Pine Monoterpenes Drive Growth, Competition, and Volatile Metabolism of Symbiotic Fungi Associated with the Mountain Pine Beetle

Monoterpenes are key conifer defences that shape interactions between the mountain pine beetle and its symbiotic phytopathogenic fungi. Conifer monoterpenes vary both within and between tree species, yet how this variation affects symbiotic fungi remains poorly understood. We tested how monoterpene blends from three geographically distinct subspecies of Pinus contorta, including subsp. contorta, latifolia, and murrayana can affect the growth and volatile emissions of two fungal symbionts, Grosmannia clavigera and Ophiostoma montium, of the mountain pine beetle. We conducted a factorial experiment using monoterpene blends representing the three subspecies in combination with three fungal cultures (two alone and one mixed) to quantify fungal biomass and volatile organic compounds. Monoterpene blends reduced fungal biomass, with the subsp. murrayana blend is the most inhibitory Grosmannia clavigera tolerated monoterpenes better than O. montium, suggesting enhanced virulence. Fungal volatile emissions varied by blend and culture; 2-methyl-1-butanol and isobutanol were positively correlated with growth, whereas verbenone and cis-grandisol were negatively correlated. These findings show that the monoterpene diversity of host trees differentially modulates fungal growth and volatile emission, revealing chemical mechanisms underlying host colonization by the mountain pine beetle across regions.