COLONIZATION OF TOBACCO SEEDLING ROOTS BY FLUORESCENT PSEUDOMONAD SUPPRESSIVE TO BLACK ROOT-ROT CAUSED BY THIELAVIOPSIS-BASICOLA

Fluorescent pseudomonad strain RD: 1, isolated from soil containing decomposing rye residues, was introduced into the rhizosphere of tobacco seedlings by root bacterization. Bacterized and non-bacterized seedlings were planted in non-sterilized fallow soil and in soil containing decomposing rye residues artificially infested with the chlamydospores of Thielaviopsis basicola. Root bacterization with strain RD: 1 significantly increased shoot and root dry weights of tobacco seedlings after 45 days when planted in fallow soil and in soil containing decomposing rye residues, compared with non-bacterized controls. Strain RD: 1 established population densities ranging from 1.0 x 10(3) to 3.6 x 10(5) c.f.u. g-1 dry weight of roots on rhizosphere of tobacco seedlings following introduction. It also significantly reduced the severity of black root rot disease in both soils. Populations of fluorescent and non-fluorescent pseudomonads and total aerobic bacteria associated with the seedling rhizospheres were significantly reduced by root bacterization with RD: 1. Strain RD: 1 also inhibited vegetative growth of T. basicola in vitro. The growth-promoting and disease-suppressing effects of RD: 1 make it potentially useful in biocontrol of T. basicola.