Jack pine dwarf mistletoe (Arceuthobium americanum Nutt. ex Engelm) may be associated with Armillaria root disease (Armillaria ostoyae (Romagn.) Herink) in accounting for significant decline and mortality of jack pine (Pinus banksiana Lamb.) in the Belair Provincial Forest, Manitoba, Canada. Sixty jack pines were randomly selected from three mortality centers and categorized into three vigor classes, each with and without dwarf mistletoe. For each tree, the root collar and all primary lateral roots within 1m of the bole were excavated and examined for signs and symptoms of Armillaria root disease. Cultures were made from symptomatic roots to confirm the presence of Armillaria. Preliminary nonparametric analysis suggests that the odds of jack pine being infected with both dwarf mistletoe and Armillaria are not significantly greater than jack pines being infected with dwarf mistletoe only (p=0.05). Other analyses indicated a weak tendency for increased Armillaria colonization of dwarf mistletoe infected jack pines. Trends observed within the data suggest that Armillaria ostoyae acts opportunistically, attacking jack pines weakened by dwarf mistletoe. For example, vigorous dominant and codominant trees, with or without dwarf mistletoe, were less frequently and less extensively colonized by Armillaria. Declining trees with dwarf mistletoe had more extensive colonization than trees without dwarf mistletoe. Root systems of dead trees regardless of dwarf mistletoe rating were completely colonized. Crown position also appears to play a role in the extent of Armillaria colonization of this shade intolerant species. However, small sample size reduced the power of statistical tests for detecting significant results.
