Timing and Root Absorption Affecting Efficiency of Metalaxyl in Controlling Phytophthora infestans on Potato in Northwestern Washington State. Gene D. Easton, Plant Pathologist, Department of Plant Pathology, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser 99350. Michael E. Nagle, Agricultural Technologist III, Department of Plant Pathology, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser 99350. Plant Dis. 69:499-500. Accepted for publication 2 January 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-499.

Late blight of potato (cultivar White Rose) was controlled in plots where metalaxyl applied 18 hr previously to foliage was washed into the soil by rain and sprinkler irrigation. Foliage application gave less effective control in plots where a plastic tarp on the soil surface prevented entry of metalaxyl into the soil, where it could be absorbed by roots. Control of late blight was better and yields were significantly higher in plots sprayed before late blight appeared than in plots sprayed after late blight developed on leaves. Foliage sprays prevented most tuber rot from late blight at harvest. For maximum control of P. infestans, metalaxyl should be applied to foliage before symptoms appear. The fungicide is then washed off the leaves by rain or sprinkler irrigation and absorbed by the roots.