Strategies for increasing ferrous sulfate efficacy against dollar spot of creeping bentgrass

Dollar spot (Clarireedia spp.) is a damaging turfgrass disease. Ferrous sulfate (FeSO4) suppresses dollar spot with little risk of resistance. Optimizing FeSO4 applications is crucial for adequate dollar spot suppression. Supplementing FeSO4 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 FeSO4, 2) proper nozzle selection for FeSO4 applications, 3) most efficacious rate combination of FeSO4 and chlorothalonil, and 4) longevity of chlorothalonil efficacy when applied in conjunction with FeSO4 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 FeSO4. In Experiment 4, 48.8 kg ha–1 of FeSO4 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, FeSO4 reduced effective chlorothalonil rates by up to 68.5% and FeSO4 increased longevity of chlorothalonil efficacy.