Abstract
A branch and stem canker disease of white and Norway spruce was investigated in southern Ontario. The disease was found to be associated with the occurrence of Valsa Kunzei var. picea, a wound parasite commonly occurring on spruce in central and eastern United States and Canada. A study of tree growth in relation to the development of stem cankers revealed that stem-cankered trees had been predisposed to the attack, as indicated by a sudden decrease in diameter growth one or more years in advance of the infection.The predisposing factor is believed to be drought, which weakens trees with a poorly developed root system, thus causing the reduced diameter growth and permitting fungus infection of the main stem and the later developmnet of stem cankers.Spruce grown on sites with a continuous supply of aerated soil water will not be affected by the disease if crowding of trees in the stand is avoided.
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