Graduate Thesis Or Dissertation
 

Insects associated with black stain root disease of Douglas-fir in western Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2801pk21g

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  • The root systems of Douglas-fir trees infected with Verticicladiella wagenerii and assigned by crown color and terminal growth characteristics to several stages of decline were excavated at three widely separated sites in the Coast Range of Oregon. Data were gathered on insect species present, extent of colonization of the root system and lower stem by V. wagenerii, and the presence or absence of viable V. wagenerii inoculum, either as hyphae in wood or as conidia, within diseased trees. The weevils Steremnius carinatus and Pissodes fasciatus and the scolytid Hylastes nigrinus were commonly associated with diseased roots and root collar-lower stem portions of diseased trees. In established pockets of V. wagenerii-induced mortality, damage by S. carinatus and P. fasciatus was always noted in trees in the year of death and H. nigrinus damage occurred in 92% of these same trees. H. nigrinus and S. carinatus, associated primarily with root damage, were the first insects to invade diseased trees. Insects sequentially colonized roots of the diseased trees as each root succumbed to infection; the colonization process spanned two to four years in most cases. P. fasciatus attack was associated with tree death. V. wagenerii occurred as viable inoculum throughout the decline of host trees. Viable V. wagenerii hyphae were found in 75% of the trees recently killed by the disease. Nearly 100% of all declining trees harbored viable V. wagenerii inoculum. Trees exhibiting severe crown symptoms had nearly their entire root system colonized by the pathogen. Conidiophores of V. wagenerii were observed in galleries and pupal cells of all three beetle species. The coincidence of viable inocula and adults of these three species of root-inhabiting insects at the time of their dispersal makes H. nigrinus, P. fasciatus, and S. carinatus potential vectors of black stain root disease in Douglas-fir.
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