Relationship Between Sclerotial Spatial Pattern and Density ofSclerotinia minorand the Incidence of Lettuce Drop

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Abstract
Soil samples taken from 15 naturally infested field plots near Salinas [California, USA] indicated inoculum densities at lettuce planting time ranging from 1.66-11.35 sclerotia of S. minor/100 cm3 of soil. The spatial pattern of sclerotia within plots was best described by the negative binomial distribution. In all but 4 plots, variance-to-mean ratios were significantly greater than unity, indicating a clustering of inoculum. Infected plants were mapped weekly in 5 of the 15 plots, each with .apprx. 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/100 cm3 of soil at planting and with the percentage of soil samples with .gtoreq. 7 sclerotia at planting (r = 0.94).