The North Dakota State University Diagnostic Lab and field scouts are reporting an increase in wheat streak mosaic virus (WSMV) this year.
This disease has been reported statewide in winter and spring wheat, plus durum. The levels of severity range from slight to severe. There are three possible reasons for the increase:
- Fall 2008 was wet, so many green grasses, volunteers and corn were present late into the season. This allowed for a continuous green bridge for the survival of the mite vector and virus.
- A heavy snow cover in many areas gave the mites and virus better chances of surviving.
- A cool, wet spring hampered the herbicide "burn-down" of weeds and volunteers and delayed spring planting.
To reduce the risk of carrying this disease into winter wheat this fall and subsequent spring crops next year, management steps must be taken now to break the green bridge that allows the mites and virus to survive, according to Joel Ransom, NDSU Extension Service agronomist and Marcia McMullen, NDSU Extension Service plant pathologist. The green bridge consists of host plants, such as wheat, corn, volunteer wheat and grassy weed hosts.
McMullen and Ransom suggest three management steps:
- Plant winter wheat during the recommended planting windows of Sept. 1 through 15 for the northern half of the state and Sept. 15 through 30 for the southern half. Earlier planting will more likely result in winter wheat emerging while the mites still are very active and when grassy weeds and volunteers in the field or adjacent fields have not been controlled or destroyed adequately.
- Control volunteer wheat and grassy weeds in a field two weeks prior to planting the next susceptible crop. A two-week window of not having a host present assures that the mite has gone through its lifecycle and not found a subsequent host to feed on and transmit the virus.
- If planning to plant winter wheat following spring wheat this fall, especially in the northern tier of counties where the spring wheat harvest will be late, use a glyphosate burn-down on the crop prior to harvest to help control late-maturing tillers and green grassy weeds.
McMullen and Ransom say the late harvesting of wheat and corn this year may perpetuate the risk of the mites and virus surviving along the green bridge until freeze-up. Weeds and volunteers may be difficult to control in some areas prior to planting winter wheat or winter freeze-up. Corn also will be green late into the season.
With all these risks, it is essential that growers do the best job possible in controlling volunteers and following the winter wheat planting date recommendations. Next spring, the earlier planted spring wheat will avoid the highest risk of subsequent WSMV infection.
McMullen and Ransom say research is available that discusses mite movement from a source field, which may help producers plan for where they should plant their winter wheat this fall if they have had a WSMV confirmation this year.
Research shows the risk of mite dispersal and virus spread from a source field tends to follow an oval-shaped pattern extending from the northwest to the southeast according to prevailing winds. The size of the oval pattern depends on the size and mite density of the mite source. If mite populations in the source fields are low, the spread to neighboring fields will occur only for a short distance and an edge effect of the spread will be evident. If mite populations in the source field are high and the mite source field is large, the mites and virus will be spread across entire fields. The extent of this distance is not known, but anecdotal evidence indicates that in some instances, it may be up to one to two miles.
The highest risk for WSMV is planting winter wheat into a field with remaining green wheat volunteers or some grassy weed hosts, such as downy brome, barnyard grass or green foxtail. Barley and oats, although not commonly thought of as good hosts, also may harbor the disease, so these volunteers also should be controlled.
More information about wheat streak mosaic virus may be found in the NDSU Extension publication PP- 646, "Wheat Streak Mosaic Virus." The publication is available on the Web at www.ag.ndsu.edu/pubs/plantsci/smgrains/pp646.pdf.
Also available is the University of Nebraska Extension Service Publication "Managing Wheat Streak Mosaic." It is available at www.ianrpubs.unl.edu/epublic/live/ec1871/build/ec1871.pdf.
Source: NDSU Extension Communications