Application of thiophanate-methyl at different host growth stages for management of sclerotinia stem rot in soybean

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is an important disease of soybean (Glycine max L.) in the North Central United States. The incidence of SSR can be reduced by planting partially resistant cultivars and by implementation of cultural practices that limit pathogen activity. Fungicides such as thiophanate-methyl are another option for control of SSR, but usually recommended in situations where susceptible cultivars must be grown or modification of cultural practices are not disease control options. Previous studies have shown that control of SSR with fungicides is possible, but that the degree of control is inconsistent especially when incidence of SSR surpasses 50%. This study was designed to evaluate the efficacy of single or multiple applications of thiophanate-methyl in relation to timing of inoculation and early reproductive growth stages of the host. Inoculum was introduced at growth stage R2 at the Illinois location, and naturally occurring inoculum was relied upon to initiate SSR at the Wisconsin location. Thiophanate-methyl was effective when applied at R1 prior to inoculation at R2, but not if applied at R3, after inoculation. Two applications of thiophanate-methyl starting at R1 or the R2 growth stage and a second 2 weeks later lowered the incidence of SSR at the naturally infested Wisconsin site compared to one application with one exception. The incidence of SSR was identical for one and two applications starting at R1.5. Soybean yield was improved by fungicides at both experimental locations, and coincided with timing, and number of applications. Data from both locations suggested that fungicides need to be applied prior to inoculum arrival to the infection court. One application was effective in the controlled inoculation study, but two applications were needed at the naturally infested location. Two applications of thiophanate-methyl improved yield of a partially resistant soybean cultivar even though the incidence of SSR was <10% at the Wisconsin location. Thiophanate-methyl is an effective option for control of SSR, especially to reduce seedborne inoculum in commercial seed production systems.