LOCAL-SOURCES OF CLAVIBACTER-MICHIGANENSIS SSP MICHIGANENSIS IN THE DEVELOPMENT OF BACTERIAL CANKER ON TOMATOES

Seed transmission, overwinter survival, and dispersal of Clavibacter michiganensis ssp. michiganensis on processing tomatoes (Lycopersicon esculentum) and spread of C. m. michiganensis on alternative hosts and nonhost plants were evaluated with rifampicin-resistant strains of C. m. michiganensis (Rif+ C. m. michiganensis) and a selective medium. The bacterium was transmitted at a low rate from seed to transplants by sowing infested seed in the greenhouse and transplanting the seedlings to a production field. Populations of Rif+ C. m. michiganensis detected on seed harvested from systemically infected plants ranged from about 10(2) to 10(5) cfu/g seed. Survival of Rif+ C. m. michiganensis associated with infested tomato debris was greater for debris on the soil surface than for debris that was buried. Populations of Rif+ C. m. michiganensis declined at about the same rate in samples buried 10, 20, and 30 cm. Populations of Rif+ C. m. michiganensis on alternative hosts and nonhost plants fluctuated from 0 to about 10(9) cfu/g fresh weight on solanaceous plants (Capsicum annuum, Datura stramonium, Lycopersicon esculentum, Nicotiana tabacum, Solanum melongena, S. nigrum, and S. tuberosum) and from 0 to about 10(3) cfu/g fresh weight on nonsolanaceous weeds (Amaranthus retroflexus, Chenopodium album, and Xanthium saccharatum). Leaf surface populations of Rif+ C. m. michiganensis on nonhost solanaceous plants and nonsolanaceous plants were significantly lower than on tomatoes. Symptoms of secondary infection were not observed on plants of these species. Susceptible (Heinz 1810) and moderately resistant (Heinz 7417) tomato cultivars supported leaf surface populations of about 10(7) to 10(9) cfu/g fresh weight. Symptoms of secondary infection, marginal scorch of leaflets or bird's-eye spots of fruit, were not observed on tomatoes until populations exceeded 10(6) and 10(7) cfu/g fresh weight for the moderately resistant and susceptible cultivars, respectively. Symptoms of systemic infection were not observed in either cultivar when Rif+ C. m. michiganensis spread from foci of infection, but symptoms of secondary infection occurred earlier and were more severe on the susceptible cultivar.