Snow mold can be a dramatic and devastating plant disease. In the Pacific Northwest (PNW), snow molds are important in areas where snow falls on unfrozen or lightly frozen soil and persists for 100 days or more. Four different snow mold diseases, all caused by soil-borne fungi, occur in the PNW: pink snow mold, speckled snow mold, snow scald, and snow rot. Pink snow mold is the most widespread of these diseases, occurring on wild grasses, lawns, and winter wheat throughout the PNW. On wheat, however, pink snow mold is less destructive than speckled snow mold. Snow scald and snow rot are limited in distribution and their overall impact on winter wheat production is minimal.
Snow mold diseases destroy the leaves and crowns of host grasses under snow. Following snowmelt, the leaves of plants with speckled snow mold are matted to the soil and covered with a whitish gray fungal growth. The fungal growth disappears after a few days of dry, sunny weather, and numerous dark-colored bodies the size of radish seeds known as sclerotia become visible over the surface of infected plants. Depending on the specific fungi, some sclerotia are round and dark brown to black in color, while others are irregularly shaped and reddish brown, occurring primarily on roots and between sheaths in the crown.
Immediately following snowmelt, plants with pink snow mold have a whitish fungal growth covering the leaves. The fungal growth soon turns a characteristic salmon color, resulting in the name “pink” snow mold (Fig. 3). Leaves and leaf sheaths with pink snow mold remain intact and turn a light to dark brown color, as opposed to the disintegration that occurs with speckled snow mold. Disease severity ranges from relatively small lesions on leaves to complete destruction of the foliage and dead plants.
Development of snow mold is favored by rain during the autumn and snow falling on unfrozen or lightly frozen soil that persists for approximately 100 days or more. Deep snow cover insulates plants and soil, maintaining temperatures close to 32° F with relative humidity near saturation, both of which are favorable to growth of these fungi. Deep snow also ensures contact between leaves and soil, thus allowing an entry point for the fungi, while at the same time preventing photosynthesis, which is thought to make plants more susceptible to infection due to a depletion of carbohydrates in the crown. Frozen soil, intermittent snow cover, or less persistent snow cover reduce the severity of snow mold.
The fungi causing speckled snow mold survive between crops as sclerotia in soil and infested host debris. Germination of sclerotia and infection of plants begin within one month after snowfall and continue as long as snow cover persists. Invasion of crowns and death of plants occur after about three months of snow cover; consequently, damage from speckled snow mold increases with longer snow cover.
The fungus causing pink snow mold survives between crops primarily in residue from previously infected plants. Infection of leaves occurs during cool, wet weather in the fall before and after snowfall. Fungal filaments growing from infested residue near the soil surface penetrate leaves and continue to grow in infected plants as long as the snow persists.
As the snow cover retreats and damage from snow mold becomes evident, the decision of whether or not to replant is difficult for growers. The wheat will look very bad as the snow melts and may even look worse for the first few days after the snow is gone. Cold conditions after snowmelt can further weaken the wheat plants. Warm conditions hasten the decomposition of the badly affected leaves. It takes two to three weeks for the stressed plants to show signs of recovery.
Severe snow mold years are generally good moisture years. The season is delayed, warming up later than usual, hence the long snow cover. Wait two weeks after the snow has left most of the field, then carefully survey the field. Small patches of dead wheat may not be worth replanting. Large acreage may require further consideration.
- If the wheat is totally dead, then the replant decision is a little easier. There should be good moisture and fertilizer remaining in the field, although starter fertilizer may help as last year’s fertilizer may have leached a foot or two through the soil profile.
- If there are more than eight plants per square foot on average, it may not pay to replant.
- Four to eight plants per square foot require careful consideration. How soon can you work the field? What does the long-term weather look like, and how well does spring wheat normally yield in the field? What seed can you get? What price will wheat bring? Then comes the balancing act. Will the reduced yield from the damaged winter wheat cover the costs already invested in the field? Or will the costs of replanting and the expected increased harvest bring a better bottom line?
Excerpted from “Snow Mold Diseases of Winter Wheat in Washington,” WSU EB1880, by Timothy Murray, Stephen Jones, and Ed Adams. For more information, or to download the document, go to http://tinyurl.com/PNWsnowmold.