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Distribution of Wheat Powdery Mildew Incidence in Field Plots and Relationship to Disease Severity. D. I. Rouse, Former Graduate Research Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802, Present address of senior author: Department of Plant Pathology, University of Wisconsin, Madison 53706; D. R. MacKenzie(2), R. R. Nelson(3), and V. J. Elliott(4). (2)(3)(4)Associate Professor, Evan Pugh Professor, and Research Assistant, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 71:1015-1020. Accepted for publication 24 March 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1015.

Disease severity and incidence of powdery mildew of wheat were determined each week on individual leaves in quadrats consisting of 10 tillers at 10 randomly selected locations in each of 30 field plots of the winter wheat cultivar Chancellor for 8 wk beginning 28 April 1978. A linear relationship was observed between disease severity on tillers and disease severity on individual leaves throughout the epidemic. Disease severity was logarithmically related to disease incidence on individual leaves and tillers. Disease incidence, the proportion of diseased leaves to total number of leaves examined, did not reach 100% until the end of the season, when disease severities were between 30 and 40%. Relationships between disease incidence and disease severity were determined for leaves at four positions in the plant canopy. For the first 4 wk of the epidemic, the spatial frequency distribution of disease incidence on leaves was best described by the negative binomial distribution, indicating that diseased leaves tended to be aggregated. The degree of aggregation decreased as the season progressed, and by the fifth or sixth weeks of the epidemic, disease incidence on leaves was randomly distributed. The negative binomial coefficient k or the mean to variance ratio provided a quantitative measure of degree of aggregation in foci. The results of this study are used as the basis for discussing sequential sampling procedures that might improve precision of estimation of disease severity and yield loss for wheat.

Additional keywords: epidemiology, Erysiphe graminis f. sp. tritici, sampling, Triticum aestivum.