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Attenuation of Metalaxyl on Potato Leaves by Simulated Acidic Rain and Residence Time. Ariena H. C. van Bruggen, Postdoctoral associate, Boyce Thompson Institute for Plant Research at Cornell University, Ithaca, NY 14853; M. G. Milgroom(2), J. F. Osmeloski(3), W. E. Fry(4), and J. S. Jacobson(5). (2)(4)Graduate research assistant and Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; (3)(5)Research technician, and Plant physiologist, Boyce Thompson Institute for Plant Research at Cornell University, Ithaca, NY 14853. Phytopathology 77:401-406. Accepted for publication 23 April 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-401.

Greenhouse-grown rooted potato cuttings (cultivars Norchip and Monona) were sprayed with 200 ppm metalaxyl 78, 30, 6, or 0 hr before exposure to simulated rain at pH 2.8, 3.4, 4.0, or 4.6 at an intensity of 8.8 mm/hr for 0, 10, 20, or 30 min. The experiment was repeated with shorter time intervals between applications of the fungicide and simulated rain (0, 3, 6, 30, and 54 hr) and shorter rainfall durations (0, 3, 6, 10, and 20 min). Metalaxyl content of foliar tissue was determined by a bioassay with Phytophthora boehmeriae after chemical extraction. At the end of each experiment, plants treated with metalaxyl and simulated rain were inoculated with P. infestans to determine residual activity of the fungicide in situ. The concentration of metalaxyl declined exponentially with time after application and with rainfall duration, until asymptotes were reached about 30 hr after spraying or after 10 min of simulated rain (1.47 mm of rain). Fungicide wash-off was not affected by the acidity of rain or cultivar. Attenuation of metalaxyl over time and by rain was predicted accurately by an exponential decay model with an asymptote. Despite a rapid initial decrease in metalaxyl concentration, residual concentrations (510% of the original deposit) were still sufficient to control late blight.

Additional keywords: acid rain, retention, Solanum tuberosum.