Lesion distribution, multiple infection, and the logistic increase of plant disease.

The lesions of few of the 112 observed cases of plant disease were randomly distributed; most were fitted by the negative binomial function with exponent approximately 1. The proportion of healthy hosts generally equaled the frequency of zero in the negative binomial. The same course of an epidemic is calculated for the logistic increase of infected plants or for plants infected by propagules produced in proportion to lesions and distributed with exponent equal 1.