Optimum timing of thiophanate-methyl application in the control of purple stain on soybean seed with consideration of development of resistance

摘要

Thiophanate-methyl protected soybean seeds (Glycine max (L.) Merr.) from purple stain (Cercospora kikuchii Matsumoto et Tomoyasu). It also had a curative effect and killed the pathogen or suppressed its growth below the necessary level for the development of the purple discolorations on seeds at harvest maturity. From results of the infection rates of seeds during their developmental stages, the residual effect and the mode of action of thiophanate-methyl, only a one-time fungicide application between 30 and 50 days after soybean flowering significantly controlled the seed disease both protectively and curatively. More than six applications of thiophanate-methyl throughout the soybean growing period stimulated the development of strains resistant to the chemical. The disease on seeds infected with the resistant strains was not suppressed by thiophanate-methyl. Therefore the application of thiophanatemethyl needs to be restricted to one properly timed application in a cultivation season to avoid the development of resistant strains.