Graduate Thesis Or Dissertation
 

Pathogenesis and intertree transmission of Verticicladiella wageneri in Douglas-fir (Pseudotsuga menziesii)

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  • Verticicladiella wageneri Kendr. is a vascular wilt pathogen of Douglas-fir in the Pacific Northwest, The disease is characterized by black staining of colonized sapwood; crown symptoms are those typical of a wilting syndrome. Histopathological studies revealed that the pathogen is limited to the xylem but causes vascular dysfunction in both the xylem and phloem. In xylem, hyphae grew in lumens of mature tracheids increasing resistance to the flow of xylem sap; bordered pit penetration facilitated intertracheal growth. Living host cells were never invaded by hyphae and evidence for the primary involvement of translocateabie phytotoxins was lacking. Increased vertical and circumferential extension of the fungus, systematically reduced the capacity of vascular tissue to conduct water. Phloem vascular dysfunction occurred with no evidence of mycelial invasion of phloem tissues. The appearance of engorged sieve cells and flattened albuminous cells, adjacent only to regions of heavily ramified xylem, suggested that this xylem colonization indirectly impeded centripetal transport of photosynthate through rays. Xylem pressure potential and transpiration water uptake were periodically measured on V. wageneri inoculated and control seedling groups to indicate the earliest significant consequence of vascular tissue colonization. Circumferential colonization of inoculated seedling roots consistently exceeded 90% when significant differences in pressure potential and water uptake were first apparent; radial colonization was proportionally less (35-61%). This pattern, supported by the histopathological evidence, suggested that foliage wilting was related to vascular occlusion. Root infections of dip-inoculated Douglas-fir seedlings were initiated through artificial wounds and natural openings to exposed xylem, and living bark and cambial tissues were never directly penetrated by hyphae, Root graft transmission of V. wageneri in Douglas-fir was verified from field excavations in natural infection centers. In potted seedling experiments, healthy seedlings regularly became infected whether intertree root contact was allowed or completely restricted. In growth chamber experiments, cool soil temperatures favored infection and establishment of V. wageneri in inoculated Douglas-fir seedlings; warm temperatures decreased the likelihood of infection. Vertical growth rate varied predictably with soil temperature fluctuations in the greenhouse; soil temperatures within or above the growth optimum range favored faster growth of V. wageneri in xylem. Growth rates in roots of older trees compared favorably with estimates of the annual rate of radial increase of infection centers.
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