Bidirectional interactions between Grosmannia abietina and hybrid white spruce: Pathogenicity, monoterpene defense responses, and fungal growth and reproduction

Bark beetle-vectored phytopathogenic fungi can play critical roles in how beetle outbreaks affect the health of forest trees. However, trees can defend themselves against fungal infection. How Grosmannia abietina, a symbiotic fungus of spruce beetle (Dendroctonus rufipennis), affects the health of mature hybrid white spruce (Picea engelmannii x glauca) and is in turn affected by the tree's defenses are unknown. We conducted field inoculations of this spruce to study the degree of pathogenicity of G. abietina, characterized the trees' resulting defensive monoterpene responses, and assessed monoterpene effects on fungal growth and reproduction in laboratory bioassays. Our results indicated that G. abietina is phytopathogenic to hybrid white spruce, which induced monoterpenes in response to infection. Dominant induced monoterpenes generally inhibited fungal growth but stimulated spore production. These findings provide insights into the bidirectional effects between spruce beetle-vectored fungi and host trees, highlighting the complex role of monoterpenes in modulating fungal activities.