2013 Annual Report
The primary mode of crown gall infection of walnut seedlings is hypothesized to occur when A. tumefaciens infected seeds are planted in fumigated soils where pre-plant fumigation creates an environment which facilitates enhanced development of A. tumefaciens populations. This results in increased crown gall incidence. Adding at least 50% vermicompost (w/w) to fumigated soils significantly reduced A. tumefaciens population development. This antagonistic impact on A. tumefaciens populations appears to be the result of changes in microbial activity since heat-treated vermicompost amendments exhibited no suppressive effects. The reduction in A. tumefaciens abundance after vermicompost amendment occurred throughout the 28-day incubation period. The biologically driven suppressive effects of vermicompost appear to be the result of changes in both microbial community composition and total microbial abundance as revealed by a culture-independent analysis. There was a significant decrease in A. tumefaciens abundance in soils with 50% or greater vermicompost application, and no significant difference in A. tumefaciens abundance between the 50%, 80% and 100% applications. However, there was no significant decrease in A. tuemfaciens populations in soils with 2% or 10% vermicompost application. Both standard compost and vermicomposts have significantly greater microbial diversity compared to fumigated agricultural soils. In contrast to the effect of the vermicompost addition, the two commercial microbial fermentation mixtures had no suppressive effect on A. tumefaciens. The vermicompost used in this study provided a consistently similar microbial community in each batch obtained from the commercial operation. Discriminate analysis revealed vermicompost samples clustered as a function of source material and not sample time from a given source suggesting it may be possible to provide a microbially consistent source of vermicompost.