The current epidemic of Dutch Elm disease is caused by the fungus Ophiostoma novoulmi.

It is disseminated by various elm bark beetles within the genus Scolytus.

The fungus blocks the water conduction system of trees resulting in wilting and death of the foliage.

Symptoms of the disease first appear in early summer as clusters of wilting or yellowing leaves which then turn brown and fall (Figures 1 and 2).

The Argus: Dutch Elm 1 The Argus: Dutch Elm 2
Fig 1 Fig 2

Affected shoots die back from the tip and the twigs sometimes turn down to form ‘shepherd’s crooks’.

Because the disease is progressive an affected tree may have a mixture of healthy foliage, yellow or brown foliage and defoliated shoots, showing infection in different branch systems.

Confirmation of the disease can be obtained by peeling the bark from symptomatic live twigs which show dark brown or purple longitudinal streaks in the outer wood (Figure 4).

The Argus: Dutch Elm 3
Fig 3

Although the streaking may not be as pronounced as in the photograph.

Cutting across the twig should reveal a ring of dark brown staining in the outer wood.

This twig also has a ring of staining in a previous growth ring. Note that the brown streaking may not be present in all affected branches, especially in lower branches of large trees.

Information and photos from the Forestry Commission. For more information and to report a diseased tree visit their website