Be wary of the wheat streak mosaic virus

This week, I borrowed excerpts from a great article written by my co-workers, Madalyn Shires, Connie Strunk, Adam Varenhorst and Anna Hagemann. Wheat harvest is happening, and winter grain planting will be underway soon, making this info very applicative this week.

As wheat harvest proceeds, it wont be long until winter wheat planting is underway in South Dakota. It is important to note that disease transmission of wheat streak mosaic virus during the fall is a greater threat to yield than spring transmission. The continued reproduction of the virus in alternate hosts develops a condition called the green bridge. Wheat streak mosaic virus causes a disease among grasses within the Poaceae family called wheat streak mosaic disease complex. Wheat steak mosaic virus is transmitted by the wheat curl mite. The WSMV is often harbored in late maturing spring wheat, alternative hosts, or volunteer wheat in late summer with less noticeable symptoms. These same plants often serve as hosts for the wheat curl mite. Unfortunately, the decline of the alternative hosts drives the movement of mites to new hosts; this generally lines up with the emergence of winter wheat. As a result, WSMV and other viruses in the disease complex are also transmitted into the new crop through feeding, creating the green bridge effect. The green bridge effect is also persistent in spring wheat where wheat curl mite moves from winter wheat to spring wheat in the spring. Spring wheat has little to no resistance to WSMV.

To effectively reduce WSMV, it is important to manage the green bridge effect to slow spread of disease in newly emerging wheat fields.

Wheat curl mite and transmission

Wheat streak mosaic virus (and other viruses in the disease complex) transmission occurs when wheat curl mites that were previously feeding on infected plants disperse to healthy plants and begin feeding.

Wheat curl mites rely on wind dispersal to find new host plants. This results in a random distribution of infected plants in wheat fields. Wheat curl mites require live plant tissues for survival, but this isnt generally an issue for them as their alternative hosts are common (ex: corn, sorghum, millet, weedy grasses, volunteer wheat, etc.). When the quality of their host begins decline, they move up the plant and get carried off in the wind. Wheat curl mites normally infest field edges first as these act as natural wind barriers.

Any life stage of the wheat curl mites can overwinter and with snow cover can survive temperatures below freezing. Once temperatures begin to warm up in the spring, the wheat curl mites become active again and continue their life cycle, and ultimately, continue the disease cycle. The wheat curl mites reproduce rapidly at temperatures between 75-80 F. There are no miticidal recommendations for wheat curl mite population reductions and managing the green bridge is the only recommendation available for wheat curl mite management. For identification of wheat curl mites, visit extension.sdstate.edu and search for Identifying the mites in wheat fields.

Managing the green bridge

The green bridge is the literal plant tissues that are present during the period of spring wheat harvest and winter wheat planting. The green bridge aids in the transmission of the virus between one crop to the following crop, moving further in the next growing season. In order to prevent the spread of the virus, the green bridge must be broken. There are multiple avenues that can be taken to dimmish the green bridge, but work best when used together.

• The planting date of winter wheat should be delayed to mid- to late September when wheat curl mite populations are lower. The SDSU Winter Wheat Breeding program recommends no earlier than Sept. 15 to Sept. 20. Delayed planting dates will also reduce the risk of Hessian fly infestation.
• There should be ample time between alternative host termination and the next planting. All volunteer wheat and grasses should be terminated following the harvest of spring wheat. There should be a two-week window between herbicide applications, and the planting of winter wheat to ensure that termination was successful.
• Herbicides can be very effective at managing WSMV hosts. Glyphosate (i.e., Roundup) can kill volunteer wheat and grass weeds carrying mites. For glyphosate specifically, there should be a 14-day window before planting the following wheat crop as mite population will temporarily increase since they will be leaving their previous hosts. In addition, glyphosate has no soil residual activity that can injure crops after harvest.
• Environmental and weather patterns should be analyzed when determining the proper periods to plant. Since mites travel primarily by wind, be sure to understand wind direction when planting your crops. If a neighboring field has just harvested a wheat crop or mowed a pasture area, be prepared to manage alternate grassy hosts as mites may move into your crop. Temperature is also important. Mite production is highest at 75 degrees F to 80 F. When temperatures reach lows of 50 F, mite reproduction slows down. Therefore, you must be aware of the impact of increased fall temperatures that occur for a prolonged duration as it favors the disease.
• Genetic resistance is equally important to reducing WSMV.

Identification

Correctly identifying this disease or any other fungal, bacterial, or viral disease is extremely important, as it aids in management decisions.

Send plant disease questions to SDSU Extension Plant Pathology Specialist Madalyn Shires or SDSU Extension Plant Pathology Field Specialist Connie Strunk (www.extension.sdstate.edu/about/our-experts). Samples can also be sent to the SDSU Plant Diagnostic Clinic. The clinics postal address is 1148 Medary Ave, 2207-D, 1451 Stadium Road, Brookings, SD 57007-1090 and the physical address for sample drop off is Berg Ag Hall Room 203 (business hours) or 001 (after hours and weekends). For more information, go here.