Written collaboratively by Emmanuel Byamukama, Associate Professor and SDSU Extension Plant Pathologist, Ruth Beck, former SDSU Extension Agronomy Field Specialist.
Much of the winter wheat in South Dakota has come through the winter in good condition. A cool fall coupled with some cool nights in late April have slowed spring growth, however, recent rains and warmer temperatures have changed that. Although the majority of winter wheat in the state is rated good to excellent in the recent USDA-NASS report, a few winter wheat fields in Central South Dakota have been diagnosed with wheat streak mosaic disease (WSMD) caused by wheat streak mosaic virus (WSMV). WSMD that is developing now is most likely the result of infections that occurred in the fall before winter wheat went into dormancy. Winter wheat fields that were planted early last fall, due to prevent planting or in areas where hail may have destroyed the wheat previous crop have an increased risk for WSMV.
WSMV symptoms start as small chlorotic streaks, which expand to form pale interrupted green and yellow stripes. The green and yellow patterns form the typical mosaic symptom. Depending on the time of infection and the cultivar planted, infected plants may be stunted, heavily yellowed, and have few tillers (Figure 1).
Symptoms worsen with stress caused by dry and hot weather conditions. The extent of yield loss depends on the severity of the symptoms and the proportion of the field with symptoms. Sometimes an entire field can be infected while in other instances, a few scattered plants may be symptomatic.
WSMV is transmitted by wheat curl mites (tiny ~0.3 mm long mites that can only be seen under high magnification such as 20x hand lens). These mites are wingless (Figure 2) but are blown by wind from neighboring wheat fields, areas of volunteer wheat or grassy weeds, into emerging winter wheat in the fall. Mite movement is often triggered by plant maturity. The mites acquire the virus when they feed on infected wheat or grass hosts and transmit it when they land and feed on uninfected wheat plants. Various conditions can lead wheat curl mite populations to build. One of these conditions is the when hail occurs on grain that is near maturity. This can often result in localized but serious infestations of WSMV in new wheat crops in those areas the following year. This is the situation that has occurred this spring. A local crop consultant from the Pierre area noted that winter wheat which looked completely healthy in early April was showing symptoms of WSMD along one edge of the field in late April. This area had received hail in early July 2019. Volunteer winter wheat in hailed winter wheat fields may have led to increased wheat curl mite populations. The mites most likely moved into nearby newly planted winter wheat later in the fall. Wheat that is infected when it is young is most likely to suffer more from WSMV infections.
Wheat curl mites are most active at the end of the wheat growing season when because of deteriorating food source (ripe wheat plants) they are forced to move. They crawl towards the tip of the host plants where they are picked by wind. That’s why planting early (before mid-September) in the fall increases the risk of WSMV.
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, several practices can be used to prevent or lessen chances of wheat getting infected by WSMV for future wheat seasons.
- 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 a burn-down herbicide application.
- For areas prone to WSMV infection, delay winter wheat planting in fall. Planting early in the fall especially when temperatures are mild increases the risk of WCMs landing and transmitting viruses in emerging winter wheat.
- Plant wheat varieties that are resistant/tolerant to WSMV. The Virology program at SDSU does screen varieties for resistance to WSMV. Virus screening shows consistent differences between wheat cultivars for grain yield, reduced symptoms, and extent of virus replication in a cultivar.
- The wheat curl mites can survive on other cereal crops like corn, millet, barley, and sorghum. Therefore, for areas with frequent WSMV epidemics, planting non-host broadleaf crops like field peas, lentils, sunflower etc. will help keep WSMV pressure low.