The winter of 2016-17 has been unusually long compared to the past few years, and the prolonged snow cover has raised concerns over potential for snow mold development in eastern Washington.
On the Waterville Plateau in Douglas County where snow mold of wheat has been a chronic occurrence since the 1940s, snow has been on the ground for nearly 100 days since sometime in early November.
In Pullman, near the opposite corner of wheat producing area, snow has covered the ground continuously since December 6, or about 60 days. Snow cover is like putting a blanket on your bed – it keeps the warmth in and the cold out.
For the wheat crop, a blanket of snow cover provides a layer of insulation that helps wheat plants survive the winter by protecting them from bitterly cold temperatures and wind that can result in winterkill. However, if the snow stays too long, the potential for damaging snow mold can occur.
Snow mold is a generic name for any one of several diseases that can develop on wheat under snow.
The pathogens that cause these diseases are specialized and able to grow in the wet, near-freezing environment under the snow and destroy most of the aboveground foliage. In eastern Washington, snow mold can be caused by one of four different fungal or fungal-like pathogens; however, speckled snow mold and pink snow mold are the most common and destructive.
Speckled snow mold is the most destructive disease in eastern Washington and needs about 100 days of continuous snow cover with unfrozen soil before damage is likely to occur. This disease is most common in the wheat-growing area north of highway 2 beginning about Almira west to Waterville.
Pink snow mold can occur alongside speckled snow mold, but is favored by slightly wetter conditions and usually doesn’t cause damage to wheat over large areas. Pink snow mold is more widespread than speckled snow mold because it doesn’t require as much snow cover to develop, so it’s also common in golf course turf and home lawns.
There is another snow mold fungus that resembles the speckled snow mold fungus that occurs widely in eastern Washington; it is often called gray snow mold or Typhula root and crown rot because it often develops below ground and is distinguished by the reddish-colored speckles that develop on infected plants. This disease causes little damage to wheat, but can be very destructive to winter barley.
Most growers in the traditional snow mold area know that planting a resistant variety early is the best way to control snow mold. Outside of the Waterville Plateau, pink and gray snow mold likely will be apparent in some spots when the snow melts.
However, they are unlikely to cause significant damage where snow cover was less than about 8” deep because the near-zero temperatures we experienced in late December and early January will have frozen the soil, which limits disease development. It is also likely that there will be some leaves killed by the cold temperatures.
To determine whether snow mold was the cause, look closely at the dead plant parts for evidence of a cob-web-like growth on the surface. With pink snow mold, plants will have a salmon-pink color soon after snow melt; the color will fade leaving brown-colored tissue. For the speckled snow molds, evidence of disease is the presence of dark-colored survival structures known as sclerotia.
Plant parts killed by cold may appear light brown or bleached without any of the signs of snow mold. Samples can also be submitted to Plant Pest Diagnostic Clinic for confirmation.
For more information, consult extension bulletin Snow Mold Disease of Winter Wheat in Washington (EB1880) available under publications in the Disease Resources section of the WSU Wheat and Small Grains website.