Influence of crop production practices on Pasteuria penetrans and suppression of Meloidogyne incognita

Abstract

Pasteuria penetrans is a parasite of root-knot nematodes (Meloidogyne spp.). Infected nematodes are not killed by the bacterium, but instead of producing eggs, females produce millions of infectious endospores. In addition to sterilizing females, P. penetrans can reduce nematode infection of roots when spore densities in soil are high because juveniles become so heavily encumbered, movement is restricted. A 4-year field study was conducted to determine 1) if fumigation with 1,3-dichloropropene (1,3-D, 28L/ha) would have a negative effect on P. penetrans and general suppression of nematodes, 2) if the occasional use of 1,3-D would be as detrimental to P. penetrans as a yearly application, 3) if tillage influenced the abundance of P. penetrans spores, and 4) if the P. penetrans at the field site was contributing to suppression of M. incognita. Fumigation with 1,3-D reduced the abundance of P. penetrans spores in the soil compared to the no-fumigation control and there was no difference between a yearly application of the fumigant and occasional applications. Tillage (conventional and strip) did not affect spore abundance. The reduction of P. penetrans spores by fumigation was small compared to the year-to-year fluctuations in spore densities. Spores per assay nematode varied from 6.3 in 2012 to 0.8 in 2014. In 2012, P. penetrans appeared to suppress populations of M. incognita to very low levels (⩽10% of the root system with galls). The extreme changes in spore densities over the course of this study were likely due to a combination of factors including the density-dependent dynamics between the nematode and bacterium, the self-limiting effect of the bacterium at high spore densities, and leaching of spores out of the root zone during rain and irrigation events. Within each fumigation treatment, there was an inverse relationship (P⩽0.003, r=−0.40 to −0.58) between spore abundance and root galling indicating that the bioassay to estimate spore numbers in the spring was a good predictor of the level of nematode suppression.