Sdhi Resistance Screening in Sclerotinia Homoeocarpa.

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 have the potential to develop resistance to fungicides quickly. In other fungal pathogens resistance and cross-resistance has been documented within the SDHI fungicides. Many new SDHIs are being registered for turf, but the potential for resistance development and the potential for cross-resistance is not well characterized. During a fungicide trial in the spring of 2016, three SDHI fungicides (boscalid, fluxapyroxad, and penthiopyrad) failed to control dollar spot in field plots at the NCSU Lake Wheeler Research Station, Raleigh, NC. Twenty-four isolates were collected from dollar spot foci in plots where disease developed including the non-treated controls. These isolates were then used to determine in vitro sensitivity to SDHI fungicides. Six different concentrations of each fungicide were tested and replicated three times. An isolate that had not been exposed to SDHIs was used as a baseline, along with 3 isolates that were collected from less frequently treated rough areas of various golf courses. The fungicides tested included boscalid, fluxapyroxad, and penthiopyrad at 0, 0.1, 1, 50, and 100 mg/L. Two mm plugs of 3-day-old isolates were transferred to fungicide-amended media (1/4 PDA media) and radial growth was measured after 40 hours. In addition, an in plantae experiment was performed in which 3.8 cm diameter plugs of creeping bentgrass were grown and maintained in a greenhouse. These were sprayed with the fungicides above and inoculated with isolates used in the in vitro studies. The plugs were moved to a growth chamber and given enhanced disease conditions. Although failures in the field were observed, in vitro assays did not support the field observations.