How can Dutch elm disease be identified?
In an early stage of infection, an infected elm tree can be identified by yellow, dried leaves at the ends of branches. In an advanced stage, entire branches dry out; finally the entire crown turns yellow, brown and bare. The symptoms of root graft infections differ from the bark beetle infections. Instead of localised top-down wilting, the entire canopy wilts from the bottom-up.
The only way to rule out whether a tree is infected with Dutch Elm Disease is to cross cut a suspected branch. As a reaction to the infection, the tree forms dark brown to brownish-black structures in its vascular bundles. Just beneath the bark, these appear as stripes running lengthwise along the branch; in cross-section, they appear as speckles or a partial or entire ring.
In some cases you can see ring-shaped discolouring in a cross-section of a branch. This is usually caused by a previous infection that the elm successfully survived. This is possible when a tree becomes infected late in the growing season (August-September). It will then enter its dormant period of winter rest early, having formed buds and having built up sufficient reserves of nutrients. By next spring, when the sap begins to flow again, the fungus has become encapsulated in the wood. Research has shown however that these encapsulated fungal spores can still germinate years later.
Which elms are susceptible to Dutch elm disease?
All elms are susceptible to Dutch Elm Disease but there is a difference in susceptibility within the species. Various new clones for example appear to be less susceptible. Ulmus leavis is the only species that doesn’t die as a result of Dutch Elm Disease, but the exact reason for this is unknown.