Efficacy of Metalaxyl in Controlling Phytophthora Root and Stalk Rot of Soybean Cultivars Differing in Field Tolerance

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Abstract
Chemical control of root and stalk rot of soybean caused by Phytophthora megasperma Drechs. f. sp. glycinea (Hildeb.) Kuan and Erwin was tested under field conditions with 4 soybean cultivars [''OX20-8'', ''Steele'', ''Amsoy 71'' and ''Harcer''] differing in field tolerance to the disease. The systemic fungicide metalaxyl was used as an in-furrow treatment at 0.25, 0.5, and 0.75 kg a.i.[active ingredient]/ha applied with the seed at planting. Metalaxyl did not affect emergence. Metalaxyl treatments reduced plant losses during the season in extremely susceptible and susceptible cultivars but not in moderately susceptible and tolerant soybean cultivars. Rates of metalaxyl up to 0.75 kg a.i./ha did not reduce percentage of plant loss in extremely susceptible cultivar plots of the level equivalent to plant loss in the tolerant cultivar plots. Yield was increased and plant loss decreased in the extremely susceptible OX20-8 by metalaxyl treatments ranging 0.5-1.5 kg a.i./ha.