By Cody Creech, Gary Hein, Stephen Wegulo, Jeff Bradshaw, and Justin McMechan

Wheat streak mosaic virus has been extensive throughout the western Great Plains this year, significantly reducing the yield potential of many fields. In Nebraska, most of the serious virus problems have occurred in areas where preharvest hail occurred last summer. However, a critical factor contributing to the issues this year was the warm, extended fall.

The wheat curl mite is a vector of a complex of three viruses in wheat, the most common being WSMV. Wheat infected with WSMV is characterized as being stunted with a yellowing mosaic pattern on the leaves. Head size and number of tillers is often reduced, which lowers wheat yields. These viruses can only become a problem if mites survive the summer on a living bridge host and become numerous in the fall to carry the virus into the new crop wheat.

The extended fall in 2016 allowed mite populations more time to build up to greater densities and increased movement into the new crop, thus increasing risk. In addition, the warm fall enabled greater virus replication and buildup within the infected wheat plants, and therefore, increased virus impact.

Mites have many hosts
About 90 different grass species have been listed as potential hosts for the wheat curl mite through field observations. Of these hosts, wheat is considered to be the primary host, with mites having high rate of reproduction. With the exception of wheat, corn is one of the most well documented and tested hosts for the wheat-mite-virus complex. Mite activity and virus spread from corn to wheat has been shown to be most severe when corn maturity is delayed past wheat emergence. Early maturation of corn in most areas last year minimized this risk and limited the importance of corn as a bridge host.

Preharvest volunteer wheat has by far the greatest risk for mite and virus development; therefore, managing this green bridge host is critical to reduce the risks from this virus complex. It is important to identify the presence of this volunteer at harvest and focus efforts on eliminating this serious threat and avoid harvest losses. Without these sources of mites, serious virus problems will not result, even if environmental conditions are quite favorable.

Preharvest volunteer wheat and other mite hosts must be controlled throughout the summer to reduce suitable mite hosts and should be completely controlled at a minimum two weeks prior to planting wheat. Mites can only survive about a day off a green host so they must find a new host to enable their survival.

Control volunteer wheat
Tillage or herbicides can both be effective at controlling hosts. Both operations would need to be completed about a month before planting wheat to allow sufficient time for the hosts to completely dry up and cease being a suitable host.

If not adequately controlled, mite populations will continue to build throughout the fall. Mites will move from these green bridge source fields in all directions as the wind changes directions through the fall. However, studies have shown that a greater potential for movement to the south and east of source fields, and this is likely due to the prevalence of winds from the northwest during the fall movement period (see Mite Movement Animation).

Fields adjacent to the source fields are at greatest risk, and in many situations, a gradient of movement and virus spread can be seen from these source fields. However, under serious mite-virus presence in volunteer wheat, mites and virus can spread well beyond the neighboring field.

Controlling volunteer wheat and other grass hosts is the key to managing WSMV. Some wheat varieties have some resistance to WSMV and could be used when in a high-risk scenario. Wheat infected after planting in the fall will likely suffer some level of yield loss. Once infected, no options remain to prevent yield loss.

Watch a Nebraska Extension video that describes more about the wheat curl mite, WSMV and how to manage it.

Creech is a dryland cropping systems specialist at the Panhandle Research and Extension Center; Hein is director of the Doctor of Plant Health Professional Program at UNL; Wegulo is a Nebraska Extension plant pathologist; Bradshaw is an Extension entomologist; and McMechan is an Extension crop protection and cropping systems specialist.