Feature Story
The Influence of Iron, Sulfur and pH on
Dollar Spot Development By Nate Reams; Erik Ervin, Ph.D.; David McCall; and Shawn Askew, Ph.D., Virginia Tech
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reeping bentgrass provides an ideal putting surface for golf courses located in temperate climates because it provides a uniform playing surface, moderately tolerates heat and disease pressure, has a medium irrigation requirement and is aesthetically pleasing. Dollar spot is the most common disease of creeping bentgrass putting greens. Sclerotinia homoeocarpa, the causal agent, degrades bentgrass greens by creating small, silver-dollar-sized depressions of dead turf and necrotic tissues that may persist throughout the winter and into the spring. To prevent disease symptoms, frequent fungicide applications are required throughout the growing season. Even though many fungicides are labeled for dollar spot control, they have become limited due to government regulation, resistant fungal pop-
ulations or a failure to provide consistent control. Cultural practices alone are not effective at eliminating dollar spot, but they can increase fungicide efficacy and longevity. Research into alternative methods for dollar spot control is required for improved integrated approaches to future disease control.
Previous research
Previous researchers have investigated nitrogen sources for dollar spot suppression, but limited information is available on the use of sulfur, iron or ferrous sulfate as alternatives. One published report indicated ferrous sulfate as a mild fungistat. Iron can assist in the suppression of pathogens that infect plants, or it can aid in the infection process of certain pathogens. Iron deficiency in wheat has been shown to cause a breakdown in patho-
Figure 1. Dollar spot infection center counts affected by the bi-weekly application of ferrous sulfate on a creeping bentgrass putting green.
22 | Virginia Turfgrass Journal November/December 2014 www.vaturf.org
gen resistance, while iron application was shown to cause pathogen resistance in normally susceptible plants. The combination of iron and sulfur as ferrous sulfate has also been shown to decrease or suppress the infection of rust in wheat. It is unknown, however, if rates of iron, sulfur or ferrous sulfate are toxic to the S. homoeocarpa or hinder the infection process. Because of the chemical structure of ferrous sulfate, soil pH may be lowered with repeat applications. Previous investigations of pH effects on dollar spot epidemics are inconsistent. For these reasons, our recent study investigated which element or elements of ferrous sulfate are responsible for dollar spot suppression at various pH levels.
Our research
In previous research at the Virginia Tech Turfgrass Research Center (VT TRC) during fall 2011, various rates of ferrous sulfate (0.25, 0.5 and 1.0 lbs./1,000 ft2 of 19% iron plus 21% sulfur, Hi-YieldÂŽ) were evaluated in combination with paclobutrazol and seaweed extract as an option for annual bluegrass removal from creeping bentgrass. After a fungicide failure with thiophanate-methyl due to resistance, a visual comparison between the control and high ferrous sulfate plots indicated that ferrous sulfate may reduce dollar spot incidence. A subsequent count of individual infection centers from each plot was conducted. The medium (0.5 lb./1,000 ft2) and high (1 lb./1,000 ft2) ferrous sulfate rates had significantly lower dollar spot infection centers than the low rate (0.25 lb./1,000 ft2) and no ferrous sulfate (Figure 1). Higher ferrous sulfate rates suppressed dollar spot