Gray snow mold has been spotted on lawns already this year. Snow mold fungi work under a protective blanket of snow and make themselves known when the snow begins to melt in the spring. Circular bleached patches a few inches to a foot in diameter become obvious. These patches sometimes overlap and cause large irregular areas of browning.
Problems commonly occur in years with prolonged snow cover. Snow molds get an early start when a wet, deep snow falls on unfrozen ground. The fall and winter weather this season was favorable for snow mold fungi.
In Iowa, two different types of snow mold diseases may develop, pink snow mold and gray snow mold. The common names describe the damage they cause. When wet, the bleached patches of grass caused by the pink snow mold fungus may show a light pink fluffy growth, especially at the outer edge of the patch. Patches caused by the gray snow mold fungus tend to be covered by whitish gray strands of fungus that glue the grass blades together.
The gray snow mold fungus produces tough survival structures called sclerotia. They're about the size of a pinhead and tend to be an orange-brown color. You'll find them embedded in the leaf tissue. A hand lens or magnifying glass as well as a little patience often are helpful when looking for sclerotia. The pink snow mold fungus does not produce sclerotia
If a snow mold problem appears on your lawn this spring, several cultural practices can help manage the disease. Injury usually can be repaired by raking the affected areas and by lightly fertilizing to encourage new growth. An excessive layer of thatch (more than 1/2 inch) should be controlled because it provides a nice place for the fungus to hide during the hot summer months. Also improve drainage if necessary, because areas that stay wet can provide favorable conditions for a number of disease organisms.
To prevent snow mold damage from reoccurring, avoid late fall applications of quick-release nitrogen fertilizers, since lush growth late in the fall is more susceptible to attack. Keeping the grass mowed until growth has stopped in the fall also will help prevent lush growth going into winter. Snow fences can be used to prevent drifting in key lawn areas. Keep in mind that injury can be made worse by activities that compact snow, such as sledding, skiing, or snowmobiling. Fungicides labeled for snow molds sometimes are applied on high-value areas, such as golf course greens and tees, and on areas where snow mold is a problem year after year. These products are applied in the fall, before snow cover.
This article originally appeared in the March 23, 2001 issue, p. 25.