A certain but non-exclusive association between Polymyxa graminis special forms and cereals

Polymyxo graminis, a ubiquitous plasmodiophorid obligate root endoparasite, is recognized as the vector of about 15 viruses on cereals and groundnut in temperate and tropical areas. Within the species, five special forms have been distinguished on the basis of specific ribotypes. Three of them occur in tropical areas: P. graminis f.sp. colombiana on rice, P. graminis f.sp. subtropicalis on cereals cropped in the tropics such as maize, pearl millet and sorghum but also on barley and/or wheat, and P. graminis f.sp. tropicalis mainly on maize, pearl millet and sorghum. Their particular host ranges distinguish them significantly from P. graminis f.sp. temperata and P. graminis f.sp. tepida found in temperate areas on barley and wheat. In order to assess whether these special forms commonly infect these cereals, barley and wheat plants were grown under controlled conditions on two soils from Belgium and France and both infested by P. graminis f.sp. temperata and P. graminis f.sp. tepida. The infection of each cereal species by each form was quantified by real-time quantitative PCR with specific primers and Taqman probes. The infection of P. graminis f.sp. temperata was significantly higher on barley than on wheat, whereas the quantities of P. graminis f.sp. tepida on wheat were higher than on barley. These results show that the distinction between these special forms, based on the ribotype, reflects differences in ecological features.