Management of Wheat Streak Mosaic Virus in Cereals Requires Integrated Pest Management
Wheat streak mosaic virus (WSMV) in cereals was a major issue in Montana in 2016, where it was identified in 24 counties. Annual losses due to WSMV are typically minor, averaging around 1% of wheat yields across the Great Plains, but were much higher in Montana during 2016. A combination of factors favored a WSMV epidemic including abundant volunteer wheat due to widespread hail in 2015 and reduced control of volunteer wheat, early planting dates to achieve high yields, timely rains which allowed early planted wheat to germinate and grow, and a long extended fall and open winter which favored reproduction and spread of the wheat curl mite (WCM) that vectors WSMV. In 2016 we observed widespread destruction of infected winter and spring wheat crops and yield losses in severely affected areas. Even organic production, which is normally protected from this disease due to intensive tillage and crop rotation, has been impacted by this disease due to the widespread nature of the problem.
Montana State University has been working on WSMV and other WCM-transmitted viruses since 2007, along with the University of Nebraska, Kansas State University, Texas A&M, and other cooperators. We are currently involved in a cooperative effort to develop models that will predict these epidemics with better accuracy, but there are several known risk factors and management practices that can be integrated to reduced risk of WSMV in cereal crops. WSMV and the WCM can only survive on living grasses and thus need a ‘green bridge,’ to be perpetuated from harvest to the new crop (Figure 2). Traditionally, the green bridge is made up of wheat seed knocked to the ground by hail that then germinates. This is also called a ‘volunteer’ crop. Grassy weeds such as downy brome that host the vector and disease can also create a green bridge between crops. The wheat stem sawfly (WSS) is an often underappreciated, but serious contributor to the green bridge. The insect cuts stems at crop maturity, leading to wheat stems falling to the ground, with resulting seed shatter and subsequent germination (volunteer).
Figure 2. The green bridge occurs when green plant material is present between planting and harvest. This is facilitated by wheat stem sawfly and downy brome (cheatgrass). Thrips and parasitoids are biocontrols for wheat curl mites and wheat stem sawfly.
The most reliable management option for WSMV and the WCM is eliminating the green bridge. This is accomplished both by 1.) terminating any green material in the field and surrounding area with herbicide or tillage at least 2-3 weeks prior to planting winter wheat, and 2.) avoiding planting too early in the fall. Most mites are moving during the harvest period as they leave the maturing crop. Colder temperatures reduce mite reproduction, movement and spread from source plants. A hard frost can significantly reduce mite numbers (Figure 3), but in the absence of a frost event, the mites continue to grow and reproduce on alternative (non-crop) host plants. Fall weather also affects WCM survival as they are moving between plants. At high temperatures (30˚C, 86˚F) and low relative humidity, mites survive off a host plant only a few hours. At low temperatures (10˚C, 50˚F) and high relative humidity, mites survive off a host plant for 5-7 days. Relative humidity seems to be more crucial than temperature. Risk may also increase in areas where winter and spring cereal crops overlap in the same area, and when a hail event causes widespread cereal crop volunteer. While delaying planting of winter wheat can help reduce the risk of plants getting infected, the opposite tactic works in the spring. Early planting of spring wheat, durum and barley reduces the risk of plants getting infected due to colder temperatures and increased plant age when the mite movement starts from winter wheat or alternative hosts.
Figure 3. Wheat curl mites in Bozeman are reduced by frost events (2013) but otherwise continue landing on spring wheat well into the fall (2014 and 2015).
Host resistance is an excellent way to manage WSMV and the WCM. Unfortunately, there are no varieties resistant to either the virus or mite currently available in Montana. There seems to be some tolerance in winter wheat varieties, but spring wheat varieties are highly susceptible. We have little data on barley, but varieties we have tested are susceptible. Yield loss is not directly related to symptom severity, but can be related to the timing of infection. Early infections, including fall infection, have much greater effects on yield than infections later in the season. Wheat infected after heading will likely experience very little loss due to WSMV. WSMV reduces root growth, which then leads to early drought and, very commonly, blank heads or light grain. This also stresses the plant and causes mites to leave the host plant.
There are no insecticides labeled for WCM management. At Montana State University, we evaluated multiple pesticides for control of WCM including organophosphates, carbamates, pyrethroids, mite growth inhibitors, soaps, oils, contact pesticides, and plant defense initiators. One active ingredient provided some control of the WCM but did not reduce spread of WSMV. This product is not labeled for WCM. Insecticides also kill non-target insects including thrips, which are predators of the WCM. This may increase disease risk. Mites die without a living host plant to feed on. In order of efficacy for host plant termination and reduction of mite survival, tillage >> paraquat > glyphosate >>> grazing.
Grazing is often suggested as an option for green bridge management, but plant termination does not occur fast enough, cow lips are not close enough to the ground to graze all plant material out of the field, and not all plants are terminated. Epidemics are often associated with incomplete plant kill and extension of the green bridge due to grazing. Herbicides can be used on a field prior to grazing, but check labels for grazing restrictions.