Rhizobacteria as biocontrol agents for soybean cyst nematode, Heterodera glycines Ichinohe

Rhizobacteria were isolated at random from Arkansas soybean fields and tested for their effects on soybean cyst nematode (SCN) in pasteurized silt loam soil in a greenhouse. Among the 201 isolates, 138 had no influence, but 36 (positive isolates) resulted in reduced and 27 (negative isolates) in increased reproduction of SCN. When 20 positive and 5 negative isolates were retested, results were highly variable. No clear relationship was detected between the inhibition of egg hatch or immobilization of J2 in vitro and an antagonistic reaction toward nematodes in vivo by the isolates. Amendment of the soil with 0.1% (w/w) peptone or casein hydrolysate did not stabilize the effects of selected positive isolates on SCN. Most of the 25 isolates were identified as Pseudomonas or Bacillus species.;Chitinolytic rhizobacteria also were isolated and tested in the soil amended with 0.6% (w/w) chitin in a greenhouse. Five of 64 isolates tested consistently resulted in reduced reproduction of SCN compared with controls. Similar results were obtained with the five isolates when chitin level was reduced to 0.4 or 0.5%. They also resulted in lower reproduction of SCN compared with controls in microplots in soil amended with 0.4% chitin. In general, their effects on plant growth were variable, ranging from inhibition to promotion. Three of them were identified as Streptomyces species. The degree of chitinolytic activity of an isolate in vitro was not directly related to the extent of its harmful effect on nematodes in soil. The mode-of-action of the five isolates on SCN is unclear.;Combinations of selected chitinolytic isolates with a fungal biocontrol agent (ARF18) for SCN suppressed the reproduction of SCN more than each organism applied alone in soil amended with none or 0.4% chitin in a greenhouse. The effects of the combinations on plant growth generally were variable. The relationship between the components that comprised effective combinations was not necessarily mutually beneficial.;Studies with one of the five isolates showed that this Streptomyces sp. C11 colonized well in a gnotobiotic system, but not in soil not amended with chitin.