Research Report
Dollar Spot Suppression with Iron Sulfate Virginia Tech Researchers: Camden Shelton, Graduate Research Assistant; David McCall, Ph.D., Research Associate; and Erik Ervin, Ph.D., Professor
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reeping bentgrass is commonly used on golf course greens, fairways and tees in temperate climates. Dollar spot, the most common disease of creeping bentgrass, degrades foliage by creating small, silver-dollar-sized depressions of dead turf that may persist throughout the winter. Fungicides are frequently applied throughout the growing season to prevent disease. The number of fungicides used to suppress dollar spot has dwindled because of government regulations, emergence of resistant pathogen populations and failure to provide consistent control.
Our research
Researchers have shown that various cultural strategies can suppress dollar spot. Previous research at Virginia Tech has shown that iron sulfate can effectively suppress the dollar spot pathogen in cultured laboratory studies and under normal field growing conditions. This discovery is being
expanded in 2016 to address whether the incorporation of iron sulfate can reduce the total needs of chlorothalonil (a broad-spectrum fungicide) throughout the growing season. Trials are underway on ‘007’ creeping bentgrass putting greens at the Turfgrass Research Center and on ‘L-93’ creeping bentgrass fairways at the Glade Road Research Facility, both in Blacksburg, Virginia. Each study is replicated on a green and fairway. Dollar spot infection centers, visual estimation of percent disease and visual estimation of turf quality are being collected twice weekly throughout the season. Cumulative disease-progression data will be used to estimate the role of iron sulfate throughout the growing season. One study investigates whether the use of iron sulfate can reduce chlorothalonil rates applied biweekly, while maintaining adequate dollar spot suppression. Five rates of chlorothalonil (0, 1, 2, 3 and 4 fl. oz./1,000
ft2 Daconil WeatherStik®, Syngenta Professional Products) are being applied to plots treated with iron sulfate (1 lb./1,000 ft2, 20% ferrous sulfate heptahydrate) or untreated controls. Very early indications show that the use of iron sulfate may result in a substantial chlorothalonil rate reduction to maintain bentgrass below an acceptable threshold. A truer estimation of chlorothalonil reductions will not be available until later in the season. The actual reduction will be dependent on what is considered an acceptable threshold for each individual facility. Other studies underway will investigate effective iron sulfate rates for acceptable dollar spot suppression and extended control with a standard chlorothalonil application. Our previous research has demonstrated a dollar spot reduction with 1 lb. iron sulfate/1,000 ft2. This suppression may be seen at lower rates, although our data is inconclusive at this point in our studies. Through June, we have been able to save one chlorothalonil application by incorporating iron sulfate. As with any chemical or culturalmanagement strategy, the use of iron sulfate will most likely not eliminate dollar spot. This tactic should be used as part of an integrated approach to maintain turf below an unacceptable limit. Each of these studies will be repeated in 2017 on creeping bentgrass putting greens and fairways. c
Chlorothalonil Rate 0 fl. oz./1,000 ft2
1 fl. oz./1,000 ft2
2 fl. oz./1,000 ft2
Untreated Control
Ferrous Sulfate 1 lb./1.000 ft2
20 | Virginia Turfgrass Journal May/June 2016 www.vaturf.org
3 fl. oz./1,000 ft2
4 fl. oz./1,000 ft2