Tree vigour and the susceptibility of Douglas fir to Armillaria root disease

In this study the effects of thinning, fertilization and pruning on the vigour of Douglas fir (Pseudotsugamenziesn) and its susceptibility to Armillaria root disease were explored. Tree vigour was definedas the relative capacity for tree growth, expressed as the above-ground biomass increment per unit ofphotosynthetic tissue, or growth efficiency (GE). It has been hypothesized that trees with higher GEcan better resist pathogen attack, and that GE can be used as a predictor of tree susceptibility to disease.In a previous study, four Douglas fir plantations were thinned, fertilized and pruned in all combinations,and the effects of these treatments on tree vigour were measured after 10 years. Root disease was not afactor in the initial study design, and mortality was ignored until 8 years after the treatments wereapplied. The results of an earlier study were utilized and the correlation between Armillaria root diseaseincidence and the effects of earlier stand treatments on tree growth was tested. Armillaria ostoyae wasthe primary mortality agent in the study area. Disease incidence on infested subplots ranged from 2 to20%. Armillaria ostoyae incidence was the highest at medium tree density (6.1%), slightly lower onthe low density (5.6%), and lowest on the unthinned plots (3.8%). There were no significant correlationsbetween disease incidence and previous tree growth. The vigour of trees that became symptomatic ordied by 1993 was not significantly different in 1983-85 from the vigour of trees that remained asympto-matic. On these sites, in areas of infestation, A. ostoyae is killing the largest, fastest growing trees, aswell as less vigorous trees. Armillaria continues to cause mortality, regardless of the growth efficiencyor growth rate of the host.