Dutch elm disease
Dutch Elm Disease is a widespread fungal infection encountered in elm trees that usually kills the infected elm within a growing season. Many elms in Northern Europe and throughout the North America have already succumbed to this disease, leaving previously beautiful tree-lined streets and parks barren.
How does Dutch elm disease kill the elm?
Trees have a natural capability of sealing off infected vascular bundles, preventing the disease spreading further. Unfortunately, the most aggressive form of Dutch Elm Disease, Ophiostoma novo-ulmi, grows faster than the tree can form these blockages. Therefore, the tree creates natural blockages in a repeating cycle. Ultimately, the elm will seal off all of its vascular bundles. This will obstruct the transport of water and nutrients to such a degree that the tree dries out and dies.
How is the Dutch elm disease spread?
The disease is most commonly spread by elm bark beetles. The beetle carries the spores of the fungus on its body and infects the elm when feeding in leaf axils in the upper crown of the tree. The infection begins in a branch or twig in the crown and gradually but quickly spreads through the entire elm.
Root grafts form naturally between adjacent elm trees – even over distances of more than 15 metres. The disease will spread from an infected tree to adjacent healthy elms via these root grafts, even after the infected elm has been removed. This is often the means of infection when avenues of elms are infected consecutively. This can only be stopped by means of root barriers.
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.