Engelmann Spruce Chemotypes in Colorado and their Effects on Symbiotic Fungi Associated with the North American Spruce Beetle

Conifer secondary metabolites play a key role in mechanisms of resistance to biotic disturbance, especially by bark beetles and beetle-associated microorganisms. Here, we describe variation in constitutive monoterpenes isolated from Engelmann spruce, Picea engelmannii, phloem across fourteen high-elevation populations in the Rocky Mountains of Colorado, and test interactions between phloem monoterpenes and an endophloedic symbiotic fungus, Leptographium abietinum, associated with the North American spruce beetle, Dendroctonus rufipennis. We consistently identified ten monoterpenes in Engelmann spruce phloem, and the trees in our samples could be classified into two geographically interspersed chemical phenotypes, or ‘chemotypes’: one in which α- and β-pinene were the most abundant monoterpenes, and one in which 3-carene was the most abundant monoterpene. Media amended with low concentrations of α-pinene, β-pinene, 3-carene, myrcene, and terpinolene stimulated growth of L. abietinum. Increasing monoterpene concentrations uniformly retarded fungal growth. Linalool completely suppressed fungal growth at all concentrations, while terpinolene completely suppressed growth at low and intermediate concentrations, indicating relatively high toxicity of these compounds. Tests with monoterpene blends representing the ‘average’ monoterpene composition of each chemotype indicated that representative chemotypes are equivalent in fungistatic activity, with chemotype blends being inhibitory even at low concentrations. Total constitutive monoterpene abundances in Engelmann spruce phloem ranged from 42 to 1796 μg/g. Induction of Engelmann spruce phloem monoterpenes in response to L. abietinum or other biotic agents has yet to be quantified, but is important for further understanding Engelmann spruce resistance to the D. rufipennis-L. abietinum complex.