Tessa Albrecht and Punya Nachappa – Department of Agricultural Biology, Colorado State University
Wheat curl mite (WCM) transmitted viruses including, Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV) and High Plains wheat mosaic virus (HPWMoV) continue to cause yield losses in the Great Plains (Fig. 1A). Plants infected with WSMV show yellow to light green streaks that coalesce to form a mosaic pattern (Fig. 1B). Symptoms are more severe if plants are infected early in their development and can include stunting, poor fertility/sterility, and reduced grain set. Average yield losses range from 2-3%, but localized affected areas can have much more dramatic losses of up to 100%.
Tessa Albrecht, a research associate in Dr. Punya Nachappa’s lab determined virus presence in wheat samples that were collected from the field by researchers or submitted by extension specialists and wheat producers for diagnosis in 2019 and 2020. We found high rates of virus incidence in symptomatic wheat samples (Fig. 2). In 2019, 95% of symptomatic field samples tested positive for one or more WCM-transmitted viruses, whereas, in 2020, 75% of samples were positive with relatively low symptom severity. Last winter brought colder than average temperatures to Colorado that may have reduced the number of mites that were able to overwinter into 2020. Furthermore, the WCM cannot survive in high temperatures and dry conditions, which maybe another reason for limited spread of the virus in 2020.
Our research revealed new WSMV isolates collected from Colorado. Fourteen out of the 22 WSMV isolates analyzed were collected from cultivars carrying either virus-resistance (Wsm2) and/or mite-resistance (CmcTam112) genes. Moreover, a couple of isolates were 100% similar to a potentially resistance-breaking isolate reported from Kansas. We plan to screen Colorado’s elite germplasm with these new variants to determine potential for resistance breakdown as part of our recently funded Colorado Wheat Research Foundation grant.
The WCM is a challenging pest to control owing to their short development time, high population growth rate, thermal tolerance and wide host range. Planting mite and virus-resistant varieties has been effective in reducing disease spread. However, there have been reports of mites adapting to resistance genes and emergence of novel, potentially resistance-breaking WSMV isolates. Hence, controlling this disease complex requires a multi-faceted approach which includes elimination of oversummering alternate hosts such as volunteer wheat, corn and wild grassy weeds, which prevents mites moving back to wheat as it begins to emerge. Delayed planting can also reduce fall infections by minimizing the overlap between wheat and alternate hosts. Research has shown that planting prior to September 15th puts wheat at a higher risk for curl mite infestation and viral infections in the fall. Another way to delay resistance breakdown is planting varieties with both mite and virus resistance such Colorado State University-developed ‘Guardian’, which will ensure improved and more durable resistance than mite or virus resistance alone.