Biology of Sclerotinia Homoeocarpa Populations and Their Resistance to Fungicides.

Dollar spot is a disease in turfgrass caused by the fungal pathogen Sclerotinia homoeocarpa. Understanding S. homoeocarpa population dynamics regarding fungicide resistance and vegetative compatibility diversity may help turfgrass managers make more informed management decisions. A total of 1025 isolates were collected from PA and the surrounding region. Isolates (n=189) were screened in vitro against four different fungicides and were subjected to vegetative compatibility analysis. Fungicides evaluated included thiophanate-methyl, iprodione, propiconazole, and boscalid using discriminatory doses 1,000 µg a.i mL^-2, 1.0 µg a.i mL^-2, 0.1 µg a.i mL^-2, and 1000 µg a.i mL^-2, respectively. Relative mycelial growth (RMG) values and the the concentrations resulting in a 50% reduction in growth (EC₅₀) were determined for each isolate across all fungicides. Resistance to thiophanate-methyl was determined qualitatively. Isolates from the Vineyard Golf Club (VGC), which has never been sprayed with a synthetic pesticide, served as a baseline for sensitivity. All isolates were paired with known isolates belonging to vegetative compatibility groups (VCGs) A to K. Of the golf courses sampled, 70% had reduced sensitivity to at least one of the four fungicides evaluated. Nearly 6% of isolates from four populations were insensitive to all four fungicides. From tees and fairways, 53% of 15 populations showed reduced sensitivity to boscalid relative to the baseline population. Resistance of these populations, however, have not been confirmed in the field. All known VCGs, except for group J, were recovered suggesting there is substantial VCG diversity present throughout our population. A total of 22 isolates were incompatible to all 11 known VCGs. Significant interactions between VCG and RMG were observed for iprodione. Remaining isolates (n=836) are currently being subjected to in vitro fungicide screening and VCG analysis.