Pathogenicity of Leptographium longiclavatum associated with Dendroctonus ponderosae to Pinus contorta

We assessed the pathogenicity of Leptographium longiclavatum sp. nov., a recently reported fungal associate of the mountain pine beetle, Dendroctonus ponderosae (Hopkins, 1902). In September 2003, 30 lodgepole pines (Pinus contorta Dougl. ex Loud, var latifolia Engelm. ex S. Wats) between 98 and 130 years old were inoculated with L. longiclavatum, the known pathogen Ophiostoma clavigerum, or an agar control. Inoculation densities consisted of 200 and 800 points/m² in a 60 cm wide band at breast height. The inoculated trees were examined in July 2004, 9 months after inoculation. Five of six trees inoculated with L. longiclavatum at high density (800 points/m²) developed yellow crowns, while the foliage of all trees inoculated at low density (200 points/m²) remained green. Both O. clavigerum and L. longiclavatum generated significantly longer necrotic lesions in the phloem and more occlusions in the sapwood than in the agar controls. In addition, the sapwood colonized by both fungi had a reduced moisture content compared with controls. These results suggest that L. longiclavatum is pathogenic and may contribute to the mortality of mountain pine beetle infested pines.