Ribotypes of Polymyxa graminis in Wheat Samples Infected with Soilborne Wheat Viruses in China

Polymyxa graminis is an obligate parasite and important vector of more than 14 soilborne plant viruses that pose a significant threat to cereal crops in Europe, North America, and Asia. Different ribotypes or formae speciales of P. graminis have been recognized and these may be associated with different cereal hosts or with transmission of different viruses. Two soilborne viruses infecting winter wheat in China have been reported and well studied (Wheat yellow mosaic virus [WYMV, genus Bymovirus] and Chinese wheat mosaic virus [CWMV, genus Furovirus]) but there has been no reported characterization of P. graminis isolates associated with them. In this study, the ribosomal DNA internal transcribed spacer (ITS) regions of P. graminis were examined from 63 wheat samples with apparent virus symptoms obtained from 16 sites within six Chinese provinces. Their associations with soilborne viruses were investigated. Ribotype I (P. graminis f. sp. temperata) and ribotype II (P. graminis f. sp. tepida) were confirmed in winter wheat regions of China for the first time. All 63 wheat root samples were infected with ribotype I of P. graminis and 11 were also infected with ribotype II. There was no obvious association between the ribotypes and infection by either WYMV or CWMV (or double infection). Phylogenetic analysis of the P. graminis ITS1-5.8S-ITS2 sequences revealed that ribotype I in China belongs to previously reported subgroup Ib, whereas ribotype II belongs to IIa. There was considerable sequence variation (pairwise distances from 0.0219 to 0.0319) between Chinese ribotype I isolates of different regions and previously reported ribotype I isolate Ken5 (accession number HE860055.1).