Utah Pests News Spring 2008

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Watch for “Moldy” Turf This Spring

 
  Pink snow mold can have a margin of pink mycelium that give this disease its name.
With all our snow this year, get ready for snow mold to appear in residential lawns and turf areas of parks, businesses, and golf courses. Snow molds are fungi that are psychrophilic, or cold-loving, and will attack plants under a layer of snow. Two of the most common snow molds are pink snow mold, caused by the fungus Microdochium nivale, and gray snow mold, caused by Typhula incarnata and T. ishikariensis. Both pink and gray snow molds can occur together and almost all turfgrass species are susceptible to both diseases.

Pink snow mold will survive unfavorable conditions in infected plants and debris of previously infected leaves. The fungus will grow slowly at first, allowing infected turf to go undetected for a long period of time. Once conditions are favorably wet and overcast with cool temperatures, the fungus will spread very rapidly. Sunny and dry conditions cause the pathogen to become inactive. Snow cover is not a requirement for pink snow mold. The disease can occur at any time of the year during cool temperatures and high humidity.

   
 
  Gray snow mold typically appears as tan to brown patches on turf, with a layer of mycelium on the leaves.
   
In contrast to pink snow mold, the fungus causing gray snow mold will survive unfavorable conditions as sclerotia (overwintering structures about the size of a pepper grain in this case) during the summer and will germinate in fall once they have been exposed to cool, wet conditions. As the leaves decompose in spring, sclerotia will fall into the thatch layer of the turf. Therefore, the disease tends to appear in the same areas year after year when conditions are favorable. Severe disease outbreaks typically only develop in areas where there is persistent snow cover in the winter.

Pink snow mold spreads slowly when the humidity is low or when there is very little moisture present on the surface of the turf. However, it spreads more rapidly and is more severe in turf that is growing slowly and has a thick layer of thatch. This usually occurs when the weather is cool and wet and when a layer of snow or heavy mulch covers unfrozen turf. The disease is also favored by poor drainage and matted grass, creating pockets of turf where the humidity is higher. Gray snow mold is similar to pink snow mold in that the disease is most severe on unfrozen turf under a layer of snow or heavy mulch. Deep snow is particularly favorable for the development of this disease; the longer the snow cover persists, the more time the pathogen has to be active. The deep snow prevents soil from freezing, increases the local relative humidity, and mats down the turf.

Symptoms

 
  Newly-formed sclerotia of Typhula (gray snow mold).
As the snow melts in spring, turf that is infested with pink snow mold may appear bleached in color, looking very similar to infection by gray snow mold. But pink snow mold begins as a small, orange to reddish-brown circular patch that changes in color to a light gray, and may grow to 8-12 inches in diameter. There may also be a faint growth of white or light pink mycelium at the patch edges, but the pink color is usually only noticeable in early daylight hours. Grass in the center of the patches may start to recover from the disease while the pathogen continues to invade the edges, which creates a “frogeye” symptom. Under long periods of low temperatures and leaf wetness, the patches can combine to create large areas of blighted turf, and small clusters of pink spores may be produced on the surface of the leaves.

 
  A severe snow mold infection on golf course turf.
   
The symptoms of gray snow mold are a little different. Once snow melt has begun, areas of light yellow, straw-colored turf will appear. The leaves are usually matted down and covered with either a thick or thin layer of white to gray mycelium. As the grass dries, the mycelium will dry out and disappear, turning the leaves gray or silver. Because the crown of the plant is not affected, new leaves will be produced during the spring. A characteristic feature of gray snow mold fungi is the production of sclerotia. These small sclerotia form on infected leaves and are often pink, white, or amber in color when they are young. As they mature, the color darkens to a reddish brown, dark brown or black.

Treatment

Management practices are similar for both pink and gray snow molds:

  • Avoid over-applying nitrogen fertilizers late in the growing season. This may facilitate disease by causing the development of succulent leaf tissue. 
  • Do not leave the grass uncut at the end of the season. In the fall, the cutting height should be 20% higher than previous cuttings. This will allow better winter survival for the turf. 
  • Avoid extreme thatch buildup.
  • Prevent large snowdrifts from forming by using snow fences, windbreaks or other types of barriers. 
  • Apply a low rate of fertilizer in the spring to promote new growth. 
  • Fungicides can be useful in controlling disease when applied in the fall, but are not as effective as applications in late winter or early spring. Fungicides for gray snow mold must be chosen carefully because of differences in efficacy between the different species. Chemicals containing PCNB (pentachloronitrobenzene) have worked well in the past but the future looks grim for this chemical option as it is scheduled to be phased out for use on turf in the near future. 
  • For pink snow mold, it is important to maintain low soil pH and balanced soil fertility. We all know how problematic that can be here in Utah with our high-pH soils. Acidifying fertilizers, such as ammonium sulfate, work to lower pH.

 

-Erin Frank, Plant Disease Diagnostician (No longer at USU)