Dutch elm disease (DED) is a serious disease of elms caused by the fungus Ophiostoma novo-ulmi. It is a type of disease known as a vascular wilt because the fungus blocks the vascular (water transport) system, causing the branches to wilt and die. It is spread by elm

Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

It only occurs in Ulmus spp. (elms) and Zelkova.

You may see the following symptoms:

• At any time in the summer months, all or part of the foliage suddenly turns yellow, then wilts, shrivels and dies
• The tips of affected young shoots sometimes curl into a 'shepherd's crook' symptom
• Peeling off the bark from affected branches will reveal brown streaks in the outer wood, which appear as a broken or continuous brown ring in the outer growth ring if the branch is cut across

Non-chemical control

All attempts to prevent the spread of DED have been long since abandoned, except in specific areas such as the Isle of Man and Brighton and Hove.  However, dead trees are a safety hazard and should be felled promptly.

We advise that native elms should not be planted, as they will almost inevitably succumb to DED. Zelkova spp. appear less badly damaged. Resistant hybrid elms have appeared on the market in recent years, but gardeners should note that whilst these are attractive trees, they usually have a different growth habit to the English elms that have been lost and do not exactly replace them. Pictures and details of many of the cultivars can be found on the website resistantelms.co.uk. Due to the risks posed by a phytoplasma disease called Elm yellows (Candidatus Phytoplasma ulmi) some nurseries are not currently importing elm trees into the UK.

The Conservation Foundation have a Native Elm Programme for propagating elms from the survivors of the last disease outbreak. Anyone who knows of a healthy mature elm (at least 190cms circumference at breast height) is encouraged to contact the Conservation Foundation.

Chemical control

No chemical control is feasible. Protectant fungicides were injected into trunks in the early stages of the outbreak, but this was required annually and soon abandoned as impractical. It is also completely impractical to control the beetle vectors.

The disease is known as ‘Dutch’ because important early research on it was carried out in the Netherlands.

It is caused by the fungus Ophiostoma novo-ulmi which was accidentally introduced to the UK from the USA in the late 1960s on imported elm logs. Prior to this, northern Europe (including the UK) already had a milder form of DED caused by a related fungus, Ophiostoma ulmi, and for some time it was not realised that the introduced fungus in the UK was different. O. novo-ulmi is not native to the USA and its true origin is unknown.

The fungus is spread by elm bark beetles, particularly Scolytus scolytus. Beetles breed in dead and dying elms, including those killed by the disease, where the larvae tunnel in the bark and outermost wood, forming galleries. The fungus produces sticky spores in these galleries, which contaminate the newly hatched adult beetles as they emerge. They then fly to healthy elms, where they feed on young bark and introduce the pathogen into the water-conducting tissue (xylem) of the tree. The fungus grows in the xylem, blocking water flow and causing rapid wilting and death. It can spread rapidly down rows of hedgerow elms through root grafts formed between adjacent trees.

The beetles tend to attack mature trees over 20-years-old, and therefore the first wave of the disease in the early 1970s was followed by a lull while the trees regenerated from suckers. But these regenerated trees have in turn succumbed. The disease has not quite reached the northern limits of elms and some remain in Scotland.