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Wheat streak mosaic worst in decades

wheat streak mosiac
WHEAT STREAK: Infestations of wheat streak mosaic killed some fields of wheat in Scott and Lane counties last fall. Now the disease is back. In central and eastern Kansas, barley yellow dwarf virus has wheat growers worried.
Commentary: After a rare appearance in the fall, wheat streak mosaic is back in full force in western Kansas.

By Vance Ehmke

If drought, poor stands and a collapsing grain market weren’t enough, now we’re looking at one of the worst wheat streak mosaic virus infections I’ve ever seen in a number of western Kansas counties.

And while wheat streak mosaic virus has traditionally been a western Kansas problem, it can occur in central and eastern Kansas as well. But even without wheat streak, early reports from those regions suggest wheat farmers there are looking at a pretty serious problem with barley yellow dwarf virus.

Kansas State University Extension plant pathologist Erick DeWolf explains that the wheat streak mosaic virus is transmitted from infected volunteer wheat to the newly planted wheat by way of windblown microscopic wheat curl mites. Usually symptoms don’t show up until in the spring when wheat starts jointing. It is very unusual to see symptoms in the fall.

However, this past fall in at least one Lane County field, the symptoms not only showed up, but also the infection was so severe that the wheat crop actually died. In addition, severe infections were also reported in Scott County last fall. Because of the visible fall infections, KSU agronomists and pathologists were bracing themselves for major problems to show up this spring.

After noticing declining conditions in some fields along with yellowing of plants over the past few weeks, DeWolf asked if I’d pull some plant samples from various fields and varieties and send them to him. I sent in about 10 wheat plant samples from western Lane County — and was shocked to find that all 10 were infected with wheat streak mosaic virus while some also were infected with High Plains virus. To me this looks like the worst viral disease situation I’ve ever seen in over 40 years of farming.

DeWolf says the KSU Plant Disease Diagnostic lab has received more samples of wheat with SWM than is normal for this time of year. “This level of activity early suggests we are likely to see a major problem with WSM this year. It is too early to tell the full extent of the problem, but some areas of the state are headed for trouble.”

In addition, he says he has heard reports of WSM in parts of Texas and sporadic reports of the disease in Oklahoma. So far in Kansas most reports are from southwest and west-central Kansas.

Why are we having the problem this year? It all traces back to the very large amount of volunteer wheat we had last fall. Volunteer wheat is the intermediate host for this and other viral diseases. If you or your neighbors don’t kill the volunteer wheat, you’re asking for trouble. And in addition to the large amount of volunteer, we also had a lot of windy weather and an extended fall. Our first hard killing frost was much delayed, as it was the previous fall.

Here on our farm in previous years I’ve seen yield losses of near 100% on fields adjacent to badly infected volunteer. DeWolf says yield loss is generally related to the timing of infection, with plants that are infected early having the greatest yield loss. “Plants already showing severe symptoms right now could have more than a 70% loss. Fields that are hit hard by the disease with most plants already infected may be a total loss.”

He also points out that the virus may continue to spread as the mites that carry the disease move with the wind. Plants that are infected later or after heading may show symptoms of yellow discoloration on the leaves but generally do not experience the same level of yield loss as plants infected as seedlings or prior to jointing.

So if you have a field of volunteer that was not killed last fall or was killed very late in the fall, how much damage could that single field do? “The wheat curl mites that spread the virus are moved by wind. The mites can move 1 to 2 miles in many situations. Thus, a single field of unmanaged volunteer wheat during the summer can affect wheat fields for miles around,” DeWolf says.

He also says uncontrolled volunteer wheat is the major host for the wheat curl mites and the disease. “Still, other grasses can be a host for the disease. Common grasses like jointed goat grass and green foxtail are also important hosts. On the other hand, common native grasses or CRP [Conservation Reserve Program] grasses like smooth big bluestem, little bluestem and indiangrass are not good hosts for the mite or disease.”

While the mosaic situation is shaping up to be a big problem in western Kansas, Jim Shroyer, KSU Extension wheat specialist professor emeritus, is also projecting a serious problem with another viral disease in central and eastern Kansas. “As in western Kansas, there was a lot of volunteer wheat last fall. Plus, those regions also had a very mild and very long fall. That, combined with an abundance of aphids, has set growers there up for serious problems with barley yellow dwarf virus this spring,” Shroyer says.

He points out, too, that if you have wheat infected with wheat streak mosaic, you should pray for wet and cool conditions. “Any stress you can take off of the plant will only help. However, if we do get the wet conditions, you need to be on the lookout for problems with foliar diseases like leaf and stripe rust. It never ends, does it?”

Ehmke is a wheat seedsman who farms in Lane County.

TAGS: Wheat
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