Suppression of growth of Ralstonia solanacearum, tomato bacterial wilt agent, on/in tomato seedlings cultivated in a suppressive soil

To identify the sites responsible for the suppressiveness of tomato bacterial wilt in a suppressive soil, population dynamics of Ralstonia solanacearum in non-rhizosphere soil, roots and stems of tomato plants was compared between a wilt-conducive soil and a suppressive soil both of which were artificially infested with the pathogenic strain SL8, Rhizobacteria were recovered as two fractions; root fraction-1 obtained after four washings in water, which was assumed to correspond to the rhizoplane, and root fraction-2 that was macerated after the washings and was assumed to be the fraction corresponding to the part inside of the roots and the firmly attached cells to the rhizoplane, In the conducive soil, all the tomato seedlings wilted while none wilted in the suppressive soil during the 31-d period of cultivation after transplanting to soils infested with the pathogen at an initial density of 10(3) cfu per g dry soil, The number of pathogens in the conducive soil increased first in root fraction-1, then in root fraction-2 and in stems up to 10(8)-10(10) cfu per g wet plant tissue during the 16-d period of cultivation after transplanting, In contrast, in the suppressive soil, the number of pathogens remained below 10(6) cfu per g tissue both in the two root fractions and in stems for 40 d, Survival rate of the pathogen inoculated into the tomato-free soils was higher in the suppressive soil than in the conducive soil, These results suggest that prior multiplication in root fraction-1 at a density above 10(6) cfu per g root is necessary for the pathogen to multiply in root fraction-2 and stems at a density above 10(6) cfu per g for the occurrence of wilt, and that the primary site for suppressiveness of the disease in tomato plants cultivated in the suppressive soil in this report was located in root fraction-1.