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The Association Between In Vitro Propiconazole Sensitivity and Field Efficacy of Five New England Sclerotinia homoeocarpa Populations

Dollar spot (Sclerotinia homoeocarpa) is a major turfgrass disease requiring fungicide application to maintain acceptable conditions for golf. A 2-year field experiment was conducted to determine the association between field efficacy of propiconazole and in vitro fungicide sensitivity of isolates from five S. homoeocarpa populations. Four golf courses with prior propiconazole exposure (Hartford Golf Club, Hickory Ridge Country Club, Shuttle Meadow Country Club, and Wintonbury Hills Golf Club), and a baseline site with no prior propiconazole exposure (Joseph Troll Turf Research Facility) were chosen as field sites. Experimental plots at each site received the following treatments at 21-day intervals: untreated, propiconazole (0.44, 0.88, 1.32, and 1.76 kg a.i. ha^–1), and chlorothalonil (8.18 kg a.i. ha^–1). S. homoeocarpa isolates were sampled at three time points during 2009 and 2010: initial (directly before fungicide treatment), 7 days after treatment (DAT), and 21 days after the last treatment. Isolates sampled from dollar spot infection centers at 7 DAT (2009 and 2010) were considered to exhibit “practical field resistance”. In parallel, S. homoeocarpa isolates from each site were assayed for in vitro sensitivity to propiconazole by determining relative mycelium growth percentages (RMG%) on potato dextrose agar amended with propiconazole at a discriminatory concentration of 0.1 μg a.i. ml^–1. S. homoeocarpa isolates from the four exposed populations displayed significantly higher RMG% values than the baseline population. In general, field efficacy at all propiconazole rates tested was lower at the four locations with prior propiconazole exposure when compared with the baseline population. Increased RMG% values on the propiconazole discriminatory concentration 0.1 μg a.i. ml^–1 were associated with decreased relative control values for all propiconazole rates in 2009 and 2010. Results suggest RMG values above 50% at the propiconazole discriminatory concentration of 0.1 μg a.i. ml^–1 may be a suitable threshold for detection of S. homoeocarpa isolates that cause practical DMI field resistance.