BROOKINGS, S.D. – Severe wheat streak mosaic disease was discovered in several winter wheat fields scouted in Central South Dakota. Due to this discovery, SDSU Extension advises that growers test for the disease, caused by Wheat streak mosaic virus, before applying nitrogen.
“This disease causes wheat leaves to yellow and plants to be stunted (Figure 1),” said Emmanuel Byamukama, Assistant Professor & SDSU Extension Plant Pathologist. “Producers may mistakenly attribute the yellowing to a nitrogen deficiency.”
Byamukama explained that yellowing of lower or older leaves in wheat can be an indication of nitrogen deficiency. However, if the yellowing is the result of Wheat streak mosaic virus, the yellow leaves will have a streaked mottled look to them.
To clearly know what is causing the yellowing, Byamukama encouraged growers to submit a sample to the diagnostic clinic to confirm Wheat streak mosaic virus before adding inputs such as fertilizer or fungicide in yellowing wheat fields.
Test plants here
Plant samples can be mailed or dropped off at the SDSU-Plant Diagnostic Clinic, SPSB 153, Box 2108, Jack Rabbit Lane, Brookings, SD 57007-1090.
Below Byamukama answers some frequently asked questions about Wheat streak mosaic virus.
How can one tell whether yellowing is due to Wheat streak mosaic virus?
The best way to know if a plant has Wheat streak mosaic virus is to submit a sample to the SDSU diagnostic lab. Wheat streak mosaic virus is systemic, meaning that all leaves have symptoms. Younger leaves may appear healthy but over time, these also show symptoms.
Older leaves show severe yellowing and streaking starting from the leaf tips (Figure 2a).
Some wheat fields scouted, however, showed just older lower leaves with yellow blotches (Figure 2b). This is not caused by Wheat streak mosaic virus; it may be some infection initiation that was halted by unfavorable environmental conditions or due to unknown environmental stress.
Producers may wonder why this disease is so prevalent this year? Wheat streak mosaic virus is transmitted by the wheat curl mite. Wheat curl mites are microscopic (0.3 mm long) and can only be seen under magnification (such as 20x hand lens).
They do not move on their own, they depend on wind to move between fields and within a field. Wheat curl mites survive from one crop to another on volunteer wheat and grassy weeds. The pattern of infection within fields can be an indication of where the disease came from. For instance, if the entire field is infected, one can assume the Wheat streak mosaic virus came from within the field. In situations where the infection is occurring along the field edge, one would assume that the mites were blown in from the adjacent field. Mites are estimated to travel up to 2 miles.
All fields found with Wheat streak mosaic virus were winter wheat planted into wheat stubble (Figure 3). It is possible that these fields were planted before all grassy weeds and volunteer wheat had completely dried up after herbicide application last fall. It is recommended that burn-down of grassy weeds and volunteer wheat is done at least two weeks before planting. The long warm fall that we had in 2016 in South Dakota and other winter wheat production areas in the central US may have allowed mite populations to build and spread the virus to more areas.
Management of Wheat Streak Mosaic
Wheat streak mosaic disease can be best managed through cultural practices. Unlike fungal diseases, nothing can be sprayed on virus-infected plants to prevent or cure virus infection. However, a few practices can be used to prevent or reduce the chances of winter wheat getting infected by Wheat streak mosaic virus before planting:
- Destroy volunteer wheat and grassy weeds at least two weeks before planting in the fall. Volunteer wheat and grassy weeds are the most important risk factor for the wheat streak mosaic disease. Volunteer wheat and grassy weeds can be destroyed through tillage or herbicide application. This will reduce the ability of the wheat curl mites to use these plants as a green-bridge to the newly emerging wheat.
- For areas prone to Wheat streak mosaic virus infection, delay winter wheat planting in fall. Planting early in the fall especially when temperatures are mild increases the risk of wheat curl mites landing and transmitting viruses in emerging winter wheat.
- Plant wheat varieties which are resistant/tolerant to Wheat streak mosaic virus. The SDSU Crop Performance Test program provides ratings of wheat cultivars against Wheat streak mosaic virus. A few cultivars are rated moderately resistant (see table below)
4. Include a broad-leaf crop in the rotation. Wheat curl mites can survive on other cereal crops including corn, millet, barley, and sorghum. Therefore, for areas with frequent Wheat streak mosaic virus epidemics, planting non-host broadleaf crops like field peas, lentils, sunflower etc. will help keep Wheat streak mosaic virus pressure low.
Media Contact:
Lura Roti, Agriculture Journalist/PR
605-695-5872
SDSU Extension Contact:
Emmanuel Byamukama, Assistant Professor & SDSU Extension Plant Pathologist
emmanuel.byamukama@sdstate.edu
Source of the table: Crop Performance Testing program
Cultivar rating score for Wheat streak mosaic virus
R= resistant, MR=moderately resistant, MS= moderately susceptible, S=susceptible. Ratings in parenthesis were provided by the entity that submitted the variety
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Courtesy of iGrow.org
Figure 1. A winter wheat field with Wheat streak mosaic virus symptomatic plants spread across the entire field (Picture taken 5/3/2017)
Link to graphic here.
Courtesy of iGrow.org
Figure 2 (A) Yellowing caused by WSMV. Notice the presence of streaks in most of the leaves.
Link to graphic here.
Courtesy of iGrow.org
Figure 2 (B) Yellowing caused by environmental factors. No streaking is observed.
Link to graphic here.
Courtesy of iGrow.org
Figure 3. A winter wheat field planted into millet stubble on the left, and into wheat stubble on the right. Notice severe yellowing on the wheat stubble side.
Link to graphic here.
This news release is available on iGrow.org.
Source: SDSU Extension