Snow Mold Disease in Turfgrass

As I gaze out my office window at my front lawn I see an unfamiliar color – brown. In the midst of the ‘winter that wasn’t’, my lawn never really retreated fully into dormancy. Normally, all things considered, the lawn will stay green through Christmas and then go to sleep. Not so this year. Although I haven’t had to cut, there most definitely has been photosynthesis going on. The thermometer is now stuck at a balmy nineteen degrees and the lawn is fast asleep. Or so you would think. Really, when we use the term ‘lawn’ we are referring not just to the turfgrass that grows in that area, but rather to the entire biosystem that the turfgrass grows in. We cannot have a healthy lawn without all of the other plants and animals that reside there. For instance, turfgrass benefits greatly from the work of earthworms, which aerate the soil as they squirm around consuming organic material and discharging it once again for another organisms benefit. Fungi, for the most part, are beneficial to the lawn. They, too, feed upon the dead organic materials in the thatch and root zones. At least most of the time they do. Most of the fungi that reside in a lawn are beneficial to the turfgrass but there are certain fungi that cause disease in lawns. During the growing season, we can observe many of these disease-causing pathogens in action when we see diseases such as Red Thread, Dollar Spot, Leaf Spot and a host of others.

Diseases That Occur At Cold Temperatures

But what you may not know is that there are diseases that infect turfgrass during the winter, too. Snow molds thrive in cold temperatures and have the potential to cause quite a bit of damage if the conditions are right. There are two broad categories of snow molds, pink and gray.

Pink Snow Mold

Pink snow mold is the more common of the two, requires no snow be present for infection and is generally the lesser of the two in severity. Known by its Latin name to turfgrass managers, Microdocium Patch begins at a single point with the infection radiating outward to form a circular shaped area of blighted leaf tissue. Infections can occur at air temperatures anywhere from 30˚ to 60˚F – quite a broad range. The fungus reproduces by means of spores called conidia that are dislodged and carried along the drainage pattern of the lawn with water – either from rainwater or snow melt. This results in some curious patterns of infections in lawns that may confuse diagnosis. The good news about this pathogen is that the infection occurs on already dead tissue and that it does not infect either the roots or the crown, the growing point of the turfgrass plant. It is most certainly unsightly, though.

Gray Snow Mold

The more significant of the two snow molds, Gray, is caused by the fungi Typhula. Within that genus of fungi, two species infect turfgrasses, incarnata and ishikariensis. (Who said that taking Latin in high school wouldn’t come in handy someday?) Gray Snow Mold does require snow cover to allow infection; indeed, turfgrass managers can estimate the severity of the disease by observing the number of days that the turf has been covered in snow. The longer the snow cover, the worse the infection. Conversely, if there isn’t much snow cover then the infection of Grey Snow Mold is generally superficial and cosmetic. A handy identification sign of Gray Snow Mold is to identify matted, blighted areas in the lawn and, getting on your hands and knees and putting your nose right down to soil level, looking for the fruiting bodies of the fungus called sclerotia.

These sclerotia will appear as little fuzzy polka dots on the blighted leaves. Don’t worry, they won’t hurt you. Where we get into trouble is with infections of the species ishkariensis that occur under prolonged snow cover. This circumstance of infection often leads to infection of the crown and roots leading to death of the plant. In a home lawn setting, this will result in a patch of dead grass that can lead to a cascade of problems including crabgrass infestation.

Disease Management

If we were maintaining a golf course or sports field at short heights of cut, we would include fungicide treatments in our program to protect against infection as it would both affect the aesthetics of the turfgrass and against losing the grass altogether. On home lawns, the diseases are usually less severe and does not warrant fungicide treatment as long as you follow the mowing recommendations that we make – 2 ½” – 3” in height and frequent enough to avoid removing more than 1/3 rd of the blade. By keeping the turfgrass at a constant height of cut we avoid the tendency for the leaves to lay over and create a mat. This mat is a perfect microclimate for disease development. Further, when infection does occur, a vigorous raking that breaks up the blighted and matted leaf tissue will allow enough light and water to reach the underlying crown of the plant. Once that occurs, rapid recovery is the norm.