Occurrence of Wheat Curl Mite and Mite-Vectored Viruses of Wheat in Colorado and Insights into the Wheat Virome

The wheat curl mite (WCM) is a vector of three important wheat viruses in the U.S. Great Plains: wheat streak mosaic virus (WSMV), triticum mosaic virus (TriMV), and High Plains wheat mosaic virus (HPWMoV). This study was conducted to determine the current profile of WCM and WCM-transmitted viruses of wheat and their occurrence in Colorado, including novel wheat viruses via virome analysis. There was a high rate of virus incidence in symptomatic wheat samples collected in 2019 (95%) and 2020 (77%). Single infection of WSMV was most common in both years, followed by coinfection with WSMV + TriMV and WSMV + HPWMoV. Both type 1 and type 2 mite genotypes were found in Colorado. There was high genetic diversity of WSMV and HPWMoV isolates, whereas TriMV isolates showed minimal sequence variation. Analysis of WSMV isolates revealed novel virus variants, including one isolate from a variety trial, where severe disease symptoms were observed on wheat varieties carrying Wsm2, a known virus resistance locus. Virome analysis identified two to four sequence variants of all eight RNA segments of HPWMoV, which suggests co-occurrence of multiple genotypes within host populations and presence of a variant of HPWMoV. A possible novel virus in the family Tombusviridae and several mycoviruses were identified. Overall, the data presented here highlight the need to define the effect of novel WCM-transmitted virus variants on disease severity and the role of novel viruses.