Reducing Chlorothalonil Inputs for Dollar Spot Suppression Using Ferrous Sulfate.

Chlorothalonil, a multi-site contact fungicide, is commonly used to suppress the dollar spot pathogen both preventatively and when disease levels become unacceptable. Annual site-use limitations of chlorothalonil may prevent turf managers from achieving acceptable dollar spot control throughout the season. Previous research has demonstrated that ferrous sulfate applied at 48.8 kg ha^-1 can suppress dollar spot epidemics without traditional fungicides, but how this may impact a chlorothalonil fungicide program is not known. Trials were established to determine how ferrous sulfate may be used to minimize chlorothalonil requirements through reducing active ingredient concentrations and extending the longevity, while still maintaining acceptable disease control, and explore lower use rates of ferrous sulfate for dollar spot suppression. Studies were conducted at the Virginia Tech Turfgrass Research Center in Blacksburg, VA in 2016 on a ‘007’ creeping bentgrass putting green built to USGA specification. Chlorothalonil treatments were applied as follows: 0, 2.28, 4.57, 6.86, and 9.16 kg ai ha^-1 (Daconil WeatherStik) across plots treated with and without 48.8 kg ha^-1 ferrous sulfate (Hi-Yield FeSO₄) applied bi-weekly. To date, chlorothalonil rates necessary to suppress dollar spot below a threshold of 30 infection centers m^-2 were reduced from 5.72 kg ai ha^-1 without ferrous sulfate to 1.15 kg ai ha^-1 in plots receiving ferrous sulfate. Our research indicates that adding ferrous sulfate to a fungicide program may extended the longevity of control, and has currently saved three applications of chlorothalonil over the season. Additionally, this research indicates that ferrous sulfate rates can be reduced from 48.8 kg ha^-1 to 24.4 kg ha^-1 without sacrificing efficacy. This ongoing research will further explore how chlorothalonil requirements can be minimized for dollar spot suppression when ferrous sulfate use-rates are reduced, both on golf course putting greens and fairways.