Indirect effects of the bacterial soil aggregation on the distribution of Pasteuria penetrans, an obligate bacterial parasite of plant-parasitic nematodes

The effects of the inoculation of two fluorescent Pseudomonas strains (P. mendocina and Pseudomonas sp.) on the soil aggregation in the rhizosphere have been tested. Their indirect effects on the conservation of spores of Pasteuria penetrans, a bacterial endospore-forming parasite of plant-parasitic nematodes Meloidogyne spp., and on the infection of Meloidogyne javanica juveniles were investigated. The amount of rhizosphere soil adhering to the root systems was higher in the soils inoculated with both bacterial strains P. mendocina and Pseudomonas sp. However, juveniles of Meloidogyne were more numerous in the soil inoculated with Pseudomonas sp. and the multiplication rate of M. javanica did not differ regardless of the treatment. Furthermore, the proportion of juveniles infested by P. penetrans spores was highest in soil inoculated with P. mendocina (B23). The spores were distributed, following a similar pattern in the soil aggregate fractions excepted in the soil inoculated with P. mendocina (B23). In the soil inoculated with B23, about 50% of the spores present in the rhizosphere were retained in the coarser structure characterized by aggregates >200 μm. The role of the bacterial extracellular polysaccharides on soil and spores aggregation and on the Meloidogyne infection may explain these results.