BROOKINGS, S.D. -- The majority of winter wheat in South Dakota is at the flag leaf stage with some fields already flowering. Inspection of winter wheat fields in Douglas, Charles Mix, Tripp, Potter, Hughes, Brookings and Lincoln counties indicate a few fields with significant disease development in the lower canopy. One of the diseases found at high levels in some fields was Wheat streak mosaic virus (WSMV).
Symptoms of WSMV ranged from mild for several fields to severe stunting, says Emmanuel Byamukama, South Dakota State University Extension plant pathology specialist.
Previous surveys indicated that WSMV is the most common virus detected in wheat in South Dakota. WSMV and two other viruses also detected in South Dakota wheat fields -- Triticum mosaic virus (TriMV) and High Plains virus -- all have the same vector -- the wheat curl mite (WCM) -- Byamukama explains.
"Wheat curl mites are very small and can only be seen with magnification. These mites feed and reproduce in the rolled wheat leaf blades and are favored by warm and dry conditions," he says. "During the summer, as wheat matures, they move to the exposed leaf surface and are blown by wind to other hosts including corn, sorghum, grass weeds and volunteer wheat."
Viruses transmitted by wheat curl mite produce similar symptoms, explains Byamukama, so it is hard to tell them apart based on visual symptoms. These symptoms can be confused with herbicide injury.
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"Normally, herbicide injury initially causes yellowing and eventually the plants die," he says. "Herbicide injury should also affect plants uniformly in the field or at least according to a drift or spray pattern. Virus infection does not kill the plants, and infected plants may either have a gradient (symptom severity decreases from the edge of the field) or may be scattered throughout the field.
Virus infected plants are usually stunted, produce fewer tillers and the tillers grow close (prostrate) to the ground."
Unfortunately, Byamukama says once virus infection has taken place, there is nothing that can be done to manage the disease. For viruses transmitted by the wheat curl mite, which are the most predominant and damaging, infections in fall are the most important, in terms of causing significant yield loss.
Manage viruses in wheat fields
Byamukama says wheat curl mite-transmitted viruses can be effectively managed through elimination of volunteer wheat and grassy weeds.
He explains that the wheat curl mite survives from one season to another on the so called "green bridge" -- the term given to volunteer wheat and grassy weeds.
"Therefore, destroying this green bridge between seasons will control WCM and ultimately WSMV. Planting of wheat in fall should be delayed until late September to early October or at least two weeks after volunteer wheat and grass weeds have been sprayed with a herbicide," Byamukama says.
Another effective management strategy for viral diseases is planting host resistant or tolerant cultivars. Evaluation for WSMV resistance among the common winter wheat cultivars has been ongoing at SDSU. Evaluation of the cultivars includes symptom severity, stunting, delay in heading, yield loss, and loss in test weight.
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Other wheat diseases found
The majority of fields scouted had minimal to no disease developing on wheat. The other diseases found include: tan spot, powdery mildew and bacterial leaf streak. These diseases are favored by wet conditions.
Another disease seen was barley yellow dwarf virus, which is transmitted by aphids.
Scout fields for Fusarium head blight
At heading through flowering, Byamukama says a disease to watch for is Fusarium head blight (FHB). The pathogen that causes FHB, Fusarium spp, survives on corn and wheat residues.
Warm and moist weather conditions favor the disease development. In addition to causing direct yield loss through bleached heads and scabby seeds, Byamukama says Fusarium spp also produce a mycotoxin, deoxynivalenol (DON). Elevated levels of DON in grain can lead to rejection at grain elevators.
He says a well-timed fungicide application can reduce FHB and DON accumulation in grain. For fields with a history of FHB, wheat following wheat or corn, risk of FHB may be high, depending on weather conditions during heading. The national Fusarium Prediction Center ( www.wheatscab.psu.edu ) provides predicted risk for FHB based on weather conditions.
To learn more, visit iGrow.org.
