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Ecology and Epidemiology

Frequency Distribution Analyses of Lettuce Drop Caused by Sclerotinia minor. J. J. Marois, Department of Plant Pathology, University of California, Davis 95616; P. B. Adams, Soilborne Diseases Laboratory, USDA-ARS, Beltsville, MD 20705. Phytopathology 75:957-961. Accepted for publication 30 April 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-957.

The location of lettuce plants with symptoms of lettuce drop, which is caused by Sclerotinia minor, was mapped in six lettuce fields 2- 3 wk before harvest. Disease incidence ranged from 2.00 to 9.16%. Frequency distribution analyses were performed with different sizes and numbers of quadrats. Eight different frequency distributions were analyzed for goodness of fit to the data by the chi square (X²) goodness-of-fit test. Quadrat size and number of quadrats influenced the type of distribution model fit and the goodness of fit of the frequency distributions. In only one instance was the Poisson distribution fit (X² P = 0.88)—when Field 1 was sampled with a quadrat size of 0.9 by 0.9 m. In all other cases, the X² probability for the Poisson distribution was less than 0.01. Of the distributions tested, the negative binomial distribution was fit most often. Lloyd's index of mean patchiness ranged from 1.10 to 3.62, indicating various degrees of aggregation. Lloyd's index of mean patchiness is a better index to compare the degree of aggregation because it does not require a goodness-of-fit test, was not affected by the mean in this study, and was not affected significantly (P≤0.05) by quadrat size. The standard-runs test also indicated that the disease was not random.