Optimizing Ferrous Sulfate Performance for Dollar Spot Suppression.

Ferrous sulfate is known to suppress dollar spot (DS) when applied at adequate use rates. Though the mode of action is not well understood, scientists have speculated that iron is reacting directly with oxalic acid produced by Sclerotinia homoeocarpa, the DS pathogen, thus preventing entrance into plant tissue. If correct, adequate foliar coverage would be necessary for maximum disease suppression. Two factors that impact spray coverage and uniformity include application spray volume and nozzle selection. Our research has concluded that DS is suppressed at rates of 24.4 to 48.8 kg product ha^-1 when applied on a 14-day interval during active epidemics, but it is unknown how spray droplet size or application volume impact performance. To address these questions, trials were at the Virginia Tech Turfgrass Research Center in Blacksburg, VA in 2017 on a ‘A4’ creeping bentgrass putting green. Ferrous sulfate (24.4 kg ha^-1) was applied at spray volumes of 281, 421, 842, and 1683 L solution ha^-1. To date, the range of spray volumes tested does not impact DS suppression, though more research is needed for confirmation. A separate study was established with 24.4 kg ha^-1 ferrous sulfate applied in 814.5 L ha^-1 solution with four common nozzles which produce different spray droplets sizes. Nozzles included XR TeeJet (XR11004-VS), Air Induction Turbo Twin Jet (AITTJ60-11004-VP), Turbo TeeJet Induction (TTI11004-VP), and TurfJet (1/4TTJ04-VS), with droplet sizes of medium, very coarse, ultra-coarse, and ultra-coarse, respectively. Results to date indicate that XR11004-VP and TTI11004-VP reduced DS more effectively than TTI11004-VP and 1/4TTJ04-VS. These data suggest that nozzles providing a finer droplet, thus greater coverage, are most effective at suppressing DS. This is congruent with the suggestion that DS is suppressed with direct contact to the pathogen rather than a physiological plant response.