Dutch elm disease (DED) is a fungal disease that attacks almost all species of elm trees. It is spread by the elm bark beetle, which carries the fungus from infected trees to healthy trees. DED is thought to have originated in Asia, where most of the elm trees are resistant to the disease. In both Europe and North America, where the fungal disease has been introduced, huge populations of elms have been wiped out. The fungus was finally isolated in 1921 by Dutch phytopathologist, Marie Beatrice Schwartz, thus the name Dutch elm disease.
DED first came to Europe in 1910, and reached North America by 1928 on diseased logs imported from overseas. The first wave of DED wasn’t as detrimental as a second epidemic in the 1960’s and 1970’s that wiped out whole forests. In the UK alone more than 25 million elms died. The problem with dutch elm disease is that once it’s present in an area, it is almost impossible to contain. Even with treatment, trees usually only survive another 5 to 10 years.
There are three different species of ascomycete microfungi – the culprits of DED. They are ophiostoma ulmi, ophiostoma himal-ulmi, and ophiostoma novo-ulmi, the latter being the most virulent. DED begins by an elm bark beetle bringing spores of the fungus to a tree, usually to the upper branches of a healthy tree where the beetle is coming to eat. The fungus attacks the vascular system of the tree. The trees natural defenses to the fungi begin working immediately, but it is these defenses that eventually kill the tree, choking off all nutrients. The elm plugs its own transport tissue with natural gum. The plugs stop the fungi from spreading, as well as all necessary water and nutrients. Although in some cases the tree survives, DED almost always ends in the death of the tree. Depending on the strain of the fungi and the species of the tree, this whole process can last anywhere from one season, to three years.
The reason that the microfungi is so easy to spread is behind the natural inclinations of the elm bark beetle. These beetles seek out dead or dying elms to lay their eggs. The beetle eggs feed on the inner bark of the tree. Once the larvae become adults, they leave the tree and move to healthier trees to eat. Before leaving however, spores of the fungus stick to the beetle.
Aside from the elm bark beetle, DED can also be spread from tree to tree if diseased roots are touching those of a healthy tree. Also, pruning a tree with equipment that may have the fungus on it can often promote dutch elm disease.
To recognize dutch elm disease, the first symptoms are usually a withering or yellowing of the upper leaves. This is followed by a browning or darkening of the branches and stems. There is a fungicide that can be injected into an infected tree. But this has to be repeated every couple of years to prevent DED from returning. Unfortunately, the only thing really to be done is to let nature take its course.