| Fifteen selected PGPR strains with induced systemic resistance (ISR) activity against several fungal, viral and bacterial plant pathogens, were initially tested during 1996--97 for their possible activity against the root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, on cucumber and tomato under greenhouse conditions. Some strains, including Pseudomonas fluorescens 89B-61 and Serratia marcescens 90-166, reduced nematode damage in initial whole root tests. Using a split-root test, reductions in nematodes suggest ISR as a possible mechanism for half of the strains. In vitro tests utilizing root-knot nematode eggs and juveniles revealed that some rhizobacteria, such as Bacillus pumilus SE-52 and B. cereus A-5, directly enhanced egg hatching and inactivated second-stage juveniles. In vitro characterization of all bacteria for production of proteases, chitinases, and siderophores suggested that biocontrol of nematodes using rhizobacteria with ISR activity is not associated with a single phenotypic profile. Further examination of rhizobacteria in combination with selected organic amendments and botanical aromatics resulted in plant growth-promotion and, under some conditions, to induction of suppressiveness to nematodes in a tomato transplant system under greenhouse conditions. Large growth promotion effects were observed in seedling height with the chitin amendment alone, and this effect was sometimes enhanced by bacterial treatments. In field trials at Florida and Alabama with soils naturally infested with Meloidogyne spp., there were low rates of root-knot nematode galling. In addition, severe outbreaks of other soilborne diseases confounded the evaluations of such trials. Significant reductions in galling, compared to the nontreated control, occurred with the following treatments: chitin, IN937a, IN937b, chitin + GB03 + IN937a + IN937b, methyl bromide and aldicarb; however, this protection was not consistent. Population studies revealed that the type of amendment or botanical aromatic used in transplant preparation changed population numbers of certain bacterial groups. For example, chitin treatments increased counts of heat-tolerant, actinomycetes, copiotrophs, and Gram-positive bacteria. All bacterial populations monitored reached their highest numbers in the plugs amended with both chitin and benzaldehyde. Two introduced spore-forming bacilli (IN937a and GB03) survived in the transplant mix and colonized the rhizosphere of tomato plug for a period of 11 weeks, based on studies with rifampicin marking and selective media. |
