Characterization of nonpathogenic Cadophora gregata, a potential biological control agent, concomitantly isolated from soil infested with Cadophora gregata f. sp. adzukicola, the cause of adzuki bean brown stem rot

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We collected 555 isolates of Cadophora gregata from adzuki bean field soils in Hokkaido, Japan, from 1997 to 2000. To identify the brown stem rot (BSR) pathogen C. gregata f. sp. adzukicola, we screened these isolates for pathogenicity to adzuki beans. Of the isolates, all of which originated in Tokachi District, Hokkaido, Japan, 23 were avirulent to adzuki bean, soybean, or mung bean. However, polymerase chain reaction (PCR) with specific primers for C. gregata f. sp. adzukicola (BSRA1 and BSRA2) detected the specific identifying DNA fragment in these isolates, and cluster analysis with inter-simple sequence repeat markers showed that the isolates were phylogenetically closer to strains that are virulent to adzuki bean. Thus, we concluded that the isolates were nonpathogenic C. gregata. A few selected isolates of the nonpathogenic C. gregata were effective at reducing BSR in vivo and show potential for development as biological control agents.