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Relationship Between Sclerotial Spatial Pattern and Density of Sclerotinia minor and the Incidence of Lettuce Drop. H. R. Dillard, University of California, Davis 95616, Present address of senior author: New York State Agricultural Experiment Station, Department of Plant Pathology, P.O. Box 462, Geneva 14456; R. G. Grogan, University of California, Davis 95616. Phytopathology 75:90-94. Accepted for publication 18 July 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-90.

Soil samples taken from 15 naturally infested field plots near Salinas, CA, indicated inoculum densities at lettuce planting time ranging from 1.66 to 11.35 sclerotia of Sclerotinia minor per 100 cm3 of soil. The spatial pattern of sclerotia within plots was best described by the negative binomial distribution. In all but four plots, variance-to-mean ratios were significantly greater than unity, indicating a clustering of inoculum. Infected plants were mapped weekly in five of the 15 plots, each with about 200 lettuce plants. At harvest, there was a random distribution of healthy and infected plants and no significant plant-to-plant spread had occurred. Disease progress curves constructed for all 15 plots showed no disease for 30 days after planting and a rapid increase during the last 10 days prior to harvest. Disease incidence at harvest was significantly correlated (r = 0.90) with the mean number of sclerotia per 100 cm3 of soil at planting and with the percentage of soil samples with seven or more sclerotia at planting (r = 0.94).