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In this issue:

Snow Molds on Turfgrass

 

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Snow Molds on Turfgrass

Most plant pathogens shut down during cold winter temperatures, but there are exceptions.  There are two cold-loving “snow mold” winter diseases of turfgrass that can develop.  The two diseases are called gray snow mold (Typhula blight) and pink snow mold (sometimes called Microdochium Patch or Fusarium Patch).  Snow molds are most likely to occur when snow falls on ground that has not yet frozen, and remains for weeks or months, like we have seen in western Kansas this year. The lush turf under the snow is an excellent nutrient source for the fungi.  The diseases cause damage on home lawns as well as golf courses, with putting greens as a particularly vulnerable site.  Both diseases can occur together at the same site (Fig 1 shows both diseases together).

Gray Snow Mold is a “true snow mold” because it primarily grows under the snow at temperatures just above freezing, and requires snow cover of at least about 2 months.  It is caused by several closely-related fungi in the genus Typhula. It causes bleached areas about 2 feet across.  After the snow melts the diseased turf is matted down, sometimes with white to gray fungal mycelium if turf is wet.  Unlike pink snow mold, gray snow mold usually produces sclerotia (light-brown to black, hard, pea-sized structures) in the turf (Fig 2).  The sclerotia are the survival structures where the fungus spends the summers.  Gray snow mold often occurs repeatedly at the same site. 

Pink Snow Mold is caused by the fungus Microdochium nivale.  Like gray snow mold it causes circular, bleached patches in the turf that are up to about 2 feet across (Fig 1).  When the turf is wet, the fungal mycelium is sometimes visible and is whitish but sometimes has a pinkish color (which is the source of the name of the disease). 

The fungus survives in plant tissue and debris.  It becomes active during cold, wet weather (32-60 F) and goes dormant when conditions are warm and dry.  Unlike gray snow mold, pink snow mold does not require snow cover.  It can develop if conditions are simply cold and wet, like in foggy or drizzly weather, or alternating periods of repeating frosts.  This snow-free phase of the disease is sometimes called Microdochium Patch (formerly called Fusarium Patch)  (Fig 3).

Cultural Control:

For both diseases, cultural practices in the fall can reduce disease pressure.

Autumn:

• Keep mowing the grass as long as it is growing.  Overly tall turf (more than 3 inches) will be matted down under the snow, making a favorable environment for the fungi.

• Don’t fertilize too late in the fall.  Late applications of N will over-stimulate the turf, making a lush growth that is a susceptible to snow molds.  Late growth will also interfere with winter hardiness.

•Rake leaves in the fall.  A layer of fallen leaves will contribute to the matting down and moisture build-up.

• Golf Courses: Use snow fences and windbreaks to prevent deep buildup of snow on golf greens

Spring:

Rake lawns to air out the turf and promote drying.

Shovel the snow off of susceptible areas like golf course putting greens.  Alternatively, you can spread something dark (like a humate product) on the snow surface to enhance melting.

Chemical Control:

Home lawns: Fungicides are not recommended for home lawns except in extreme situations.  Though symptoms look dramatic, the turf will recover eventually.  Use cultural practices described above.

Golf course/commercial:  For gray snow mold, fungicides must be applied in the fall before the first significant snow.  Fungicides in the spring will not have an effect, because the disease has already been active all winter.  For pink snow mold, spring fungicides can sometimes have an effect.  Remember, unlike gray snow mold, pink snow mold does not require snow and can grow if conditions are simply cold and wet.  Spring fungicides may reduce such development.

For pink snow mold, fungicides with the active ingredients fludioxanil, iprodione, PCNB, propiconazole, pyraclostrobin, thiophanate methyl, and trifloxystrobin have been reported to provide good to excellent control.  PCNB can cause phytotoxicity, though, especially on creeping bentgrass, and this chemical is under review by EPA and might not be available in the future.

For gray snow mold, some available products include those with PCNB, triadimefon, iprodione, flutolanil, or fludioxanil as active ingredients. (Kennelly)

Photos taken from:  http://www.uoguelph.ca/~thsiang/snow/snowmold.htm

More photos and information for commercial turf are available here:
Gray snow mold: http://www.ces.purdue.edu/extmedia/BP/BP-101-W.pdf

Pink snow mold: http://www.ces.purdue.edu/extmedia/BP/BP-102-W.pdf

Will snow mold be a problem in wheat in 2007?

The answer to the above question is probably not, but it is possible.  In conferring with Dr. Bob Bowden, USDS-ARS plant pathologist, who has been in Kansas since 1989, he only remembers one year in the early 1990's that snow mold was detected.  In that year, it was found in only low levels around the state with the exception of one severely infected field in the Victoria area.

There are several different pathogens that can cause snow mold disease, but Microdochium (synonym Fusarium) nivale, the cause of pink snow mold, is the one most likely to be found in Kansas according to Dr. Bowden.

Snow mold is most apparent when the snow first melts.  It is very typical for the disease to occur in distinctly circular patches (see image on left). At this time, the pinkish-colored mycelium may still be evident.  Small brown-black fruiting structures known as sclerotia may also be present. Some leaves may be completely dead, while others may be only partially wilted or dead (see image on the right).
 

Images from the American Phytopathological Society

Despite extensive leaf destruction, wheat plants often recover and produce satisfactory yields if crowns are not damaged.  Stands that are thinned by disease may be more susceptible to weed infestations.  Dry, warm weather after snow melt will favor plant recovery, whereas cool, damp weather permits the damage to progress. (Jardine)

Additional information can be found at the following links.
Snow Mold, Washington State University
http://pnw ag.wsu.edu/smallgrains/Snow%20Mold.html

Wheat - Snow Molds and Snow Rots, Oregon State University
http://plant disease.ippc.orst.edu/disease.cfm?RecordID=1148

Snow Mold-Plant-Antagonist Interactions: Survival of the Fittest under the Snow
http://www.apsnet.org/online/feature/snowmold/



 

Contributing authors:

Megan Kennelly ( kennelly@ksu.edu )

Doug Jardine ( jarding@ksu.edu )

Judy O’Mara  ( jomara@ksu.edu )