19 July 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-129.

The increased incidence and severity of gray leaf spot during the last several years prompted an evaluation of the variability present among 15 isolates of Cercospora zeae-maydis from several geographical regions of the eastern United States. Four susceptible corn hybrids were inoculated at the seven- to eight-leaf stage and placed in a greenhouse mist chamber. Two components of parasitic fitness, disease efficiency and lesion length were recorded 25 days after inoculation. Disease efficiency was measured as the percentage of lesions produced from a given amount of inoculum and lesion lengths were determined in millimeters. Significant differences in disease efficiency and lesion length occurred among isolates, while differences among hosts and host-by-isolate interactions were not significant. Lesion length was the only variable measured in the field. An analysis of variance over locations indicated a significant location-by-isolate and location-by-hybrid interaction. Orthogonal comparisons suggested the interactions were due primarily to three isolates and to one hybrid. Analysis of variance for each location demonstrated significant hybrid and isolate effects at both locations. The results of this study demonstrate the variability present in a collection of isolates of C. zeae-maydis and suggests that breeders should use more than one isolate when developing resistance to the pathogen.

Additional keywords: corn, gray leaf spot, maize, Zea mays.