Impact of biological amendments on Agrobacterium tumefaciens survival in soil

• Published in: Applied soil ecology : a section of Agriculture, ecosystems & environment Vol. 87; pp. 39 - 48
• Main Authors: Strauss, S.L., Stover, J.K., Kluepfel, D.A.
• Format: Journal Article
• Language: English
• Published: 01.03.2015
• Subjects: Agrobacterium tumefaciens; Juglans; USA, California
• ISSN: 0929-1393
• DOI: 10.1016/j.apsoil.2014.10.016

Paradox, the primary walnut rootstock used in California, is susceptible to Agrobacterium tumefaciens, which causes crown gall. While A. tumefaciens is susceptible to commonly used fumigants such as methyl bromide (MeBr) and Telone-C35 (1,3-dichloropropene and chloropicrin), these fumigants also significantly modify the indigenous soil microbial community, which impacts the ability to inhibit A. tumefaciens populations introduced into these soils. We hypothesized increasing microbial diversity and activity in fumigated soil would provide greater competition for A. tumefaciens, thereby reducing its abundance and limiting disease incidence. Three commercially available soil amendments at multiple rates were tested in native and Telone-C35 fumigated soils in laboratory experiments: vermicompost and two commercial microbial fermentation mixtures marketed as soil amendments. The amendment-soil mixture was infested with a rifampicin-resistant mutant of A. tumefaciens whose abundance was determined by dilution plating and quantitative PCR over the course of four weeks. The two commercial fermentation amendments had no effect on A. tumefaciens population dynamics. However, after a four-week exposure to 10% (w/w) or greater vermicompost, A. tumefaciens populations declined, and in some cases dipped, below detection limits. This suppressive effect appeared to be predominately biotic as heat-treated vermicompost had no impact on A. tumefaciens populations. In addition, culture independent analysis using the Shannon index of diversity found a positive correlation between A. tumefaciens suppression and microbial community diversity. The vermicompost used in this study also provided a consistently similar microbial community in individual batches used over a 9-month period, making it suitable for reliable commercial use.