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 bacteri al wilt in a suppressive soil, population dynamics of Ralstonia solanacearu m in non-rhizosphere soil, roots and stems of tomato plants was compared be tween a wilt-conducive soil and a suppressive soil both of which were artif icially infested with the pathogenic strain SL8, Rhizobacteria were recover ed as two fractions; root fraction-1 obtained after four washings in water, which was assumed to correspond to the rhizoplane, and root fraction-2 tha t was macerated after the washings and was assumed to be the fraction corre sponding to the part inside of the roots and the firmly attached cells to t he 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 densi ty of 10(3) cfu per g dry soil, The number of pathogens in the conducive so il 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 cu ltivation after transplanting, In contrast, in the suppressive soil, the nu mber of pathogens remained below 10(6) cfu per g tissue both in the two roo t 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 fra ction-1 at a density above 10(6) cfu per g root is necessary for the pathog en to multiply in root fraction-2 and stems at a density above 10(6) cfu pe r g for the occurrence of wilt, and that the primary site for suppressivene ss of the disease in tomato plants cultivated in the suppressive soil in th is report was located in root fraction-1.