A microbial culture collection composed of 1820 bacterial strains, including 298 actinomycete strains, was established from the roots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings harvested from conifer nurseries and forest sites. Two hundred and thirty-four strains inhibited the growth of Fusarium, Cylindrocarpon, and (or) Pythium spp. in in vitro assays. A significantly greater proportion of bacterial strains from actinomycete genera exhibited antifungal properties compared with bacterial strains from nonactinomycete genera. Eighty-nine percent of identified inhibitory strains were Streptomyces, Streptoverticillium, Bacillus, Pseudomonas, or Burkholderia species. The actinomycete species were isolated almost exclusively from forest seedlings. Recovery of inhibitory strains representing 29 microbial species was enhanced using a variety of methods to isolate microorganisms from the roots of seedlings from nursery and forest sites. Bacterial strains (including actinomycete strains) with antifungal activity were tested for in vitro growth inhibition of six clinical human bacterial pathogens (Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, Proteus mirabilis, and Pseudomonas aeruginosa). Forty-eight percent of the tested strains inhibited one or more human pathogens, Inhibitory activity towards fungal and bacterial pathogens was strain specific, not species specific, and many inhibitory strains exhibited broad-spectrum activity. Strains with antifungal activity against several conifer root pathogens were also more likely to inhibit multiple species of clinical bacterial pathogens.
The influence of inoculation with rhizosphere bacteria on hybrid spruce (Picea glauca × engelmannii) seedling growth was investigated by isolating bacteria from the rhizosphere of naturally regenerating spruce seedlings collected from two ecologically distinct zones of British Columbia. Forty bacterial strains from spruce were screened in a seedling growth experiment, and several strains were shown to stimulate spruce seedling growth. The three most effective spruce growth promoting strains from each ecological zone were then tested in a seedling growth response specificity experiment in which seed, soil, and bacteria from both spruce ecosystems were examined in all possible treatment combinations, including pasteurized soil. Spruce seedling growth was primarily affected by seed and soil source (Salmon Arm > Mackenzie for both factors), and by soil pasteurization (pasteurized > unpasteurized). Significant growth effects due to bacterial treatment in the specificity experiment occurred only in pasteurized soil, but seedling biomass accumulation was stimulated by up to 59% in response to inoculation. There was no evidence of growth response specificity that was related to plant – bacteria adaptation within spruce ecotypes; two of the Mackenzie bacterial strains significantly inhibited growth of Mackenzie seedlings in Mackenzie soil, but two strains stimulated the growth of Salmon Arm seedlings, one in each soil type. Two Salmon Arm strains significantly stimulated growth of Mackenzie seedlings on Salmon Arm soil, and two strains stimulated the growth of Salmon Arm seedlings, one in each soil type. Seedling biomass was greater when treatments of spruce seed, soil, and rhizosphere bacteria that originated from the same geographic site were pooled and compared with treatments in which at least one factor originated from the other site. However, this effect was explained by the significant seed × soil interaction in the absence of bacteria; mean seedling biomass was greatest when spruce seed and soil treatments with the same geographic origin were pooled and compared with unrelated seed – soil combinations. Seedlings from pooled treatments were also significantly larger when bacteria and soil had a common geographic origin, but only when tested in pasteurized soil. Our results indicate that spruce growth promoting rhizosphere bacteria can be isolated from naturally regenerating spruce seedlings, but growth response variability was observed between trials. Seedling growth promotion by bacterial strains was not related to the use of spruce ecotypes, soil, and (or) rhizosphere bacteria with a common geographic origin. Key words: spruce seedlings, growth, inoculation, rhizosphere bacteria.
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