On cool-season grasses, there are two diseases referred to as snow molds, though the causal pathogens are in different genera, and therefore unrelated to each other. There are some slight differences in the conditions that favor their development. But in both cases, the disease is most severe if there is snow cover over unfrozen soil.
The causal pathogen of pink snow mold is Michrodochium nivale. Conditions that favor development of this disease include cold fogs, persistent drizzle, low light (common in winter time), soil pH around 6.5 and excessive thatch. Air temperature requirements are variable with the disease, affecting turf in temperatures as low as 32 degrees and as high as 75 degrees. It’s most severe when the temperature is between 32 and 45 degrees. While not always present, fluffy pink mycelium is a good sign of pink snow mold (see Figure 1).
Grey snow mold is caused by a couple of pathogens, Typhula incarnata or T. ishikariensis. Conditions favoring its development include the presence of excessive thatch and frost-injured tissue. Like pink snow mold, it’s most severe with snow cover over unfrozen soil (with T. ishikariensis, snow cover is required). Following a spring thaw is a very common time to notice this disease. Minimum air temperature required for activity is 19 to 23 degrees and the optimum temperature for activity ranges from 32 to 59 degrees (T. incarnata), or 44 to 59 degrees (T. ishikariensis). It usually manifests as circular areas of yellow or brown turf, with matted down leaf blades, that may or may not be covered with grayish mycelium. Sclerotia, which are hardened pink, brown or black resting bodies about the size of a pencil eraser, are a tell-tale signs of this disease (see Figure 2 below).
If you have a history of either of these diseases, reduce your fertilizer applications in late fall and keep the turf height low. You can also take steps to remove snow cover and provide some air circulation to make the environment less favorable for disease formation. Finally, on fields with a history of damage due to snow mold, there are fungicides labeled for control. Given that the most severe damage is done under snow, preventative, rather than curative applications of the fungicide are recommended.