Evaluation of Fungicides and Surfactants for Control of Fairy Rings Caused by Marasmius oreades (Bolt ex. Fr.) Fr.

Marasmius oreades , a causal agent of fairy rings, is one of the most important pathogens of turfgrass in the Great Plains region of North America. Following in vitro and greenhouse screening of surfactants and fungicides, two organosilicone surfactants, Silwet L-77 and Sylgard, together with the fungicide chlorothalonil, were evaluated in the field. Treatments were applied to healthy and infested turfgrass ( Poa pratensis L., Festuca rubra L.) in either 1992, 1993, or in both years. Plots were sampled for grass production, canopy cover, mushroom production, grass chlorophyll content, soil water content, and phytotoxicity. Typically, there were no significant fungicide effects, fungicide by surfactant interactions or differences between Silwet L-77 and Sylgard. Relative to the water control, surfactants caused an approximate 3-fold increase in grass productivity on infested plots in the year of application. However, the difference in canopy cover between organosilicone-treated and control plots tended to be much less. Applying the surfactants to diseased plots in two successive years decreased the canopy cover but had no significant effect on grass production. Chlorophyll content tended to decrease in response to surfactants regardless of whether the turf was infested or healthy. Surfactants almost completely eliminated mushroom production and greatly reduced the occurrence of mycelium. Both organosilicones increased soil water content in infested areas; differences were detectable 2 years after application. Acute phytotoxicity from the surfactants was detected in infested but not in healthy plots. There was no direct evidence of chronic phytotoxicity. Organosilicone surfactants appear to have considerable potential for the management of fairy rings. Chemical names used: oxyalkylenemethylsiloxane (Silwet L-77); 2-(3-hydroxypropyl)-heptamethyltrisiloxane (Sylgard); tetrachloroisopthalonitrile (chlorothalonil).