Farm Progress

Plant pathologist Emmanuel Byamukama answers pressing questions on the wheat virus.

February 1, 2018

3 Min Read
GREEN GOOD: Winter wheat may show signs of wheat streak mosaic virus this spring. Green plants are a sign all is well. Yellowing plants might mean there’s an infection.

Wheat streak mosaic virus was a severe problem in parts of South Dakota last year. Will it be a problem again this spring?

Maybe, says Emmanuel Byamukama, South Dakota State University Extension plant pathologist,

“We had some mild conditions in the fall going into winter, and the fact that wheat streak mosaic was widespread last season, we might see some WSMV in spring. It will also depend on the temperatures in spring. If it warms up early, this will encourage early symptoms of WSMV. Symptoms seen in early spring are a result of infections that happened in fall.”

Byamukama recommends scouting fields and sending samples to the SDSU diagnostic clinic for confirmation.

“General yellowing of plants should not be solely taken as indicator of wheat streak mosaic virus,” he says. “Other factors, such as nitrogen or chloride deficiency and water logging, can cause wheat plants to look yellow. When taking samples of wheat for testing, obtain at least five samples on a line across the field in the direction of the prevailing wind. This will help determine extent of wheat streak mosaic virus spread across the field.”

Byamukama provided the following answers to questions about WSMV, many of which were asked at the Ag Horizons Conference in Pierre, S.D., through Dec. 4-6.

What is wheat streak mosaic disease?
It is a disease caused by a viral pathogen called wheat streak mosaic virus, which is transmitted by transmitted by microscopic mites called wheat curl mites. The mites can an only be seen under magnification. They are moved from field to field by wind. They do not develop wings but can crawl to neighboring plants.

Can wheat curl mites survive the winter?
Wheat curl mites can survive bitter cold winter temperatures. They overwinter as eggs, immature mites or as adult mites. The adult mites reside near the growing point of a wheat plant and can be insulated from low temperatures, especially under snow cover.

Can insecticides be used to control wheat curl mites?
Insecticides are not effective against wheat curl mites mainly because they inhabit the inner whorl of the leaf near the growing point of the plant. Insecticides used to control other types of mites, such as spider mites, will not control wheat curl mites.

Could neighboring pasture grasses be a source of inoculum?
Neighboring pasture grasses can be a source of the mites and the virus. However, the preferred host for the vector and the virus is wheat. Therefore, while pasture grasses may serve as a source of inoculum, this inoculum is limited, and usually only a few wheat plants along the field edges will be affected. Larger epidemics of this disease happen when the inoculum comes from within the field, or when wheat curl mites are blown in from a neighboring wheat fallow field during the fall shortly after winter wheat is planted.

Can it spread through seed?
Seed is not an important source of inoculum for wheat streak mosaic virus. Although research has shown that a low percentage (<0.5%) of seed harvested from infected plants can transmit wheat streak mosaic virus, the infected seedlings do not lead to significant wide spread disease within the field. Wheat curl mites remain the major source of wheat streak mosaic virus.

What can be done?
Once plants are infected with a viral disease, nothing can be done to “cure” the plants of the virus. The best way to control wheat streak mosaic virus is to destroy the volunteer wheat and grass weeds in the field to be planted to winter wheat at least two weeks before planting. Wheat curl mites cannot survive more than 48 hours without a living green tissue to feed on. The second practice is to delay planting in fall, especially following a year when wheat streak mosaic virus was widespread in an area. Infections that happen in spring do not result in significant yield loss. A few cultivars are resistant or tolerant to wheat streak mosaic virus. See wheat variety trial for information on a variety’s resistance.

SDSU Extension Service contributed information for this article.

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