Degradation of the Fungicides Iprodione and Chlorothalonil On Golf Course Turfgrass Under Winter Conditions.

Microdochium nivale is an important winter pathogen of turfgrass in temperate climates that typically requires one or two fungicide applications prior to snowfall for season-long control. Fungicide degradation rates in the winter can vary based upon intensity of sunlight reaching the turfgrass canopy, soil temperatures, and microbial activity. Determining the approximate number of days after application at which fungicide protection falls below acceptable levels both under snow cover and in the absence of snow cover is an important tool for those who manage turfgrass in harsh winter climates. To determine the length of control of iprodione and chlorothalonil under winter conditions,.creeping bentgrass maintained at a height of 1.2 cm was sprayed with iprodione, chlorothalonil, or a tank mixture of both. Each treatment was replicated four times under snow cover and in the absence of snow cover. One hour following the fungicide application on December 6th, one 10 cm diameter core was taken from each plot and taken to a growth chamber set at 12°C and an 8 hr photoperiod and inoculated with M. nivale. Inoculum was prepared by subculturing a M. nivale isolate onto full strength PDA and incubating at 18°C under full light for 4 days. Four 10 cm Petri dishes were macerated along with 150 ml deionized water, and 2 ml of M. nivale solution was pipetted onto the center of each core. Subsequent cores were sampled weekly or biweekly for the next four months and inoculated in the same fashion. Disease severity was visually rated weekly for 6 weeks following each sampling date. Under these conditions iprodione and chlorothalonil failed to provide adequate protection after 35 days, which correlates to an approximate rate of degradation. These data will be correlated with fungicide concentration analyses to determine at what fungicide concentration protection from M. nivale begins to fail.