Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/5/01
Contacts:
Paula Flynn, Extension Plant Pathology, (515) 294-0581,
pflynn@iastate.edu
Elaine Edwards, Continuing Education and Communication
Services, (515) 294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Feb. 9
Snow Molds
By Paula Flynn
Extension plant diagnostician
Extension Plant Pathology
Most of the organisms that cause problems on outdoor plants take a break during the cold winter months. Unfortunately for our lawns, a group of nonconforming fungi exist - the snow mold fungi. The term "snow mold" might bring up interesting images of a fuzzy green growth creeping across snow banks. But snow molds actually are quite discreet, making themselves known only when the snow begins to melt away.
Although snow mold fungi can infect several different types of plants, the most troublesome attack turfgrass. Because snow molds prefer to work under a protective blanket of snow, problems commonly occur in years with prolonged snow cover. Snow molds may get an early start when a wet, deep snow falls on unfrozen ground. Our fall and winter weather this season has been favorable for snow mold fungi.
In Iowa, two different types of snow mold diseases may develop, pink snow mold and gray snow mold. As you might speculate, these 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.
Both snow mold groups have similar features. As the snow melts, circular bleached patches a few inches to a foot in diameter become obvious. These patches sometimes overlap and cause large irregular areas of browning. On occasion, both types of snow mold may occur together.
If you have an eye for detail, you can fine-tune your snow mold diagnosis by taking a close look at the affected grass blades. 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 nestled in the leaf tissue. A hand lens or magnifying glass as well as a little patience often are helpful to locate sclerotia.
The pink snow mold fungus does not produce sclerotia. It's also not as fussy about the snow cover requirement, and it may become active if prolonged cool, wet weather occurs. Temperatures between 32 F and 50 F are the most favorable for this fungus to invade grass blades.
Our lawns are a tempting food source for snow mold fungi because they often are kept in a lush growing condition until snowfall arrives. Without time to properly prepare for winter conditions, the grass plants may be in a vulnerable state and less able to defend themselves from fungal attack. Injury can be made worse by activities that compact the snow, such as sledding, skiing or snowmobiling.
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 of dead plant material found between the soil and the leaf blades) 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. 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.
ml: isugarden
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