PULLMAN, Wash. – A blanket of snow protects winter wheat from freezing, but significant snow cover for too long on unfrozen ground can lead to a disease problem that farmers don’t want: snow mold.
Snow mold is a fungus-caused disease of wheat and other grasses, including lawns. The disease can cut wheat yields by 20 percent to 40 percent when severe. The fungus destroys the leaves and crown beneath the snow, according to Tim Murray, chair of the plant pathology department at Washington State University.
In Washington, snow mold shows up during severe winters in north central Washington – Chelan, Douglas, Lincoln, Okanogan and Stevens counties. It’s not normally a problem in the Palouse because snow cover does not persist long enough.
“We normally need about 100 days of snow cover with unfrozen soil for speckled snow mold to be a problem,” Murray said. “We’re approaching that in our snow mold area.”
Based on one report from a grower north of Wilbur, there’s anywhere from two- to six-feet of snow that’s approaching 90 days of cover,” Murray said. “There was some frozen soil underneath, so it’s hard to say how severe the disease will be.”
Many different fungi are capable of causing snow mold disease, but not all are found in Washington. “The three we have in Washington like soil that is not frozen under the snow, or if the soil is frozen, not frozen too firmly and will defrost under the snow.
Signs of the disease appear just after snowmelt. “Within a day of snow melt, plants will be covered with a mildew or sort of a light colored cobweb,” Murray said. “After a few days, the plants will be gray colored and look dead if the disease is severe. If it is less severe, some green leaves may be seen. As the plants dry after snow melt, small black structures that cause a speckled appearance become visible.
Fall-applied fungicides can control the disease, but are not cost-effective for wheat growers because of the relative low value of the crop and uncertainty of the occurrence of the disease, Murray said. Bred-in genetic resistance is regarded as the best option.
Some current wheat varieties, including Eltan and Bruehl, have snow mold resistance, “but we’re always looking to make improvements,” Murray said.
For the past 10 years, he has been collaborating with scientists at Japan’s National Agricultural Research Center for Hokkaido to develop what varieties that have improved resistance to snow mold.
Murray and his Japanese collaborators have been testing wheat lines for snow mold resistance in plots near Waterville and Mansfield and on the island of Hokkaido near Sapporo, where snow mold is a significant program. The lines they have tested in the field are the most promising graduates of earlier tests in growth chambers at WSU’s Plant Growth Facility in Pullman, where winter conditions can be simulated inside.
Zenta Nishio, one of the Japanese researchers, developed a set of PCR primers that are used in the lab to evaluate snow mold resistance. PCR is a laboratory technique that allows scientists to detect DNA specific sequences of an organism’s genes.
The scientists have evaluated 100 progeny lines from a cross of highly resistant wheat from Switzerland and very susceptible wheat from The Netherlands and are now developing a genetic map that should help wheat breeders identify resistant plants more quickly.
“It could save breeders years,” Murray said. “You can’t count on getting a field test every year, especially in Washington. That’s part of the reason we send our lines to be field-tested in Japan. They get the disease more frequently. Even so, it takes 12 months to go through one round of testing. With our controlled environment tests and markers, we can do multiple rounds of testing in a year.”
The research has been funded by the Washington Wheat Commission.