Characterizing Fungicide Resistance in Sclerotinia Homoeocarpa Populations of Continental United States Golf Courses.

Dollar spot is the most economically important disease of amenity turfgrass.  The causal agent of dollar spot, Sclerotinia homoeocarpa, exists around the world and is fairly diverse considering a lack of a sexual stage in nature.  These populations develop resistance to certain chemistries fairly quickly and little is known about the relationship of populations within different regions of the US.  With an extensive library of isolates collected from across the country, the characterization of these populations in terms of chemical resistance was prioritized. Using ten isolates from each of four determined regions (Southeast, Northeast, Midwest, and West Coast) to total 50 tested isolates, in vitro assays for 8 different fungicides were preformed across 6 different concentrations replicated 3 times. Isolates were selected from cool season grass fairways, tees, and rough.  Dollar spot is rarely a major issue on putting greens because fungicides are typically applied weekly or every two weeks and excellent rotations are used. The fungicides tested included propiconazole, tebuconazole, triadimefon, boscalid, fluxapyroxad, penthiopyrad, fluopyram, and fluazinam at 0, 1, 10, 50, and 100 μg mL-1. Six mm plugs of 6-day-old purely cultured isolates were transferred to fungicide amended media and radial growth was measured after 5 days. Ten isolates were collected from each of two courses within the Southeast region and were compared against older isolates to observe changes in in vitro sensitivity to fungicides.  In vitro sensitivity varied significantly among regions and among fungicide class.  With limited analyses completed, it appears that fungicide sensitivity has remained fairly stable.  However, further research is needed to examine mechanisms of fungicide resistance within this pathogen population.