The role of fluorescent pseudomonad bacteria in the resistance of paper birch (Betula papyrifera Marsh.) to armillaria ostoyae in the southern interior of British Columbia

The overall goal of this study was to investigate the role of fluorescent pseudomonad bacteria in the resistance of paper birch to infection by the root rot fungus Armillaria ostoyae in the southern interior of British Columbia. The objectives of this study were to compare the population sizes of pseudomonads in the rhizosphere of pure birch, pure Douglas-fir and mixed stands of the two species and investigate the incidence and strength of antagonism exhibited by fluorescent pseudomonad bacteria isolated from these different stand types toward Armillaria ostoyae in dual culture in vitro experiments. The pseudomonad populations were determined by growing paper birch and Douglas-fir seedlings in soils collected from young and mature birch, Douglas-fir and mixed stands and isolating the pseudomonads from the roots of these seedlings. The birch stands supported an average of 4 times more pseudomonads than the Douglas-fir stands and the mixed stands had intermediate populations. The young stands had twice as many rhizosphere pseudomonads as the mature stands. There was no difference in the populations of pseudomonads isolated from the paper birch and Douglas-fir seedlings grown in the same soil types on a per seedling basis. Th<?re was a significant positive correlation between pseudomonad populations and percent cover of birch and density of birch in the stands (stems per hectare). Populations of pseudomonads were strongly negatively correlated with basal area of Douglas-fir, percent cover of Douglas-fir and carbon:nitrogen ratio of the soil. Of the 270 pseudomonad isolates paired with Armillaria ostoyae in vitro, 53% inhibited the linear growth of the Armillaria fungus and 94% caused a reduction in the mass of the fungus compared to the unchallenged control Armillaria isolates. Overall, the pseudomonad bacteria isolated from the pure birch stands were significantly more antagonistic toward Armillaria than those isolated from the Douglas-fir stands. In the 20 pseudomonad isolates tested, the mechanism of antagonism exhibited by these bacteria appeared to be the production of antifungal substances such as antibiotics. IV This study provides evidence that paper birch provides a more favorable environment for fluorescent pseudomonad bacteria in the rhizosphere and indicates a possible mechanism by which paper birch can positively influence the health of managed forest stands.