A Perfect Storm: Kansas wheat fields hit hard by WSMV outbreak

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The wheat curl mite is a small but mighty adversary of Kansas wheat producers as the vector for Wheat Streak Mosaic Virus (WSMV), Triticum mosaic virus (TriMV) and High Plains Mosaic Virus. Fueled by a combination of the right weather conditions and ample volunteer wheat and other safe harbors, producers across the state appear to be losing the battle against these tiny critters and the diseases they carry in what could be the worst WSMV outbreak since 2017. 

“We have some fields that are close to total losses,” said Kelsey Andersen Onofre, K-State assistant professor of plant pathology, who has been across the state scouting fields. “It’s hard to walk in a wheat field in Kansas right now and not find at least low-level, trace-level symptoms of WSMV, and that’s pretty unusual.”

Andersen Onofre explained that symptoms of WSMV — symptomatic leaves with dark green coloring and light-yellow discolorations or stunted plants with a distinct yellow discoloration — are showing up more clearly over the last few weeks as temperatures warm up, but the cause happened last summer and fall. A suspected high population of mites last summer found a place to hide for the summer, fall and winter — most commonly in volunteer wheat or volunteer wheat hidden in a field of cover crops or double crop acres. Mites survive the winter in these “green bridges” and mild weather meant populations stayed fairly high — so high that mites were able to disperse over long distances, up to one or two miles away from a source, and there are now concerning levels of the diseases they carry across the state.

“If we have an extended, warm fall, like we did in 2024, it’s a perfect storm for mite populations to stay at high levels and increase later in the season,” Andersen Onofre said. “The more populations increase, the higher the chance they will be blown by wind into neighboring fields. If the weather stays warm, we can see this spread continue over long distances.”

That is certainly the case in Saline County, where more diverse cropping systems are making it an emerging issue where it hasn’t traditionally been found.

“This is by far the worst year I’ve seen for impact of wheat streak mosaic virus in our area,” said Gypsum farmer Justin Knopf. “Many fields have been affected with a high level of presence of the virus. Some fields, where they’re right next to a field where the source was, have significant impact.”

Infected fields at this point in the plant’s growth stage near flowering will often not head out well or will have low grain fill, meaning producers with infected fields should expect yield loss, especially in fields where the flag leaf has already turned yellow. 

“Unless we have a significant weather event such as hail or excessive heat during grain fill, wheat streak mosaic virus is likely to be the number one cause of yield loss in this area,” said Knopf.

Adding insult to injury, there is no effective treatment for WSMV once the infection has occurred. Producers must balance estimated yield loss, requirements for crop insurance and the economics behind terminating the most severely infected fields in favor of planting a summer crop. 

“Unfortunately, there is not much to do at this point in the season if you have a field that has severe WSMV infection. These fields are likely not good candidates for a fungicide application. Fungicides will have no effect on wheat streak, and it will be difficult to recover yield losses from foliar diseases if yield potential is already reduced by wheat streak.” Andersen Onofre said. “We do not recommend spraying insecticides or miticides. There is no immediate action you can take to slow down the spread of WSMV or preserve yield in affected fields.” 

Increased instances of the viruses this year has farmers thinking about the longer-term causes. Knopf attributed the widespread infection this year to three really dry falls and winters in a row, more hail in the area last year (areas with hail are virus hot spots) and more diverse cropping systems, including warm season cover crops and double cropping.

Producers who selected wheat varieties with known tolerance or genetic resistance to WSMV are seeing the benefit of that decision.

“I haven’t historically selected varieties based on WSMV resistance in this area, but it will certainly be on my mind for next year’s crop,” said Knopf.

“We do not have any perfect resistance, but variety selection is making a difference under this really high pressure situation,” Andersen Onofre said. However, she cautioned producers that even varieties with tolerance or genetic resistance to WSMV may still be susceptible to Triticum mosaic virus (TriMV), and dual infections of both diseases are also popping up. When fields are infected with both viruses, yield losses can be more severe. She encouraged producers to work with their local extension office or directly send samples to the K-State Plant Disease Diagnostic Clinic (https://www.plantpath.k-state.edu/extension/plant-disease-diagnostic-la…) to test for the different wheat viruses to help identify infections. 

In addition to WSMV and TriMV, she also encouraged producers to monitor their risk of Fusarium head blight (scab) as weather conditions during flowering have been conducive to the development of scab and leaf spot diseases like tan spot and Septoria in parts of the state like central and southeast Kansas. These wheat diseases can be managed with fungicide applications. A Fusarium Risk Tool is available at wheatscab.psu.edu for producers to manage their risk. Unlike WSMV, producers cannot scout fields to see FHB, so they must make application decisions based on individual field risk, which is particularly important for wheat fields planted into corn residue. Producers can review which fungicides work best for FHB at https://bookstore.ksre.ksu.edu/pubs/foliar-fungicide-efficacy-ratings-f….

“We are recommending producers watch the FHB risk pretty carefully, especially right now in central and southeast Kansas, as we are in that flowering window and we have had favorable conditions,” she said. 

Producers will have the chance to speak with K-State extension experts and local field staff and agronomists during the upcoming winter wheat variety plot tours that start May 13. The plot tours will provide a chance to see how different available and upcoming wheat varieties are reacting to the disease pressure this season. Find the full list of plot tour dates and locations in the May 1, 2025 Agronomy eUpdate. These field observations are another important tool as producers plan beyond this year’s harvest, especially as it will take a collective effort to stop the WSMV streak. 

"Now we need to look towards aggressive volunteer management and variety selection for the fall because we have had bad WSMV this year,” Andersen Onofre said. “If we don’t manage our volunteer wheat in the state after harvest, we could be right back in the same place a year from now — this is going to be absolutely critical after harvest."

Read more at kswheat.com/wheatstreak.