DISSEMINATION OF CLAVIBACTER-MICHIGANENSIS SUBSP MICHIGANENSIS BY PRACTICES USED TO PRODUCE TOMATO TRANSPLANTS

Rifampin-resistant (Rif+) strains of Clavibacter michiganensis subsp. michiganensis were used to study the epidemiology of bacterial canker of tomato (Lycopersicon esculentum). Clipping tomato seedlings in transplant beds with a rotary mower transmitted Rif+ C. m. michiganensis from inoculated seedlings to healthy seedlings. Epiphytic populations of Rif+ C. m. michiganensis were first detected on clipped, asymptomatic seedlings 9-13 days after the first clipping. Populations of Rif+ C. m. michiganensis on clipped seedlings ranged from 0 to about 10(7) cfu/g fresh weight throughout the clipping period. Symptoms did not appear on these seedlings until 14-20 days after transplanting, or about 31-40 days after the first clipping. The incidence of systemically infected plants in production fields 1 wk before harvest increased about 10% for each 0.1% increase in incidence of infected seedlings in transplant beds before clipping. When healthy and diseased seedlings were harvested and mixed in bundles at various contamination rates, Rif+ C. m. michiganensis was spread from diseased to healthy seedlings by seedling harvest practices. Populations of Rif+ C. m. michiganensis recovered from bundles of transplants, and incidence of systemically infected plants in production fields were affected significantly by the percentage of diseased seedlings per bundle (i.e., contamination rates). Symptoms of systemic infection on healthy seedlings mixed with infected seedlings were first observed 35-42 days after transplanting. Incidence of systemically infected plants in production fields 1 wk before harvest increased about 3-5% for each 1% increase in the contamination rate. Because Rif+ C. m. michiganensis is spread by clipping and seedling harvest practices used in the production of tomato transplants, bacterial canker can be a cultural practice disease. Seed transmission rates as low as 0.01-0.05% (one to five seeds per 10,000) could initiate an epidemic of bacterial canker in tomato production fields in the Midwest because of the cultural practices.