As the snow melts in the spring, you might notice pale brown or matted patches on your lawn. This could be snow mold, a disease caused by a fungus.
Snow mold occurs most often during years when the snow comes early and cover the ground with a thick layer of snow. The layer of snow will prevent the ground to freeze, which causes ideal growing conditions for the fungus during the winter months. It will most often form where excess snow accumulate, for example snow banks created from removing and piling snow. This will give the fungus more time to develop before the sun dry up the turf.
The snow mold patches are circular and can range from a couple inches to feet across. As long as the turf is wet and the temperature low, these patches will continue to grow. These patches have a white matted appearance with colored fungal growth. The fungal growth may either be covering the whole patch or developing just around the fringes. Periodically the fungal may grow into small mushrooms.
Different types of snow mold
There are a few different types of snow mold, but the two most common are gray and pink snow mold. Gray and pink snow mold develop and thrives under mostly the same conditions, but the severity of the diseases varies.
Gray snow mold
Gray snow mold (also called Typhula blight or Typhula Incarnata) is the less sever type of snow mold. The fungus only attack and kill the grass blades and leaves the crown and roots untouched.
It can be recognized by white matted areas of grass where the blades are covered with strings of fungal matter creating a web. If you look close, you might see tiny fungal fruit bodies, called sclerotia, sticking to the blades and crowns of the infected area.
During the summer, when it is relatively dry the sclerotia lay dormant in the thatch layer and on the crown of the grass plant. As the weather gets cooler and moister during the fall, the fungus starts to grow. However, gray snow mold seldom develops unless under a layer of snow.
In the US, it occurs most commonly in the Great Lakes region where the winter is cold and has a decent snowfall.
Pink snow mold
Pink snow mold (also called Micodochium nivale) is the most sever type snow mold you typical find in the U.S. The fungus usually only attack and kill the grass blades of the plant. However, under ideal growing conditions for the fungus, it might kill the whole plant by attacking both the crowns and roots as well.
As the name implies, you can recognize pink snow mold by the pink fungal spores that grow on the grass blades of the infected area.
The fungus survives the hot and dry summer lying dormant in the thatch layer or the soil. As the weather gets cooler and wetter (32° to 65°F), the spores start to grow. If pink snow mold has the chance to establish a foothold during early fall, it is more likely the fungus will continue to grow underneath the snow through the winter. When this happens the damage to the turf will most likely be severe and could be hard to get rid of.
Preventing snow mold
By following a few easy steps during the fall, you can almost eliminate the threat of snow mold all together.
- Avoid using too much fertilizer during the fall. If fertilizing during the fall, choose a type with less nitrogen and high levels of potassium. Some studies have shown that potassium can suppress the disease. Overuse of fertilizer during the fall when the grass have stopped growing will just be wasted and give nutrients to different diseases.
- Continue to mow your lawn regularly at the end of the season. The taller the grass is when the snow settles and covers the lawn, the more likely it is to create good growing conditions for the snow mold. You can stop mowing the lawn when the grass is no longer actively growing.
- Remove leaves from your lawn. By raking the lawn during the fall, you will remove all the leaves and excess clippings that snow mold thrive underneath. Some like to mulch the leaves into the lawn, but I personally rather raked them up. This brings us the next point.
- Consider dethatching your lawn during fall. Mulched clippings and leaves will break down a lot slower during late fall and too much thatch will create a great breeding ground for snow mold. A healthy layer of thatch is about ½ inch thick and will protect the crowns and roots during the winter months.
- During winter, prevent large snow piles to form. This can be difficult, as the snow from, for example, the driveway must be dump somewhere. If you do not have a choice and must make snow piles on your lawn, you can spread out the piles of snow during the spring to encourage a more rapid melting process.
- Choose grass types that are less susceptible for disease. Kentucky bluegrass and fin fescues are more resistant to snow mold than for example perennial ryegrass, tall fescue and creeping bentgrass.
Curing snow mold
In most cases, snow mold will disappear when the weather gets warmer and dryer. Nonetheless, I would recommend raking the affected area to remove the worst part of the snow mold. Make sure the turf dries up. To speed up the recovery, you can also fertilize the affected areas. For the worst areas infected by pink snow mold, you might need some fungicide to treat it.
If you have problems with snow mold (both pink and gray) recurring year after year, it might with worth using some fungicides during the fall. However, try to limit the use of fungicides to the minimum, as diseases can get resistant with overuse. There have been cases in some parts of the U.S. where various turf diseases have shown resistance to dicarboxmide and benximidazole fungicides.