Graduate Thesis Or Dissertation
 

The root insect-- black-stain root disease association in Douglas-fir : vector relationships and implications for forest management

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  • Verticicladiella wageneri Kendrick is a vascular wilt pathogen of conifers, causing a black staining of colonized sapwood of roots and lower stem. In Douglas-fir, V. wageneri is intimately associated with insects. Hylastes nigrinus, Pissodes fasciatus, and Steremnuis carinatus are commonly associated with diseased hosts, carry inoculum of V. wageneri in the field, successfully transmit the pathogen to seedlings under laboratory conditions, and create suitable infection courts in susceptible hosts. Furthermore, insect-mediated transmission of V. wageneri has been documented tor the first time. Stand density management, such as precommercial thinning, results in elevated activity of H. nigrinu, P. fasciatus, and S. carinatus in disturbed stands. Insects colonize roots and the root collar region of cut trees; these hosts are susceptible to infection by V. waqeneri. Also, crop trees are wounded on the roots and root collar region by H. nigrinus for one to two years following precommercial thinning. Some of these wounds penetrate to the xylem and are, therefore, suitable infection courts for V. wageneri. Time of precommercial thinning can be manipulated to significantly reduce immigration of vectors, i.e., by thinning plantations during early summer after the peak flight of H. nigrinus. H. nigrinius and S. carinatus are attracted to alpha-pinene, a major constituent of Douglas-fir oleoresin. Forest management activities that injure hosts, and hence cause release of alpha-pinene, may attract vectors of V. wageneri. H. niqrinus and S. carinatus also are attracted to ethanol. In addition, root sections infected with V. wageneri are more attractive to H. nigrinus and S. carinatus than uninfected roots. Aspects of injury and stress to hosts leading to the release of host attractants are discussed. A crop production/pest management system structure is developed which links pest management activities for black-stain root disease prevention with normal intensive forest management. Pest management should be addressed at all stages of forest management: the harvest-establishment, annual, precommercial, and commercial phases of crop production.
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