Effect of Vermicompost, Chemical Fumigation, and Anaerobic Soil Disinfestation on Agrobacterium Tumefaciens Abundance Under Walnut Nursery Conditions.

The primary walnut rootstock used in California, Paradox, is susceptible to infection by Agrobacterium tumefaciens, which causes crown gall. Currently, walnut nurseries control weeds and infection by A. tumefaciens and other phytopathogenic agents by using preplant chemical fumigation. While A. tumefaciens is susceptible to the common soil fumigants, methyl bromide and Telone-C35, these fumigants are either being phased out or under increasing regulation.  In addition, these fumigants do not prevent re-infection of the soil by A. tumefaciens, which can reside on the husk of walnut seeds planted into these fumigated soils.  Increasing the soil microbial diversity surrounding the walnut seeds planted into fumigated soil may provide greater competition for A. tumefaciens, thus reducing its abundance and limiting crown gall incidence. In laboratory trials we found that A. tumefaciens abundance significantly decreased in fumigated soils amended with vermicompost. We performed two field trials in which plots were pre-treated with:  1) the chemical fumigant alternative method, anaerobic soil disinfestation (ASD), 2) the chemical fumigant Telone-C35, or 3) remained without treatment as a no-treatment control. Vermicompost was then applied to half the furrows in each treatment and walnut seeds, dipped in a suspension of a rifampicin-resistant mutant of A. tumefaciens, were planted into all treatments. The abundance of A. tumefaciens on seeds was assessed over a 12 week period prior to germination using dilution plating. The abundance of A. tumefaciens on seeds planted into soils pre-treated with ASD or with Telone-C35 and amended with vermicompost decreased over time and by 12 weeks post-planting, were significantly less than populations found on seeds planted in soils without vermicompost amendments. In addition, Paradox seed germination rates were greater in fumigated soils amended with vermicompost, than in ASD-treated soils amended with vermicompost.