Wheat Streak Mosaic Virus: Should You Be Concerned Next Spring?

 

Figure 1. A winter wheat field infected with Wheat Streak Mosaic Virus in Central South Dakota 2017.

 

 

Wheat streak mosaic disease is caused by viral pathogen called Wheat Streak Mosaic Virus (WSMV). This virus is transmitted by microscopic mites called Wheat Curl Mites (WCM). WCMs can only be seen under magnification (Figure 2). WCMs move from field to field by wind. WCMs are not capable of moving on their own since they do not develop wings but can crawl to neighboring plants. WSMV can result in severe yield losses and sometimes an entire wheat field can be lost when wheat is sprayed out and a different crop planted in spring.

 

 

 

Many questions were raised on wheat streak mosaic disease of wheat during the recent Ag Horizon Conference. This was, in part, due to the widespread occurrences of this disease during the 2017 season (Figure 1). Some of the questions raised included:

 

 

On the question of whether WSMV can continue to spread during winter period, the WCMs are active when the wheat plant is actively growing. Since winter wheat is dormant at this time, the WCMS will be inactive and no further spread of WSMV happens during this time of the year. WCMs resume to be active when temperatures warm up in spring. It should be mentioned here that infections, which take place in the fall, are the most damaging to wheat. Spring infections cause mild grain yield loss.

 

 

Wheat curl mites can survive bitter cold winter temperatures. WCMs overwinter as eggs, immature or as adult mites. The adult WCMs reside near the growing point of a wheat plant and can be insulated from low temperatures especially under snow cover.

 

 

Insecticides are not effective against the WCMs curl mites mainly because WCMs are protected from exposure to insecticides as they inhabit the inner whorl of the leaf near the growing point of the plant. Insecticides used to control other types of mites such as spider mites will not control WCMs.

 

 

Neighboring pasture grasses can be a source of the WCMs and the virus. However, the preferred host for the vector and the virus is wheat. Therefore while pasture grasses may serve as a source of inoculum, this inoculum is limited and usually a few wheat plants along the field edges will be affected. Larger epidemics of this disease happen when the inoculum is from within the field or when WCMs are blown in from a neighboring wheat fallow during fall shortly after planting.

 

 

Seed is not an important source of inoculum for WSMV. Although research has shown that a low percentage (<0.5%) of seed harvested from infected plants can transmit WSMV, the infected seedlings do not lead to significant wide spread disease within the field. WCMs remain the major source of WSMV.