Iron Sulfate and Phosphorous Acid Affect Turfgrass Surface pH and Microdochium Patch Severity on Annual Bluegrass

Microdochium patch is a turfgrass disease caused by the fungal pathogen Microdochium nivale. Iron sulfate heptahydrate (FeSO₄•7H₂O) and phosphorous acid (H₃PO₃) applications have previously been shown to suppress Microdochium patch on annual bluegrass putting greens when applied alone, although either disease suppression was inadequate or turfgrass quality was reduced from the applications. A field experiment was conducted in Corvallis, Oregon, U.S.A., to evaluate the combined effects of FeSO₄•7H₂O and H₃PO₃ on Microdochium patch suppression and annual bluegrass quality. The results of this work suggest that the addition of 3.7 kg H₃PO₃ ha^-1 with 24 or 49 kg FeSO₄•7H₂O ha^-1 applied every 2 weeks improved the suppression of Microdochium patch without substantially compromising turf quality, which occurred when 98 kg FeSO₄•7H₂O ha^-1 was applied with or without H₃PO₃. Spray suspensions reduced the pH of the water carrier, therefore two additional growth chamber experiments were conducted to better understand the effects of these treatments on leaf surface pH and Microdochium patch suppression. On the application date in the first growth chamber experiment, at least a 19% leaf surface pH reduction was observed compared with the well water control when FeSO₄•7H₂O was applied alone. When 3.7 kg H₃PO₃ ha^-1 was combined with FeSO₄•7H₂O, regardless of the rate, the leaf surface pH was reduced by at least 34%. The second growth chamber experiment determined that sulfuric acid (H₂SO₄) at a 0.5% spray solution rate was always in the group that produced the lowest annual bluegrass leaf surface pH, but did not suppress Microdochium patch. Together, these results suggest that while treatments decrease leaf surface pH, this decrease in pH is not responsible for the suppression of Microdochium patch.