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A Mathematical Model of the Spatial and Temporal Dynamics of Chlorothalonil Residues on Potato Foliage. J. A. Bruhn, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address of senior author: E. I. du Pont de Nemours & Co., Biochemicals Department, Experimental Station, Wilmington, DE 19898; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:1306-1312. Accepted for publication 12 February 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1306.

The spatial and temporal dynamics of the protectant fungicide chlorothalonil on potato foliage in the greenhouse and field were described with a mathematical model. In this study, the average half-life of chlorothalonil residues for the entire potato plant canopy was 6.60 days, but half-life values ranged from 1.23 to 7.50 days depending upon weather conditions. Rainfall on the day of fungicide application had the greatest impact on the removal of chlorothalonil from potato foliage, but the effect of rain declined with aging of the fungicide deposit. The logarithmic rate at which chlorothalonil was lost from potato foliage increased linearly with increasing average daily temperature from 15 to 27 C. Chlorothalonil was lost most rapidly from foliage in the top of the plant (average half-life, 3.6 days) and least rapidly from potato leaves nearest the ground (average half-life, 21.3 days). The slower decline of fungicide from the bottom of the canopy was attributed to increased protection there and to the redistribution of fungicide from top leaves onto lower foliage. Chlorothalonil was redistributed from top to bottom foliage more effectively by light than heavy rains. No redistribution was observed in the absence of rainfall. The magnitude of initial fungicide deposit had no effect on the rate of fungicide removal, but rates of fungicide loss differed with potato cultivar. The variability of fungicide levels within a potato canopy was generally described by a gamma probability distribution under all conditions encountered in this study. A model was developed from these data to describe the day-to-day change in both average residue levels and the variability in residue levels of chlorothalonil on potato foliage located within each of four canopy strata. In the model, a gamma probability distribution accounts for the variability in fungicide residue levels, and mean residue levels decline exponentially over time as a function of rainfall, time since application, temperature, and position within the canopy. The change in average residue levels and in the associated variability observed over time in field plots of potatoes were accurately described by the model.