Many Front Range lawns are now being afflicted with gray snow mold, caused by a fungus called "Typhula incarnate." Snow mold most often occurs during periods of prolonged snow cover, but can also occur where leaves and other debris has accumulated on lawns during the fall and winter, said Tony Koski, a turf specialist with Colorado State University Cooperative Extension Agency.
North-facing lawns, shaded lawns, and areas where snowplowing or drifting snow created especially deep snow will be the most commonly affected parts of the landscape. The fungus is most active in moist lawns at temperatures just above freezing. Circular patches (6-12 inches across) will have a moldy appearance if the fungus is actively growing – usually just as the snow melts and the lawn is exposed.
As the lawn begins to dry out and fungal growth slows, the patches can take on a light tan appearance, fading to light gray or white. Under severe conditions, these smaller patches may grow together to form large, matted areas that appear to be dead. This disease rarely kills turf in the home lawn, although young seedlings from a late fall seeding may be killed (fall sod will be alright).
"Fungicide applications are not recommended when snow mold occurs in a home lawn," Koski said. "Spring fungicide applications will not effectively control or prevent the disease, nor will fungicide use hasten spring turf recovery."
The best way to speed recovery is to remove dead and matted material by light raking to promote air circulation and drying. Light spring nitrogen fertilization (especially if no fertilizer was applied the previous fall) will help speed the formation and growth of new grass from the underground stems that are not harmed by the snow mold fungus. Lawns that appear slow to recover this spring will benefit from core cultivation, followed by overseeding with the same grass species already present in the lawn.
"Contrary to popular belief, late-season fertilization — also referred to as ‘winterizer’ — neither encourages snow mold, nor increases its severity," Koski said. "In fact, late-season/fall fertilization will encourage more rapid healing and recovery when lawns are afflicted with snow mold."
Koski said gray snow mold is generally uncommon in Front Range lawns because snow must continuously cover the lawn for at least 40-60 days before the fungus becomes noticeably active – a rare occurrence on Front Range lawns.