Effects of Rhizobacteria on Root-Knot Nematodes and Gall Formation

Three hundred and fifty-four randomly selected bacteria from plant rhizospheres, when tested for activity against Meloidogyne incognita, caused a wide range of effects from a reduction to an increase of root galling on tomato and cucumber in greenhouse tests. Results were highly variable, even with strains that previously had given significant differences. A bioassay, based on selecting bacterial strains that produced nematicidal compounds in vitro, proved to be a better and more rapid means of identifying promising nematode antagonists. About 1% of more than 5,000 bacteria isolated from rhizospheres of different plants produced detectable compounds that affected the vitality of second-stage juveniles of M. incognita in an in vitro test. Twenty percent of these subsequently reduced the number of galls on cucumber in a soil-free pouch system when applied as a seed treatment. Selected strains were applied as a drench to nonsterile soil infested with M. incognita. White clover plants growing in bacteria-treated soil had fewer galls and larger root systems. Both plant top and root weights were significantly greater compared with the nontreated control