Genetic groups of Clavibacter michiganensis subsp. michiganensis identified by DNA fingerprinting and the effects of inoculation methods on disease development

Abstract

Clavibacter michiganensis subsp. michiganensis (CMM) strains collected in nine different prefectures in Japan were fingerprinted by inter-simple sequence repeats–polymerase chain reaction (PCR) and repetitive sequence–based PCR. Forty-three CMM strains corresponded to 22 DNA fingerprinting patterns, indicating that the CMM population in Japan has high genetic diversity. Tomato plants were inoculated with CMM by two different methods: defoliation using infected scissors and planting in soil containing infected plant debris. Defoliation resulted in a larger number of diseased plants, disease developed earlier, and plants survived for significantly fewer days. Upon planting in the infected soil, 33.3 % of plants were infected and 6.7 % were diseased, i.e., 20.1 % of infected plants developed disease symptoms. These results indicate that some plants are infected with CMM through plant debris in the soil as the primary inoculum and most infected plants maintain latent infection. Thus, growers may unwittingly carry out disbudding and defoliation of latently infected plants, resulting in secondary inoculum.