Effect of Anaerobic Soil Disinfestation on Soilborne Phytopathogenic Agents and Microbial Community Under Tree-Crop Nursery Conditions.

Anaerobic soil disinfestation (ASD) is a fumigation-independent strategy for controlling soilborne pathogens. While ASD has been shown to suppress soilborne pathogens for several vegetable crops, it has not been examined for tree-crop diseases. In addition, little is known about the impact on the soil microbial community composition. We investigated the potential of ASD to manage soilborne populations of Agrobacterium tumefaciens and Pythium ultimum under walnut nursery conditions. The following experiment was repeated twice: rice bran at 20 metric tons/ha was applied to ASD plots, irrigated for 24 h (13 cm of water, 1 drip emitter per 930 cm²), and covered by TIF plastic for 6 weeks. Mesh bags of sterile soil inoculated with either A. tumefaciens or P. ultimum were buried at 6”, 18”, and 30” depths prior to ASD treatments. Anaerobic conditions were generated down to 30” depth within 1 week and maintained for 6 weeks. A. tumefaciens and P. ultimum populations were reduced below detection limits during ASD in all treatment replicates at 6 and 18”, and in some cases at 30.” Next-generation sequencing of soil bacterial 16S rRNA genes revealed that the soil bacterial community composition is significantly altered at all depths after ASD. In particular, there were increases in several facultative and obligate anaerobic organisms, such as Clostridia sp., and decreases in other common soil bacteria such as Bacillius sp., and Actinobacteria sp. The ability of ASD to alter the soil microbial community and suppress specific pathogens down to depths of 30” indicates great potential for ASD as a fumigant-alternative for nursery tree-crops that require pre-plant pathogen suppression to at least 30” soil depths. The specific changes in the soil bacterial community provide a basis to determine whether these taxa contribute to soilborne disease suppression in ASD.