Growth and virulence of mountain pine beetle associated blue-stain fungi, Ophiostoma clavigerum and Ophiostoma montium

The mountain pine beetle (Dendroctonus ponderosae) is commonly associated with the blue-stain fungi Ophiostoma clavigerum and Ophiostoma montium. Ophiostoma clavigerum is the primary invader of sapwood after beetle infestation and is thought to be the most virulent of the two fungi. Growth of these fungi was studied under oxygen-deficient conditions on malt agar in test tubes and Petri dishes. In addition, growth was studied in phloem and sapwood of young living shore pines (Pinus contorta var. contorta) and western white pines (Pinus monticola) inoculated with fungus in low densities (eight inoculations per tree). In test tubes with limited oxygen O. clavigerum grew for a longer time than O. montium. Both fungi are fast growing on malt agar (maximum growth 4.4-9.0 mm/day), but O. clavigerum grew better at temperatures below 25°C. The rapid growth and the ability to tolerate low oxygen levels may be important adaptations for O. clavigerum as the primary invader of fresh sapwood. However, although O. clavigerum grew better in the phloem of both tree species, there were no differences between the two fungi in their ability to colonize the sapwood of the inoculated trees.Key words: blue-stain fungi, Dendroctonus ponderosae, growth rate, oxygen deficiency, virulence.