Typhula blight development in Poa annua and Poa pratensis as influenced by persistence of the fungicides chlorothalonil and fludioxonil under snow cover.
Typhula blight is a major problem on golf courses where snow cover persists for long periods. The disease is primarily managed by applying a mixture of fungicides in the autumn prior to winter snow cover. The fungicides chlorothalonil and fludioxonil are often included in these mixtures even though they do not completely suppress Typhula blight when applied alone. We studied whether minimal control was due to a lack of persistence of these fungicides under snow, or whether certain isolates of Typhula ishikariensis and T. incarnata exhibited tolerance to these fungicides. The persistence of chlorothalonil and fludioxonil residues in the turf was determined during the winters of 2005-06, 2006-07 and 2007-08. Concentrations of chlorothalonil and fludioxonil in the verdure decreased or remained nearly the same at most sampling dates, indicating these fungicides did not dissipate rapidly under snow. Despite this, only marginal control of Typhula blight was observed in the fungicide-treated plots. The in vitro sensitivity of T. ishikariensis and T. incarnata isolates to chlorothalonil and fludioxonil was tested. Growth of most isolates (70%) on agar amended with 1 µg mL-1 chlorothalonil was inhibited by more than 50% relative to growth on non-amended agar. However, almost all isolates exhibited some growth at concentrations as high as 500 µg mL-1. A high proportion of isolates (85%) were inhibited by greater than 80% at 1 µg mL-1 fludioxonil. Therefore, at least some growth of these isolates at high fungicide concentrations may explain why chlorothalonil and fludioxonil are not completely effective in suppressing Typhula blight.