Simulated Rainfall and Mowing Impact Fungicides Targeting Dollar Spot.

The effect of an abrupt rain event soon after fungicide application on its effectiveness is a common concern. Furthermore, little is known regarding the effects of mowing to remove leaf surface exudates prior to a fungicide application. The objective of this field study was to determine the level of dollar spot (Sclerotinia homoeocarpa F.T. Bennett) control in fairway height creeping bentgrass (Agrostis stolonifera L.) provided by four fungicides applied about 30 minutes prior to simulated rain. A second objective was to determine if mowing to displace dew prior to fungicide application would impact fungicide performance. The fungicides evaluated were boscalid (0.28 kg a.i. ha^-1); chlorothalonil (9.0 kg a.i. ha^-1); iprodione (0.25 kg a.i. ha^-1); and propiconazole (0.5 kg a.i. ha^-1). Prior to fungicide application, half of each main plot was mowed in the morning, when dew was on the canopy (AM). The following day, after the turfgrass canopy was dry, the other half of each main plot was mowed (PM). Plots in the AM and PM treatments were mowed thereafter when the canopy was either wet or dry, respectively. About 30 minutes after fungicide application, four blocks received 2.5 to 3.8 cm of simulated rainfall (rain). Four blocks were not subjected to simulated rain (rain-free). The study was conducted in 2007 and 2008, but only 2007 data are discussed. Plots subjected to simulated rain had more S. homoeocarpa infection centers than rain-free plots. Iprodione and chlorothalonil were the least and most affected by simulated rain, respectively.  Percent difference of disease reduction in rain vs. rain-free plots were as follows: iprodione = 30%; propiconazole = 44%; boscalid = 47%; and chlorothalonil = 67%. Percent difference of disease reduction in AM vs. PM mown plots were as follows: iprodione = 48%; propiconazole = 51%; boscalid = 64%; and chlorothalonil = 64%.