Mowing Frequency and Plant Growth Regulator Effects on Dollar Spot Severity and on Duration of Dollar Spot Control by Fungicides.

Management of dollar spot (incited by Sclerotinia homoeocarpa) on golf course fairways is increasingly challenging. The objectives of this study were to determine the influence of mowing frequency and plant growth regulators (PGRs) on dollar spot severity and on the residual efficacy of fungicides for control of dollar spot. Two 4-month-long studies were conducted on 'Putter' creeping bentgrass (Agrostis stolonifera) maintained as a fairway at the University of Connecticut. Treatments were arranged in a three-by-three-by-five factorial that assessed the influence of mowing frequency (2, 4, or 6 days week^-1) and PGRs (paclobutrazol, trinexapac-ethyl, or none) on dollar spot control by five fungicide treatments (boscalid, chlorothalonil, iprodione, propiconazole, or none). Turf was mowed in the afternoon hours to minimize the confounding effect of mowing frequency on leaf wetness duration. Treatments were initiated in the late spring of 2007 and 2008, and each fungicide treatment was reapplied only when dollar spot exceeded a threshold of five infection centers plot^-1. In the absence of fungicides, dollar spot severity was reduced by 63 to 90% in plots treated with paclobutrazol and by 13 to 55% in plots treated with trinexapac-ethyl. Dollar spot severity was 23 to 50% lower in plots mown 2 days week^-1 compared with those mown 6 days week^-1. In cases where a significant interaction was observed between mowing frequency and PGRs, dollar spot was reduced on most rating dates in plots treated with trinexapacethyl that were mown 2 days week^-1 compared with those mown 6 days week^-1. Survival analysis of days until threshold was met revealed that duration of control of fungicides in plots receiving paclobutrazol were 28 to 84% longer compared with plots not receiving PGR. Duration of control by fungicides was generally similar between plots treated with trinexapac-ethyl and no PGR. In general, mowing frequency did not influence duration of control. Results from this study indicate that paclobutrazol could be used to increase the treatment interval of fungicides and that mowing frequency in the absence of dew is likely to have little influence on fungicide residual efficacy. When used without fungicides, PGRs and less frequent mowing may reduce dollar spot in situations where fungicide use is limited.