After the wettest winter in a decade, snow mold on turfgrass is likely to be evident over much of the state. Golf course superintendents are especially concerned about this disease, but it can affect home lawns, too.
Snow mold is actually a complex of diseases caused by several fungi, all of which thrive on cold, wet turf. "Gray snow mold" is caused by two species of fungi in the genus Typhula that can create significant damage in locations where a snow covering has lasted for three months or more (a large portion of Iowa in 1992-93). As the snow melts off, gray snow mold appears as ashy gray areas of matted grass varying from a few inches to a few feet across. Close examination of the matted grass, particularly before it dries out, will often reveal brick red, match-head size fungal structures called sclerotia. "Pink snow mold," caused by the fungus Microdochium nivalis, produces smaller (less than 6 inches across), round spots on the turf; often, the outer margin or the entire patch has a salmon-pink tinge. In most cases, the patches caused by gray and pink snow molds are temporary, and the turf recovers a normal appearance within 4 to 6 weeks after growth resumes. However, severe outbreaks can kill turf and will require overseeding or resodding.
Snow mold control begins with fertility. Although very high nitrogen rates in fall can make snow mold symptoms worse, typical moderate rates of fall nitrogen do not make the disease worse, and actually speed recovery the following spring. Where drifted snow makes snow mold a common problem, snow fences can be erected to control drifting. Snow can even be shoveled from greens, but this may expose the turf to damage from winter desiccation. The key tactic for snow mold control on golf courses is a late fall application of fungicide. The standard snow mold fungicides for many years have been mercury-based, but although a few such products are still registered, regulatory pressures appear certain to drive the mercurics off the market within a few years. For years, superintendents and agrichemical companies have been trying tank-mixes of various fungicides such as chlorothalonil, vinclozolin, PCNB, chloroneb, iprodione, myclobutanil, and others, with some success. However, a consensus on the most effective combinations has not emerged, probably because of variability in the disease complex from place to place and year to year. An exciting possibility for biological control of gray snow mold, using the nonpathogenic fungus Typhula phacorrhiza, was demonstrated in Canada several years ago, but the status of commercialization of the tactic is uncertain at this time. . .
This article originally appeared in the March 24, 1993 issue, pp. , 1993 issue, pp. 28-29.