6 Common Fall and Winter Diseases
Lawn diseases don't always appear in the summer. There are many that prefer the cool, wet weather of fall and winter. Here is a brief background on six of the most common diseases you could face during the fall and winter and how to deal with them.
Necrotic Ring Spot
This disease can occur when temperatures are anywhere from 65 to 85ºF. Often the damage will not become apparent until the summer when the grass is stressed, even though the disease began in the fall. You will see patches of grass 6 to 24 inches in diameter, whose color varies between green, brown, and red in the outer ring. The patches also might look sunken compared to the rest of the lawn. This is because the disease causes the thatch layer to decompose more quickly. The best ways to prevent necrotic ring spot are to keep the thatch layer less than 1/2" through cultural management and dethatching, watering the lawn appropriately to avoid drought stress, and avoiding heavy fertilization in the spring. If patches do die out, obtain some grass seed and reseed the areas quickly to keep weeds from appearing. If you have an ongoing problem with necrotic ring spot in your grass, you may apply a fungicide in the fall.
Red thread tends to appear when there have been long periods of humid, wet weather, with temperatures ranging from 40 to 75 degrees F. Red thread also prefers soils with low fertility levels, especially low levels of nitrogen. The fungus begins as small pink or red strands that appear on the tips of grass leaves, which can be spread to other areas by mowing or foot traffic. As the disease continues its attack on the lawn, there will be patches of water-soaked grass, as small as an inch or as wide as a foot in diameter. Eventually these patches will bleach out and die. To prevent red thread, apply adequate amounts of fertilizer, and keep the soil pH neutral, between 6.5 and 7.0. Aerate as often as needed to maintain good soil drainage. Water your yard in the early morning, so that you don't extend the dew period and thereby increase the humidity around your lawn. If you do suspect that red thread has infected your lawn, then bag grass clippings and avoid walking across your yard until the disease has subsided. Fungicides are not necessary unless the infection becomes widespread and severe.
Stripe smut is prevalent when temperatures have consistently been 50-60 degrees F. The fungus will cause yellow-green streaks to appear on the grass blades. These streaks will then turn gray, and finally black. When the streaks are black, a black powder will also be released. Last of all, the leaves will split into several long pieces, turn brown, and die. You will notice thin, uneven growth in your lawn while this is happening. Even though the symptoms of the disease are unpleasant, the disease does not often inflict widespread, permanent damage. Watering deeply and infrequently will help prevent strip smut, as well as not watering during the evenings, which extends the dew period. Extra fertilization and irrigation can help encourage new growth and mask damage if the disease does appear in your grass. If you have dealt with stripe smut before, you may use a fungicide in the late fall or early spring to prevent it from appearing again.
Helminthosporium melting out and Leaf Spot
These two diseases are actually a chronological progression of the fungus Helminthosporium, which takes advantage of wet, humid weather and temperatures ranging from 50 to 90 degrees F. The first phase is the leaf spot. Dark red-brown or purple black spots will appear on the grass leaves, while the overall lawn becomes thin and brown. As the grass approaches the melting out phase, the crowns, roots, and rhizomes of the lawn will begin to rot and large areas of grass will die out. To prevent this disease, do not scalp the lawn. Mow it as high as is recommended. Keep the thatch layer small, fertilize and irrigate correctly, aerate when necessary, and apply organic topdressings to the soil. If you do notice leaf spot in your lawn, apply a fungicide early on, before the lawn enters the melt out phase.
Gray snow mold
Gray snow mold thrives when the temperatures are between 32 and 60 degrees F, there is snow cover on the ground, and the ground is unfrozen. Once the snow melts in the spring, you will see bleached patches of grass that are as big as 2 feet in diameter. There also might be white, cottony mold. Gray snow mold usually doesn't kill grass, but is unsightly and worth avoiding. Mow the grass shorter than usual in the fall, so there is no matted grass in the winter to encourage mold to develop. Don't over fertilize either, and keep the thatch layer 1/2" thick or less. During the winter, avoid piling snow onto the grass, or heavily compacting it. If you do find evidence of gray snow mold in the spring, try to rake over the patches and lightly fertilize the areas. This will encourage new grass growth.
Pink snow mold
Pink snow mold and gray snow mold are very similar in the way they affect lawns, although they area caused by different fungi. Pink snow mold will grow in temperatures from 40 to 60 degrees F and when the weather is moist and humid. It needs the ground to be unfrozen in order to grow, but does not require snow cover like gray snow mold does. You will notice patches that are yellowing and that are anywhere from several inches to one foot in diameter. There could also be cottony mold that is pink in color. Unlike gray snow mold, pink snow mold has the potential to kill grass. To prevent pink snow mold, you should keep the grass height shorter than usual, avoid fertilizing too much in the fall, and dethatch if the thatch layer becomes too thick. Remove leaves from the lawn quickly, so that moisture isn't trapped on the grass. Don't apply a fungicide unless you have had to deal with pink snow mold before. If this is the case, apply the fungicide late in the fall. If you notice damage in the spring, apply small amounts of fertilizer to encourage new growth.