6 Wood Discoloration

8.1 Fungal Damage to Living Trees 169
Fig.8.4. Pathogenesis of Dutch elm disease (translated from Nierhaus-Wunderwald and
Engesser 2003, with permission of Swiss Federal Institute for Forest, Snow and Landscape
Research)
other vector species are recognized (Wingfield et al. 1999). In North America,
vectors are the imported S. multistriatus and the American elm bark beetle
Hylurgopinus rufipes. The females select almost exclusively diseased, dying, or
already dead elms for their breeding galleries. The larvae take up the pathogen,
which is passed on alive via the pupa to the young beetle. The young beetles
contaminated with spores (conidia or ascospores) infect healthy trees in twig
crotches of small branches during maturation feeding. Since this bark is too
thin for oviposition, the beetles leave the healthy tree and use the thick-barked
parts of diseased elms for their breeding galleries. The change between the stem
of infected elms and the branches of healthy trees makes the Scolytus beetles
effective vectors (v. Keyserlingk 1982). Root graft transmission via connections
from adjacent trees is the major cause of elm death in urban areas.
The principal reaction compounds developing in elms following invasion
by the fungus are cadinane sesquiterpenoids [mansonones, elm phytoalex-
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