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UBC Theses and Dissertations

Identification of phosphate solubilizing bacteria and evaluation of their application with insoluble phosphorus fertilizers to soils from certified organic orchards affected by replant disease Thurston, Molly Adair


Replant disease (RD) affects the growth and establishment of young fruit trees in old orchard soils. Organic management strategies are needed as an alternative to chemical controls. Improved phosphorus (P) nutrition to stimulate root growth and improve tree establishment is a well-established strategy; however, only insoluble Rock Phosphate (RP) and Bone Meal (BM) inputs are available to Canadian organic growers. The ability of specific plant growth promoting rhizobacteria to solubilize phosphate may improve P availability and its uptake in young apple trees, replanted into inoculated orchards. In this study, 101 bacteria isolated from the roots of legumes from Saskatchewan soils were screened for P solubilization. Thirty-four of these bacteria were positive for P solubilization as measured by halo diameter production on calcium phosphate medium. Twelve isolates showing the largest halo diameters and three known P solubilizing bacteria (PSB) were compared on three media: calcium phosphate, Pikovskaya (PVK) and PVK with bromophenol blue. All twelve isolates previously identified as Pseudomonas, Rhanella, Serratia and Klebsiella spp. solubilized P on all media, although the halo diameters varied among media. The isolates were tested in liquid culture, where a marked decrease in the pH of the solution was observed and six isolates were identified for further testing in growth pouch assays in the presence of insoluble P. The root growth of apple seedlings inoculated with one of the six bacterial isolates, showed significant increases in total root length, surface area and the number of root tips compared to the control after four weeks of incubation. Three isolates were selected for greenhouse bioassays using five RD-affected soils collected from organic orchards. These isolates were inoculated onto apple trees, alone or in combination with RP or BM. Two field trials were simultaneously conducted in organic apple orchards, using iii the same treatments; however there were no significant effects of the isolate treatments in either set of experiments. Although the strongest P solubilizers did not enhance tree growth in the greenhouse and orchard trials, the in vitro work showed the potential of PSB as a tool to mitigate the impact of RD.

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Attribution-NoDerivs 2.5 Canada