Don’s 3,000-acre mixed farm can be found south of Weyburn, Sask., near the U.S. border. He called me last June after he discovered patches of yellowing durum wheat plants in his field.
Don had sprayed the crop at the three-leaf stage with a tank mix of Groups 1, 2 and 4 herbicides to control grasses and broadleaf weeds. Now, two weeks later, he was concerned about patches of injured plants scattered across his field.
“After I sprayed my durum, the crop seems to be set back and yellowing in patches,” he told me. I immediately set out for Don’s farm, as I was concerned the plants had suffered some form of herbicide injury.
From the road, the field looked much worse than it was, because most of the yellowing plants were, in fact, the dying wild oats. The herbicide tank mix Don had sprayed had worked well.
Overall, I estimated about 10 per cent of the wheat plants were affected, while the majority were healthy.
Up close, I observed the wheat plants’ leaf margins were yellowing, and some, to a lesser degree, had a reddish-purple tinge. The plants’ leaf veins and mid-ribs remained greener than the rest of the leaves.
I found the yellowing wheat plants in patches, scattered throughout the field, and injured plant patches were found next to healthy ones.
My initial concern was Don’s wheat had suffered a herbicide injury, but I ruled this out at once because the damage occurred in patches scattered randomly throughout the field, and only a small percentage of the crop was affected.
Because Don’s fertility program was adequate for the crop’s needs, I also eliminated a nutrient issue as a possible cause of the yellowing leaves. In addition, the region’s growing conditions this season, so far, were good, so I didn’t believe any environmental stressors were affecting the crop, either.
However, these symptoms — the leaf yellowing, low percentage of affected plants, and restriction of injured plants to patches scattered randomly throughout the field — were textbook for a certain wheat disease.
“Do you know if you have any winter wheat crops nearby?” I asked.
“Sure is,” said Don. “There’s a neighbouring field just west of here.”
That confirmed it for me.
Crop Advisor’s Solution: Avoid seeding durum near WSMV-infected winter wheat
That clinched it. The symptoms presented in Don’s durum wheat crop were caused by wheat streak mosaic, which is a viral disease of wheat. It was coincidence Don checked his wheat field for herbicide efficacy at the onset of wheat streak mosaic disease symptoms, which, at that stage, looked like herbicide injury.
The wheat streak mosaic virus is transmitted by the wheat curl mite. The mite is small enough to be blown from one field to another by wind. The mite and the virus need living plant material to survive, and winter wheat is one of their favourite hosts for overwintering. They can also survive through the winter on volunteer cereals and perennial grasses.
The virus enters the plant through the leaves, destroying chlorophyll. Often, the chlorosis caused by the virus results in light green or yellow streaks in the plant leaves. The virus spreads to all plant parts. And the earlier the infection, the greater the plant damage.
In the spring, surviving mites multiply rapidly and are blown by the wind to neighbouring fields of spring wheat, again providing living hosts for both mites and viruses. If winter wheat is sown near fields infected with the disease, mites can carry the virus to the winter wheat crop, completing the disease cycle.
To confirm wheat streak mosaic virus in Don’s field, I sent plant samples to the Crop Protection Laboratory in Regina. Lab results indicated the virus was present in the plants. I also knew there were several confirmed cases in the area.
Currently, there are no pesticides for mite control, and no form of chemical control for the wheat streak mosaic virus.
In Don’s case, because the percentage of infected durum wheat plants was low, yield loss was minimal. However, planting spring wheat near infected winter wheat puts the crop at greater risk of infection. Thus, avoid seeding cereals near an infected winter wheat crop. In addition, it is important to control volunteer cereals in the fall and spring prior to planting to reduce the risk of infection.
Ultimately, a green bridge is the biggest contributing factor to increased infection risk. Eliminating green bridges, even by a week or two, helps lower the risk of infection because mites and viruses can’t survive without living hosts.
Early seeding of cereals also decreases the risk of infection as mite populations only increase with rising temperatures in the spring. Therefore, early seeding also helps prevent early viral infection and greater yield losses.
Bethany Wyatt works for Richardson Pioneer Ltd. in Weyburn, Sask.