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Best Time to Scout for Wheat Streak Mosaic Virus
South Dakota Ag Connection - 01/12/2018

Due to wheat streak mosaic disease showing up in wheat fields across South Dakota in 2017, many wheat growers wonder if they need to worry about the disease spreading this winter and wheat streak mosaic virus showing up in their fields spring 2018.

"Since winter wheat is dormant throughout the winter months, and the wheat curl mites which transmit the disease are only active during the growing season, no further spread of wheat streak mosaic virus will occur this winter," said Emmanuel Byamukama, Assistant Professor & SDSU Extension Plant Pathologist.

Wheat streak mosaic virus can result in severe yield losses and sometimes an entire wheat field can be lost when wheat is sprayed out and a different crop planted in spring.

Byamukama explained that wheat growers will need to begin scouting their fields for the disease as temperatures warm up this spring and wheat curl mites resume activity.

"The best time to send a sample confirming wheat streak mosaic virus, is spring once wheat has resumed growing," Byamukama said. "It is important to confirm presence of wheat streak mosaic virus before management decisions such as spraying out wheat and planting something else are made."

Byamukama added that general yellowing of plants should not be solely taken as indicator of wheat streak mosaic virus as other factors such as nitrogen deficiency, chloride deficiency and water logging can cause wheat plants to look yellow.

When taking samples of wheat for wheat streak mosaic virus testing, obtain at least five samples on a transect across the field in the direction of the prevailing wind.

"This will help to gauge the extent of wheat streak mosaic virus spread across the field," Byamukama said. "I want to remind growers that spring infections of wheat streak mosaic virus cause mild grain yield loss. It is the infections that take place in the fall, that are the most damaging to wheat."

What can wheat growers do about wheat streak mosaic virus?

Once plants are infected with a viral disease, like wheat streak mosaic virus, Byamukama said nothing can be done to 'cure' the plants of the virus.

"Wheat streak mosaic virus management requires pre-planting practices that prevent/limit infection from taking place in the fall, he said. "The best practice to manage is to destroy the volunteer wheat and grass weeds, or vegetation I like to refer to as the "green bridge," 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 Byamukama encouraged growers to consider, is to delay planting in fall, especially following a year when wheat streak mosaic virus was widespread in an area.

"Once wheat plants escape fall wheat streak mosaic virus infections, infections that happen in spring do not result in significant yield loss," he said.

There are a few cultivars which are resistant or tolerant to wheat streak mosaic virus. Learn more about these, by reviewing the 2017 South Dakota Winter Wheat Variety Trial Results for ratings for wheat streak mosaic virus (WSMV) at this iGrow link.

Wheat streak mosaic disease is caused by viral pathogen called wheat streak mosaic virus. This virus is transmitted by microscopic mites called wheat curl mites. Wheat curl mites can only be seen under magnification.

Wheat curl mites are carried from field to field by the wind. Wheat curl mites are not capable of moving on their own since they do not develop wings but can crawl to neighboring plants.

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

Insecticides are not effective against the wheat curl mites mainly because the mites are protected from exposure to insecticides as 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.

Wheat curl mites remain the major source of wheat streak mosaic virus.

Neighboring pasture grasses can be a source of the wheat curl 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 a few wheat plants along the field edges will be affected.

Larger epidemics of this disease happen when the inoculum is from within the field or when wheat curl mites are blown in from a neighboring wheat fallow during fall shortly after planting.

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.

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