Now is the time to prepare fields to be planted to winter wheat by controlling grassy weeds and volunteer cereal crops, says Joel Ransom, North Dakota State University Extension Service agronomist for cereal crops.
"Killing plants that harbor the wheat curl mite at least two weeks prior to planting winter wheat is the only practical way of controlling the wheat streak mosaic virus that is vectored by this microscopic mite," Ransom says.
According to Marcia McMullen, NDSU Extension Service plant pathologist, WSMV is transmitted by wheat curl mites when they move from infected plants to feed on newly emerging winter wheat plants. These mites are able to move only short distances and need actively growing grasses to survive and reproduce.
"If the WSMV is passed to the winter wheat crop in the fall, it will overwinter on the crop," McMullen says. "Not only can the winter wheat crop be devastated by this disease, an infected crop can serve as a source of infection for cereals planted in the spring. The chances that WSMV will be problematic is very high if the green bridge is not broken, as WSMV and wheat curls mites commonly are found in grassy plants, even those not showing symptoms, at this time of the year in North Dakota."
Kent McKay, Extension Service area specialist at the North Central Research Extension Center in Minot, identified three reasons why WSMV was damaging winter wheat fields in north- central and northwestern North Dakota in 2007.
The first, farmers that had few volunteer plants in their fields, as low as one plant in 100 square feet in one case, did not apply a burn-down herbicide because it was erroneously presumed that a low density of volunteers could not effectively harbor the mites that transmit the virus.
The second occurred when farmers applied glyphosate the same day they planted their winter wheat. In this case, dying volunteers were able to support the mites until the winter wheat emerged, at which time the mites moved to the winter wheat.
The third case involved planting winter wheat into canola residue without a burn-down herbicide application. Unfortunately, there were sufficient mite-infested volunteer wheat plants from the previous wheat crop in the canola residue to result in the infection of the emerging winter wheat. This crop had to be destroyed in the spring.
"The take-home message from these experiences is that all fields to be planted to winter wheat should be sprayed with glyphosate at least two weeks before planting, no matter what the previous crop or the level of volunteer growth present," McKay says.