Extended snow cover on ground that is not completely frozen is conducive to turf diseases called snow molds. Two snow mold diseases, gray snow mold and pink snow mold, occur in Iowa and typically appear at this time of year. Gray snow mold is caused by two species of the fungus Typhula, while pink snow mold is caused by the fungus Microdochium nivale.
Symptoms of snow molds first appear when snow melts in late winter or early spring. Circular, straw-colored patches appear in the lawn as the snow recedes. Patches caused by gray snow mold may be a few inches to a few feet in diameter, while those caused by pink snow mold tend to be smaller, less than six inches across. These patches may continue to enlarge if the grass remains cool and wet. Grass in the patch may be matted and wet, with pink or gray colored fungal growth over the patch or on the edge. Gray snow mold causes small, pinhead-sized round structures (sclerotia) to develop on the leaves and crowns of the grass plants.
Damage caused by snow mold usually is not serious, and affected areas typically green up eventually, though more slowly than the rest of the lawn. Raking the affected areas gently can help to dry them out and prevent further fungal growth. In some cases, affected areas will need to be overseeded if they fail to recover.
Problems with snow mold can be minimized in the future by:
- Avoiding excessive nitrogen fertilization in the fall
- Mowing the lawn at the recommended height throughout the fall
- Keeping the thatch layer to 1/2 inch or less
- Raking up fallen leaves in the fall
- Preventing excessive snow accumulation on problem areas, by using snow fencing and spreading out large snow piles to promote melting
- Applying fungicide late in the fall on golf courses and other high-value turfs