This study characterized the chemical response of healthy red pine to artificial inoculation with the bark beetle-vectored fungus Leptographium terebrantis. In addition, we sought to determine whether stress altered this induced response and to understand the implications of these interactions to the study of decline diseases. Twenty-five-year-old trees responded to mechanical wounding or inoculation with L. terebrantis by producing resinous reaction lesions in the phloem. Aseptically wounded and wound-inoculated phloem contained higher concentrations of phenolics than did constitutive tissue. Trees inoculated with L. terebrantis also contained higher concentrations of six monoterpenes, alpha-pinene, beta-pinene, 3-carene, limonene, camphene, and myrcene, and higher total monoterpenes than did trees that were mechanically wounded or left unwounded. Concentrations of these moneterpenes increased with time after inoculation. Total phenolic concentrations in unwounded stem tissue did not differ between healthy and root-diseased trees. Likewise, constitutive monoterpene concentrations in stem phloem were similar between healthy and root-diseased trees. However, when stem phloem tissue was challenged with fungal inoculations, reaction tissue from root-diseased trees contained lower concentrations of alpha-pinene, the predominant monoterpene in red pine, than did reaction tissue from healthy trees. Seedlings stressed by exposure to low light levels exhibited less extensive induced chemical changes when challenge inoculated with L. terebrantis than did seedlings growing under higher light. Stem phloem tissue in these seedlings contained lower concentrations of ol-pinene than did nonstressed seedlings also challenge inoculated with L. terebrantis. It is hypothesized that monoterpenes and phenolics play a role in the defensive response of red pine against insect-fungal attack, that stress may predispose red pine to attack by insect-fungal complexes, and that such interactions are involved in red pine decline disease. Implications to plant defense theory and interactions among multiple stress agents in forest decline are discussed.
