Residual Efficacy of Fungicides for Controlling Brown Patch on Creeping Bentgrass Fairways

Residual efficacy of five fungicides (azoxystrobin, flutolanil, metconazole, polyoxin D, and pyraclostrobin) applied to creeping bentgrass (Agrostis stolonifera) maintained under golf course fairway conditions was determined using a bioassay method. During 2010 and 2011, six different field experiments were conducted. Each consisted of a single fungicide application followed by periodic (0, 3, 7, 10, 14, 17, and 21 days after application) turf sampling, inoculation of samples with an isolate of Rhizoctonia solani, and incubation in a controlled environment chamber for 48 h. For each sample date, fungicide efficacy was determined by measuring the extent of symptom expansion on fungicide treated and nontreated samples. Efficacy half-life values based on a two-parameter Weibull function were 3.1 to 14.0 days for the fungicides used in this study. Residual efficacy was further examined in 2011 by analyzing residues from creeping bentgrass verdure using liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS). Quantitative analysis from LC/TOF-MS revealed that fungicide residues were depleted rapidly following application to turfgrass and reinforced the precipitous decline in fungicide efficacy demonstrated by the bioassays. Regardless of fungicide, more than 90% of active ingredient applied was depleted from the verdure between 3 and 8 days after application, and more than 99% of fungicide was depleted at 17 days after application. This research provides a quantitative description of the temporal nature of loss of fungicide and fungicide protection from turf.