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
