Distribution of Wheat Powdery Mildew Incidence in Field Plots and Relationship to Disease Severity

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Abstract
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 cv. 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 log related to disease incidence on individual leaves to total number of leaves examined, did not reach 100% until the end of the season when disease severities were 30-40%. Relationships between disease incidence and disease severity were determined for leaves at 4 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 5th or 6th wk 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.