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Fungi associated with the mountain pine beetle, Dendroctonus ponderosae Lee, Sangwon

Abstract

The mountain pine beetle (MPB; Dendroctonus ponderosae) and its fungal associates have infested large areas of lodgepole pine (Pinus contorta var. latifolia) forests in British Columbia. In order to understand how the fungi affect the beetle epidemics and tree defenses [i.e. tree defences], in this work we identified the fungal species associated with the MPB, characterized their pathogenicity, and investigated the genetic structure of a fungal species, one of the major pathogens. Identifying ophiostomatoid fungi by a classical morphology approach is often inconclusive. For accurate identification, molecular approaches were developed for Ophiostoma montium and Ophiostoma clavigerum. O. montium could be differentiated from a synonymous species, O. ips, using a single polymerase chain reaction of either the β-tubulin gene or ribosomal DNA. Similarly, restriction fragment length polymorphism analysis of the β-tubulin gene using the enzyme Hin fI could distinguish O. clavigerum from other morphologically related ophiostomatoid fungi. Using molecular and morphological approaches, we characterized the diversity of MPB fungal associates. Fungi were isolated from beetles, beetle galleries and sapwood of infested lodgepole pines in six epidemic sites across British Columbia. A total of 1042 fungi that belong to nine species were recognized. Unexpectedly, an O. minutum-like species was frequently isolated from the beetle. Unknown Leptographium and Entomocorticium species were also isolated in addition to the known MPB associates O. clavigerum and O. montium. The unknown Leptographium species was reported as Leptographium longiclavatum sp.nov. The pathogenicity of L. longiclavatum to mature lodgepole pines was estimated and compared to the pathogenicity of O. clavigerum, an MPB associate that is known to kill mature pines when inoculated at high density. For this test, foliage colour, phloem lesions, occlusions, and sapwood moisture content of inoculated trees were examined. The data showed that L. longiclavatum was a pathogen, although it seemed slightly less virulent than O. clavigerum. The genetic diversity of 170 O. clavigerum isolates from five sites in Canada and two sites in the USA was characterized using amplified fragment length polymorphism (AFLP). While the genetic diversity was low, and the geographic and genetic distances were not significantly correlated, we identified two genetically distinct groups in the population.

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