From the Viette's Views Gardening Blog
Snow mold is often seen on the lawn in the spring after the snow melts. It is especially common when heavy snow has fallen on unfrozen ground.
There are two types of snow mold:
Snow mold is often more common after late winter or early spring snow storms.
Snow mold (and powdery mildew for that matter) is generally not a serious problem and fungicide applications are usually not recommended. Normally, after a few windy days with lots of sun, the grass dries out and the snow mold will disappear with no treatment at all.
For denser patches of snow mold, the normal recommendation is to simply rake the area lightly to allow the grass to dry more quickly. The raking also disrupts the growth of the fungi.
Increasing the amount of sunlight that reaches the lawn through the selective pruning of a few trees can help reduce the growth of mold and mildew on the lawn.
In most cases, the grass will recover and green up – perhaps just a little slower than the rest of the lawn. However, sometimes small patches of grass may be killed by snow mold. These areas can be overseeded and top dressed with a thin layer of good quality compost in the spring.