Tree vigor and the susceptibility of Douglas-fir to Armillaria root disease

The main objective of this thesis was to determine the influence of tree vigoron susceptibility to Armillaria root disease. First, the effect of thinning, fertilizingand pruning on tree vigor of four young Douglas-fir (Pseudotsuga menziesii var.menziesii) plantations was explored. Tree vigor was calculated by measuring woodproduction per unit of leaf area per year. Then, the presence of Armillaria rootdisease was compared to tree vigor at plot and individual tree levels, to determinethe relationship between tree vigor and the susceptibility to the disease. Matingtests, somatic compatibility tests, and isozyme analysis were used to determine thespecies and genets of Armillaria occuring in the study sites. Tree vigor and growthwere substantially increased by thinning, and neither fertilization nor pruningshowed an important effect. Armillaria ostoyae was the only Armillaria speciesfound causing tree mortality. Few, large genets characterized the populationstructure of this species, suggesting that vegetative spread over a long period oftime was the dominant event in occupying the area. Ai-miliaria ostoyae seemed tobe aggressive enough to produce mortality where it was present, regardless of the
vigor status of the host. It is concluded that increasing tree vigor as a strategy toreduce disease losses may not be a feasible option.