Dutch Elm Disease
Dutch elm disease is a common and widespread fungal infection of elm trees, which in most cases proves fatal for the infected elm after a short period of time (several months). Many elms in Northern Europe and throughout the United States of America have succumbed to this disease, leaving previously beautiful tree-lined streets and parks barren.
What is Dutch elm disease (DED)?
Why does the elm die from DED?
Upon infection, death is caused by the fact that a tree's natural defence mechanisms start working to compartmentalize the growing fungus by sealing off the infected vascular bundles with thyllae to prevent further spread of the disease. Unfortunately, Dutch elm disease grows faster than the tree is able to form these blockages, causing the tree to form these natural protective barriers in a repeating cycle. In the end, the elm will block virtually all of its vascular tissue with these barriers, causing the tree to dry out and die, because upward transport of water and nutrients is obstructed.
How is the disease spread?
Elm bark beetle
The disease is mainly spread by the Elm bark beetle (the vector), Scolytus scolytus,Scolytus multistriatus and Hylurgopinus rufipes in Europe and North America. The beetle carries the spores of the fungus on its body and infects the tree when feeding in the leaf crotches in the upper canopy of the elm. The infection will therefore start in one specific branch or part of the upper canopy and "work its way down" to gradually (but swiftly) infect the entire elm tree.
Elm trees growing next to each other are often root-grafted, even over distances of up to 50 feet. The disease will disperse itself from one infected tree to adjacent healthy elms via these root grafts, even after the diseased elm has been removed! The symptoms of these root graft infections differ from the bark beetle infections in that instead of localised top-down wilting, the entire canopy wilts from the bottom-up. This type of infection causes the "roll-up" effect by the disease in elm-lined streets, and can only be stopped by severing these root grafts by means of trenching.
A report on preventing root graft infections can be found on thedownload page.
Where Does Dutch elm disease come from?
The English name (Dutch elm disease or DED) leads people to assume that the disease comes from the Netherlands. This is incorrect since it is not a Dutch disease of elms, but a disease whose causal organism (Ophiostona novo-ulmi) was discovered by a female Dutch scientist (Ms. Christine Buisman) in the 1920's. The fungus is probably Himalayan in origin, but there have been many ‘migratory' epidemics in Europe and North America. The disease is not selective, which means that many types of elm (such as American and English elms) are vulnerable. Over the past few decades, the fungus (Ophiostoma novo-ulmi) has mutated several times and in its current form is considered to be more aggressive than the original strain.
How to Recognize Dutch elm disease?
In an early stage of infection, a sick elm tree can be seen from the yellow/brown dried out leaves at the ends of branches. These leaves stay on the branch tip, forming small "flags". In a more advanced stage, whole branches in the crown dry out and then in the final stage the whole crown of the tree becomes yellow, brown and finally bare.
Only the physical removal of a suspect branch and cross-cutting it can really tell if it is infected with Dutch elm disease. A speckled ring of black discoloured xylem tissue (the clogged up vascular bundles) on the branch's cross section directly under the bark shows that the tree is infected. After peeling back the bark, you should also see dark stripes along the length of the branch (another telltale sign of clogged vascular bundles).
In some cases, you might see a ring-shaped discolouring - that usually does not form a complete circle - closer to the core in the cross-section of a branch. This is probably caused by a previous infection that the tree successfully survived. This is possible if a tree becomes infected late in the growing season (August-September). It will then enter its dormant period of winter-rest early, still having formed buds and will have built up sufficient reserves of nutrients. The following spring, when the tree forms new sapwood, the fungus remains compartmentalized in the heartwood from the previous summer. Research has however shown that the spores of the fungus can still germinate many years (up to 14!) after being sealed off this way!