Timing and Root Absorption Affecting Efficiency of Metalaxyl in ControllingPhytophthora infestanson Potato in Northwestern Washington State

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Abstract
Late blight of potato (cultivar White Rose) was controlled in plots where metalaxyl applied 18 h 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 spinkler irrigation and absorbed by the roots.