VIEW ARTICLE

Ecology and Epidemiology

Effects of Simulated Acidic Rain on Wash-off of Fungicides and Control of Late Blight on Potato Leaves. Ariena H. C. van Bruggen, Postdoctoral associate, Boyce Thompson Institute for Plant Research, Ithaca, NY 14853-1801; J. F. Osmeloski(2), and J. S. Jacobson(3). (2)(3)Research technician, and plant physiologist, respectively, Boyce Thompson Institute for Plant Research, Ithaca, NY 14853-1801. Phytopathology 76:800-804. Accepted for publication 11 February 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-800.

The effects of simulated acidic rain at two pH levels (2.8 and 4.6) on removal of five fungicides in each of two formulations (wettable powders and flowables) on three potato cultivars (Norchip, Monona, and Katahdin) were investigated in a series of factorial experiments. Residual fungicide on leaf disks was determined by atomic absorption spectroscopy for maneb, mancozeb, and copper hydroxide, by gas chromatography with electron capture detection for chlorothalonil, and by yeast bioassay for triphenyltin hydroxide (TPTH). Wash-off of TPTH and copper hydroxide from potato leaves was significantly increased by simulated rain at pH 2.8 compared with rain at pH 4.6 regardless of fungicide formulation and potato cultivar. Removal of maneb, mancozeb, and chlorothalonil was not affected by acidity of simulated rain. Regardless of acidity of simulated rain, wettable powders were removed more effectively than flowables. Despite 13-83% removal of fungicides from foliage by simulated acidic rain, late blight control was still significant compared with plants not treated with fungicide. Phytophthora infestans infection of potato leaves sprayed with TPTH was higher after an acid rain treatment at pH 2.8 than at pH 4.6, but the increased infection was ascribed to enhanced susceptibility of foliage to late blight caused by rain at pH 2.8 rather than to increased removal of the fungicide per se.