Polymyxa graminis f. sp. temperata and P. graminis f. sp. tepida are distinguished on the basis of their specific ribosomal DNA sequences. In order to evaluate whether or not host specialization is associated with the special form, the occurrence of infection of both forms on barley and wheat was studied. P. graminis inocula were obtained from soils collected in Belgium and France. Their ribotypes were characterized using molecular tools specific to P. graminis f. sp. temperata or P. graminis f. sp. tepida such as restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction (PCR)-amplified rDNA, nested and multiplex PCR. Both special forms were found in each country and coexisted in some soils. The host specificity of P. graminis special forms for barley and wheat was studied from two soils collected at Gembloux (Belgium) and Chambon-sur-Cisse (France), each infested by bymo- and furoviruses. P. graminis f. sp. temperata is more frequent on barley and P. graminis f. sp. tepida on wheat. Furthermore, the quantification of each form on barley and wheat by two separated real-time quantitative PCR assays confirms the observations on the vector specialization. These results suggest a certain but not exclusive host specificity of P. graminis special forms.

In order to assess the occurrence of Wheat spindle streak mosaic virus (WSSMV) in Belgium, a reverse-transcription polymerase chain reaction (RT-PCR) was developed, targeting WSSMV isolates from Canada, France, Germany, Italy, and the United States. The primers also were designed for virus quantification by real-time RT-PCR with SYBR-Green. No cross-reaction with soilborne cereal viruses such as Barley mild mosaic virus, Barley yellow mosaic virus, Soilborne cereal mosaic virus, and Soil-borne wheat mosaic virus was observed. The RT-PCR and real-time quantitative RT-PCR allowed a more sensitive detection of WSSMV than enzymelinked immunosorbent assay. The incidence of WSSMV in Belgium was evaluated using a bioassay with wheat cvs. Cezanne and Savannah and rye cv. Halo, grown in 104 Belgian soils. The presence of WSSMV was detected from plants grown in 32% of the soils. The RT-PCR methods developed here, combined with large sampling, allowed WSSMV to be detected for the first time in Belgium. The real-time quantitative RT-PCR was developed as a tool for evaluating the resistance to WSSMV by quantifying the virus concentration in wheat cultivars.

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