to 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 preventive fungicide applications in fall prior to winter snow cover. However, fungicide rates necessary to control snow mold are often much higher than those needed to suppress other turf diseases during the summer.  The persistence of chlorothalonil and fludioxonil residues in the turf during the winters 2005-06 through 2007-08 was measured by gas chromatography/mass spectrometry.   In 2005-06, chlorothalonil concentrations decreased by approximately 50% the first week after application and then decreased at a rate of 0.7-1.0 µg per g tissue per day during snow cover.    The rapid decrease in chlorothalonil concentration after the first week was not observed in other years.   Instead, concentrations of chlorothalonil and fludioxonil in the verdure decreased at less than 1.0 µg per g tissue per day 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 Typhula incarnata and T. ishikariensis isolates to chlorothalonil and fludioxonil was tested.   Growth of most isolates (70%) on potato dextrose agar (PDA) amended with 1 µg per ml chlorothalonil was inhibited by more than 50% relative to growth on non-amended PDA.  However, almost all isolates exhibited at least some growth at concentrations as high as 500 µg per ml.   A high proportion (85%) of isolates were inhibited   by >80% to 1 µg per ml fludioxonil.