The term "Snow Mold" refers to lawn diseases that occur primarily from late fall to mid-spring. Snow Mold or Snow Mould appears as a fluffy white, pink or grey residue that seems to follow the retreating snow line. Cool, wet conditions with or without snow cover cause this disease to flourish.
A properly fertilized, well-maintained lawn is most resistant to damage from snow mold. A vigorous raking in early spring through the snow mold residue will help reduce this disease and quicken the turf grass healing process.
The two most important diseases that occur at low temperatures are Pink and Grey Snow Mold.
PINK SNOW MOLD
Symptoms begin as small, circular spots which can grow to rusty-brown patches up to 12 inches in diameter. The diseased areas are initially pale yellow and later turn tan or whitish-grey as the leaves die. Infected leaves have a bleached appearance and feel slimy when wet. The dead grass blades have a matted appearance. The disease spread rapidly when under snow cover, or during periods of cool, wet weather. The grass at the edges of the spots may be covered with a white cottony growth called mycelium. This mycelium will often develop a salmon-pink or rusty brown colour when exposed to light.
GRAY SNOW MOLD
This disease often occurs when there is an early, deep snowfall covering the lawn before the ground has frozen in the fall. The symptoms are visible when the snow melts from the lawn. The matted patches are light yellow to bleach brown in colour and up to 24 inches in diameter. Around the margin of the infected area, a greyish-white, aerial mycelium can be observed during damp weather. Leaves of infected plants are often matted together and break free easily. Injury to the turf is aggravated when the snow is compacted by walking, skiing, snowmobiling, or sledding.
Control options are the same for both diseases:
1. Avoid thatch build-up in lawns, as this serves as an ideal environment for survival and growth of these fungi. Thatch (the layer of decaying grass residues and roots between the leaves and the soil) can be reduced by core aeration and proper lawn care practices.
2. Avoid applying fertilizers with excessive quick release nitrogen to the lawn during late summer and fall, before the grass goes dormant. Too much nitrogen promotes lush growth, which will not harden off properly, and is therefore very susceptible to snow mold and other types of winter injury.
3. Cut grass to recommended heights until growth stops in fall. Excessive foliage under snow cover provides ideal conditions for snow mold fungi.
4. Make sure leaves are removed from the lawn in the fall. Avoid piling snow that you have shoveled off walkways onto your lawn.
5. In areas with a history of snow mold problems, apply a recommended fungicide in late fall just before snow cover.
6. No matter how tempted you are to start fussing with your lawn fungus, wait until the grass has had a chance to dry out before attempting any first aid.
7. Rake the area affected with snow mold and fertilize lightly to encourage new growth. Re-seed areas that have heavy damage.