Frequency Distribution Analyses of Lettuce Drop Caused bySclerotinia minor

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Abstract
The location of lettuce plants with symptoms of lettuce drop, which is caused by S. minor, was mapped in 6 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 (.chi.2) 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 (.chi.2 P = 0.88)-when Field 1 was sampled with a quadrat size of 0.9 by 0.9 m. In all other cases, the .chi.2 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 .ltoreq. 0.05) by quadrat size. The standard-runs test also indicated that the disease was not random.