Survival ofPseudomonas solanacearumin Selected North Carolina Soils

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Abstract
Survival of P. solanacearum was monitored from 1 growing season to the next in microplots containing soils from 4 North Carolina sites that varied greatly in the occurrence of bacterial wilt of tomato [Lycopersicon esculentum] from year to year. The microplots were infested with 2.5 .times. 109 colony-forming units of P. solanacearum (race 1, biotype 1) per gram of soil in Sept. after soil in 1/2 of the plots was fumigated with methyl bromide. Survival was monitored by using a selective medium and susceptible tomato plants. The bacterial population declined more rapidly in the soils collected from nonpersistent sites than in soils from persistent sites. Fumigation of soils from nonpersistent sites was only partially effective in improving persistance and slowing the rate of decline of P. solanacearum. Populations of P. solanacearum declined rapidly upon addition to soil. Six months after infestation, tomato seedlings were planted directly into the microplots, because the bacterium was no longer detectable by testing it with the selective medium. Bacterial wilt on tomatoes did not develop in the nonfumigated soils collected from nonpersistent sites, but was common within 30 days after transplanting in fumigated soils and in both fumigated and nonfumigated soils collected from persistent sites. In greenhouse experiments, disease development was similar in steamed and nonsteamed soils regardless of the collection site, if tomato plants and inoculum of P. solanacearum were added simultaneously. However, when soils were infested with the bacteria and held for 60 days before the tomato seedlings were transplanted, significantly less disease developed in nonsteamed soils collected from nonpersistent sites, compared to steamed soils from the same site or steamed and nonsteamed soils from the persistent site. Suppressive soil factors (possibly of biological origin) exist at sites where P. solanacearum does not persist from season to season.