VIEW ARTICLE

Ecology and Epidemiology

Patterns of Rhizoctonia Foliar Blight on Soybean and Effect of Aggregation on Disease Development. X. B. Yang, Former graduate research assistant, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experimental Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803; J. P. Snow, and G. T. Berggren. Professors, respectively, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experimental Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803. Phytopathology 81:287-293. Accepted for publication 30 August 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-287.

Each of six soybean fields with a history of Rhizoctonia foliar blight was divided into 0.75- × 0.75-m quadrats. Diseased leaves in each quadrat were initially counted at soybean growth stage V8-V11. At soybean growth stage R4, disease incidence in each quadrat was assessed. Lloyd’s index of patchiness (LIP) of diseased leaves ranged from 1.56 to 4.54 at the first rating. At the second disease rating, LIP of disease incidence for each plot ranged from 1.32 to 2.15, indicating a decrease in disease aggregation (except for 1987 Lake Arthur). Cluster size was examined using paired-quadrat variance analysis. Different cluster sizes were detected in different fields, indicating an environmental effect on pathogen dispersal. For each field, paired-quadrat variance curves from two rating dates had similar trends. The apparent infection rates of the disease were negatively correlated with the LIP at first rating (P < 0.05), indicating that spatial pattern influenced disease development. Disease incidence late in the season was predicted with a logistic model, using early infection as a predictor (P < 0.01). Incorporation of LIP into the logistic growth model to correct aggregation effect improved the prediction.