REGULAR ARTICLE Effect of anaerobic soil disinfestation on the bacterial community and key soilborne phytopathogenic agents under walnut tree-crop nursery conditions S.L. Strauss & R.F. Greenhut & A.E. McClean & D.A. Kluepfel Received: 8 June 2016 /Accepted: 24 November 2016 # Springer International Publishing Switzerland (outside the USA) 2017 Abstract Background and aims Anaerobic Soil Disinfestation (ASD) is a chemical-independent approach to managing soilborne phytopathogens. While it has been demon- strated that ASD can suppress phytopathogens in vege- table cropping systems, it has not been examined for control of tree-crop diseases under nursery conditions. Here we report on the potential of using ASD to manage soilborne populations of Agrobacterium tumefaciens and Pythium ultimum under walnut rootstock nursery conditions and compared the efficacy of ASD with the chemical fumigant Telone-C35. Methods Mesh bags of sterile field soil inoculated with either A. tumefaciens or P. ultimum were buried in the first of two trials at 7.6 and 15.2 cm soil depths and in the second trial at 15.2, 45.7, and 76.2 cm soil depths prior to ASD or fumigation treatments. Populations of A. tumefaciens and P. ultimum in the mesh bags were determined immediately after ASD treatment. Bacterial rRNA was extracted from bulk soils at these depths prior, immediately following ASD, and 4 months post-ASD to determine the effect of ASD on the bacterial community composition. Results and conclusions After seven days of ASD treat- ment, populations of both phytopathogens fell below detection limits at 7.6 and 15.2 cm soil depths. After completion of the six-week ASD treatment or 4-week fumigation period, populations of A. tumefaciens remained below detection limits at 7.6 and 15.2 cm depths and were not significantly different from A. tumefaciens population levels detected in the fumi- gation treatment. Futhermore, A. tumefaciens popula- tions were significantly reduced, compared to no- treatment control soils, in both ASD and fumigation treatments at the 45.7 and 76.2 cm depths. Pythium ultimum populations dipped below detection limits after ASD treatment at 7.6 and 15.2 cm soil depths, and were significantly lower than no-treatment controls at the 45.7 cm depth. The bacterial community composition of ASD-treated soils also differed significantly as a function of soil depth, and the effect of ASD on the bacterial community composition persisted over time. The documented ASD-induced changes in the soilborne bacterial communities may have contributed to the pop- ulation reductions observed for both A. tumefaciens and P. ultimum. The significant reductions of A. tumefaciens and P. ultimum, down to a depth of 76.2 cm, were similar for both ASD and Telone C-35 treated soils. This illustrates the potential of ASD to replace chemical Plant Soil DOI 10.1007/s11104-016-3126-4 Responsible Editor: Stéphane Compant. Electronic supplementary material The online version of this article (doi:10.1007/s11104-016-3126-4) contains supplementary material, which is available to authorized users. S. Strauss : R. Greenhut : A. McClean : D. Kluepfel (*) USDA-ARS Crops Pathology and Genetics Research Unit, University of California, 1 Shields Avenue, Davis, CA 95616, USA e-mail: Daniel.kluepfel@ars.usda.gov Present Address: S. Strauss Southwest Florida Research and Education Center, University of Florida, 2685 State Rd. 29 N, Immokalee, FL 34142, USA