Pathogenicity to Douglas-fir ofOphiostoma pseudotsugaeandLeptographium abietinum, fungi associated with the Douglas-fir beetle

Pole-size Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) were inoculated with two fungi (Ophiostoma pseudotsugae (Rumb.) von Arx and Leptographium abietinum (Peck) Wingf.) associated with the Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) to evaluate their pathogenicity. Pruning the lowermost 30% of the live crown had no effect on host tree defenses. Inoculation with O. pseudotsugae produced significantly longer lesions in the phloem and resulted in a significantly greater percentage of necrotic phloem than inoculation with L. abietinum. The percentage of occluded sapwood was also greater following O. pseudotsugae inoculations, but the difference was not statistically significant. Individual lesion lengths declined significantly with increasing inoculation density, but the total percentage of necrotic phloem increased significantly. Both fungi appeared to be better adapted to grow in sapwood than in phloem. None of the inoculated trees were dead after 5 months, but some were chlorotic with less than 30% functional sapwood within the inoculation band. The results suggest that these fungi may assist the Douglas-fir beetle in overcoming the defenses of live trees. Resume : Des Douglas taxifoliOs (Pseudotsuga menziesii (Mirb.) Franco) au stade de perchis ont OtO inoculOs avec deux champignons, Ophiostoma pseudotsugae (Rumb.) von Arx et Leptographium abietinum (Peck) Wingf., associOs au dendroctone du Douglas taxifoliO (Dendroctonus pseudotsugae Hopkins) dans le but diOvaluer leur pathogOnOcitO. Le fait diOlaguer 30% de la cime dans sa partie infOrieure niavait pas dieffet sur les rOactions de dOfense de lihUte. Liinoculation avec O. pseudotsugae a causO des lOsions significativement plus longues dans le phlome et un pourcentage significativement plus OlevO de phlome nOcrotique quiavec L. abietinum. Le pourcentage de bois diaubier obstruO Otait Ogalement plus OlevO dans le cas des inoculations avec O. pseudotsugae mais la diffOrence niOtait pas statistiquement significative. La longueur des lOsions diminuait significativement avec liaugmentation de la densitO des inoculations mais le pourcentage total de phlome nOcrotique augmentait de faAon significative. Les deux champignons semblaient mieux adaptOs pour se dOvelopper dans le bois diaubier que dans le phlome. Aucun des arbres inoculOs niOtait mort aprs 5 mois mais certains Otaient chlorosOs et avaient moins de 30% de bois diaubier fonctionnel dans la zone diinoculation. Les rOsultats suggrent que ces champignons pourraient aider le dendroctone du Douglas taxifoliO ‡ surmonter les dOfenses des arbres vivants. (Traduit par la ROdaction)