Disease progress of late leafspot of peanut caused by Cercosporidium personatum was monitored on the variety ‘Florunner’ in unsprayed plots and in plots sprayed weekly with chlorothalonil (343 μg active ingredient/ml H₂O). The canopy was divided into three vertical semicircular leaf layers, each 15 cm high. Proportions of visible disease (x_v) and defoliation (d) per leaf layer were estimated with a modified Horsfall-Barratt rating system. Total disease (x_t) in each leaf layer and per plant were calculated with the equation

x_t = [(1-d)^* x_v + d].

Apparent infection rates (sensu Vanderplank) were calculated for x_v, d, and x_t in each leaf layer and for the total plant canopy. Rates of disease increase and defoliation on sprayed and unsprayed tissue were not significantly different (P = 0.05). Disease severity in plots sprayed with chlorothalonil usually lagged three to five days behind disease in unsprayed plots. The disease components, x_v, d, and x_t were greatest in the bottom canopy layer. The rate of defoliation in the top leaf layer of unsprayed plots was significantly slower (P = 0.05) than that in the other two leaf layers, possibly because of dilution by the new plant growth. Chlorothalonil sprays reduced initial disease but did not reduce the rate of increase of C. personatum.