Dollar spot (caused by Sclerotinia homoeocarpa F.T. Bennett) is a common disease of Agrostis stolonifera L. and is especially devastating on putting greens. Sequential fungicide applications are often required throughout the growing season for adequate control. This research was conducted to determine if ferrous sulfate (FeSO4) and its elemental components are capable of reducing dollar spot development. Trials were established in situ and in vitro to address this objective. Putting green trials were conducted in Blacksburg, VA, during the summers of 2012 and 2015. Biweekly (May to September) foliar treatments were applied in four randomized complete blocks as: control, FeSO4 (48.8 kg ha−1), sulfur (10.3 kg ha−1), and iron chelate (11.2 kg Fe ha−1). Dollar spot infection centers and visual turf quality were recorded throughout the summer. When disease exceeded a threshold of 75 infection centers per plot, chlorothalonil (2,4,5,6‐tetrachloroisophthalonitrile, 12.5 kg ha−1 a.i.) and nitrogen fertility (12.2 kg ha−1) were applied to encourage recovery. After recovery, inputs were minimized to encourage dollar spot development. Ferrous sulfate reduced dollar spot pressure each year. Turf quality was improved with FeSO4 in 2012, but not in 2015. Iron chelate suppressed dollar spot in 2012, but not in 2015. Sulfur had no impact on dollar spot or turf quality in either year. Direct fungitoxic activity of FeSO4 against the dollar spot pathogen was evaluated in vitro across a range of pH. Pure cultures of S. homoeocarpa obtained from the field test site were plated on 0.25× potato dextrose agar amended with 0, 10, 100, and 1000 mg kg−1 FeSO4. Mycelial growth was enhanced at 10 mg kg−1, was not affected at 100 mg kg−1, and was completely inhibited at 1000 mg kg−1. These data suggest FeSO4 suppresses dollar spot by acting as a fungistat against the pathogen.

Dollar spot is a common disease of both warm‐ and cool‐season amenity turfgrasses caused by Clarireedia species. Previous field studies have shown dollar spot suppression with 48.8 kg ha−1 ferrous sulfate heptahydrate. In vitro research suggests that 100–1,000 mg L−1 ferrous sulfate concentrations suspended in agar will directly inhibit Clarireedia mycelial growth. The impact of ferrous sulfate concentration on dollar spot development in situ is not clearly defined. Our research explored five field rates and five concentrations of ferrous sulfate against dollar spot development in situ and Clarireedia growth in‐vitro, respectively. Field‐applied rates included 0, 4.88, 24.4, 48.8, and 97.6 kg ha−1 ferrous sulfate. Our data indicate a nonlinear relationship between ferrous sulfate rate and dollar spot development, with 26.4 kg ha−1 required for 50% dollar spot suppression and only a 10% increase in suppression with previously reported rates of 48.8 kg ha−1. Ferrous sulfate suppressed dollar spot on both golf course fairways and putting greens. Radial mycelial growth of four isolates were modeled using ferrous sulfate‐amended agar at concentrations of 0, 200, 400, 600, and 800 mg kg−1. Effective concentration required to suppress Clarireedia growth by 50% ranged from 511 to 687 mg L−1 and varied by isolate, though the relationships for all had a negative linear relationship of mycelial growth to concentration. Radial growth of the two isolates collected from warm‐season grasses grew faster than two isolates collected from cool‐season grasses.

Dollar spot (Clarireedia spp.) is a damaging turfgrass disease. Ferrous sulfate (FeSO 4 ) suppresses dollar spot with little risk of resistance. Optimizing FeSO 4 applications is crucial for adequate dollar spot suppression. Supplementing FeSO 4 applications with chlorothalonil fungicide could also increase efficacy against dollar spot and longevity of disease suppression. Four experiments were conducted to determine the 1) optimal water carrier volume for FeSO 4 , 2) proper nozzle selection for FeSO 4 applications, 3) most efficacious rate combination of FeSO 4 and chlorothalonil, and 4) longevity of chlorothalonil efficacy when applied in conjunction with FeSO 4 for dollar spot suppression on creeping bentgrass (Agrostis stolonifera L.). These studies were conducted on either putting greens mown at 3.2 to 6.4 mm or fairways mown at 16.5 mm. The following carrier volumes were tested in Experiment 1: 281, 421, 842 and 1684 L ha -1 . For Experiment 2, Turbo FloodJet (TF), Turbo TeeJet Induction (TTI), Air Induction Turbo TwinJet (AITTJ), and Extended Range Flat Fan (XRFF) nozzles were used to determine optimal nozzle type. In Experiment 3, chlorothalonil was applied at 0, 2.28, 4.57, 6.86 and 9.16 kg a.i. ha -1 with or without 48.8 kg ha -1 of FeSO 4 . In Experiment 4, 48.8 kg ha -1 of FeSO 4 was applied biweekly and chlorothalonil was curatively applied at 8.2 kg a.i. ha -1 when infection centers per plot reached 30. Water carrier volume had no effect on dollar spot suppression, the AITTJ and XRFF nozzles provided the greatest disease suppression and turf quality, FeSO 4 reduced effective chlorothalonil rates by up to 68.5% and FeSO 4 increased longevity of chlorothalonil efficacy.