Development of a Pathotype Specific SCAR Marker in Plasmodiophora brassicae

Plasmodiophora brassicae is an obligate biotroph that causes clubroot, one of the most damaging diseases of crucifers. Breeding of clubroot-resistant plants has been hampered by the large variation of pathogenicity in P. brassicae and by the lack of an efficient means for detecting specific isolates. To improve the practicality of P. brassicae pathotype-identification, a molecular approach was developed. RAPD profiles of 37 single-spore-derived isolates belonging to seven different pathotypes were compared. A RAPD marker, OPL14₁₂₀₀, was found in the molecular pattern of all the isolates belonging to one particular pathotype (P1), pathogenic on all differential hosts tested. The DNA band corresponding to this marker was cloned and sequenced. No significant homology to previously characterised nucleotide sequences was found. Primers were designed to specifically amplify the OPL14₁₂₀₀ band. The SCAR marker was observed in all isolates belonging to pathotype P1 and was absent in isolates belonging to other pathotypes and in the different plant hosts analysed. The SCAR marker was also generated from direct amplification of DNA from clubs (mixture of host and pathogen DNA) developed after infection by P1 isolates. This molecular marker may be a valuable tool for rapid and reliable identification of P. brassicae P1 isolates in areas where resistant varieties are cultivated.