The moldy growth known as snow mold can be prevented by keeping your lawn mowed to two inches, removing raked leaves, and avoiding nitrogen fertilizer in September and October. They survive in thatch and on plants as sclerotia or as mycelial threads, and only begin to grow and infect grasses when cold wet weather begins. You may only know it was there in the spring, when snow melts and you discover circles of dead grasses which will disappear with new (grass) growth. The time for prevention is in the fall, not with chemicals but chiefly with behavior. You might want to treat composted leaves, however, before using them as mulch, to avoid spreading a snow mold infestation.
If you’re in California, you might want to give us a call and see about getting an assessment from Byebyemold.