Abstract
Clubroot caused by Plasmodiophora brassicae is one of the most severe diseases of Chinese cabbage in Nagano, Japan. The fungi are classified into several races based on host-parasite relationships. However, determination of the race of P. brassicae isolated from an individual clubroot requires several time-consuming steps such as a cross-inoculation test using several host plants. Genetic diversity among races in P. brassicae is not understood well. In this study, to understand whether phylogenetic relatedness of Japanese isolates of P. brassicae is directly reflected in differences of host specificity among those isolates, we compared the nucleotide sequence of 18S, 5.8S and the ITS region of the rRNA of ingenous isolates of P. brassicae obtained from Nagano Prefecture with those of several strains obtained from the United States and England. Most of the nucleotide sequence of the ITS region of 7 fungal isolates and the partial nucleotide sequence of the 18S rDNA region of 5 fungal isolates were sequenced. The nucleotide sequence of 18S rDNA did not show clear differences among 5 fungal isolates. However, 18S rRNA sequences of P. brassicae (U18981) of the United States were remote from those of Nagano isolates. The nucleotide sequences of the ITS region of 7 Nagano isolates were similar to those of P. brassicae isolated from Australia, England and Korea. Therefore, we found no relationship between phylogenetic relatedness of the ITS region and host specificity of Nagano isolates. However concerning the 5.8S rDNA region, phylogenetic differences based on sequences of 5.8S could distinguish race 9 and 4 isolates from race 8 and 16 isolates classified by the Williams methods.
