Gray Snow Mold is a common winter disease that affects a number of cool-season grass species in the northern region. It is caused by the fungi Typhula incarnata (and a few other species) It occurs during the winter, usually under snow cover. We generally see circular patches of cottony mycelia six to eight inches in diameter as the snow melts in the spring.
This year was particularly bad for Gray Snow Mold because of the heavy snow and extended snow cover of turf areas. There is also a Pink Snow Mold that attacks turf in the winter. It is caused by fungi in the Microdochium genera. One of the main, visible differences between the two is that Gray has sclerotia. These are large, leathery fruiting bodies that appear on the tissue (see below). Pink does not have sclerotia.
I have had a lot of calls this spring on the snow mold, both from golf course superintendents and lawn care specialist. On golf greens and fairways, it can cause damage that will be present until as late as June. For that reason, we generally treat these areas with fungicide in the fall to prevent it. On golf course roughs and lawns, the damage can be very apparent in the spring as the snow melts, but it usually goes away quickly as the areas come out of dormancy. Fungicides are very expensive and we generally do not treat lawns and golf course roughs. In the rough, spring fertilizer and mowing will help to clear these areas. Once you see the symptoms in the spring, it is too late to apply a fungicide. This needs to be done in the fall.
If you are in lawn care, explain to your customer that the damage will recover quickly with a little fertilizer, mowing, and warm-weather. Raking some of the debris away may improve the appearance of the areas, but it is not necessary.
Figure 1. Circular Patches of mycelia on golf course rough. Picture by Timothy Christians.
Figure 2. Close up of mycelia. Picture by Timothy Christians.
Figure 3. Picture of snow mold on Kentucky bluegrass lawn in Des Moines. Picture by Nicolas May of Trugreen Lawn Care.
Figure 4. Sclerotia on Kentucky bluegrass. Pictue by Nick Christians.
Figure 5. Close up of sclerotia. Picture by Nick Christians.