in Vitro Variability of Sclerotinia Homoeocarpa Exposed to Ferrous Sulfate.

Dollar spot is one of the most common diseases found on both warm and cool-season turfgrass species around the world. Ferrous sulfate has been demonstrated to be a viable option to suppress dollar spot on creeping bentgrass under normal field conditions at rates of 24.4 to 48.8 kg ha^-1. In vitro studies have also shown 100 to 1,000 mg kg^-1 ferrous sulfate-heptahydrate concentrations to directly suppress growth of Sclerotinia homoeocarpa collected from a creeping bentgrass putting green that had received repeated fungicide applications over many seasons. Genetic diversity of S. homoeocarpa segregates isolates into two groups; strains associated with warm-season and cool-season grasses. It is unknown whether S. homoeocarpa of each group will react similarly in the presence of ferrous sulfate. More research is needed to determine the effective concentration of ferrous sulfate required to suppress isolates collected from warm-season and cool-season grasses. Multiple isolates were collected from warm-season and cool-season grasses across Virginia, with representative isolates selected for laboratory studies. In vitro trials were conducted at the Glade Road Research Center in Blacksburg, VA in 2017. Isolates of each group from areas with no previous fungicide history and intensive fungicide-application history. Pure cultures of S. homoeocarpa were grown on ¼ strength potato dextrose agar amended with 0, 200, 400, 600, and 800 mg kg^-1 of ferrous sulfate heptahydrate, with mycelial growth recorded after three days incubation. These data indicate a strong linear relationship between mycelial growth of S. homoeocarpa and ferrous sulfate concentration (r ≥ 0.74). Some growth was observed at the highest concentration of ferrous sulfate. Extrapolation of this linear relationship indicated concentrations of 1024 to 1448 mg kg^-1 are needed to completely suppress mycelial growth in vitro.