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Dutch elm disease (Ophiostoma novo-ulmi)

 

Ceratocystis ulmi.  Section of elm root showing Dutch elm disease streaking.Dutch elm disease (DED) was first observed in north-west Europe about 1910, caused by the fungus Ophiostoma ulmi. It is called "Dutch" elm disease because the fungus was first described by Dutch scientists, although it is believed to be of Asian origin. By the 1940s this first epidemic had died down after causing losses of 10—40% of elms in a number of European countries.

A second and much more destructive outbreak of the disease began in the 1960s. The new outbreak was caused by an entirely different, far more aggressive fungus, Ophiostoma novo-ulmi, which had been introduced into Britain on imported, infected elm logs.
 
Within a decade about 20 million elms out of an estimated UK elm population of 30 million were dead. By the 1990s the number was probably well over 25 million. In lowland central and southern Britain, the new epidemic took rapid hold, especially of 'English' elm trees (Ulmus procera), during the early to mid-1970s, leading to the death of most mature English elms by the early 1980s.

The fungus is spread from infected trees to healthy trees by elm bark beetles, and is known to be present in Europe, North America and New Zealand.

Outlook

When an English elm is killed by the disease, some roots remain alive and new elms regenerate from these. A single dead elm is therefore often replaced by a thicket of vigorous young elms growing from these roots. Other elm species are prolific seed producers from an early age, and seedling elms grow rapidly. So although not many big elms remain in Britain, there are many millions of young elms growing around the countryside, especially in hedgerows. Unfortunately once the young elms reach a certain size they can be detected by the beetles which spread the fungus, and many of them become infected.

There is no effective cure available, but early sanitation felling or removal of infected trees and branches can slow the spread of the disease. This has been effective in helping to retaining good populations of mature elms in some places in Britain, especially in Brighton. Fungicides, tree vaccines and chemical and biological controls have been or are being developed, but these treatments have limitations, such as expense, difficulty of application, and the need to be repeated at intervals.

Some work is being done to identify and breed elms trees which show resistance to, or tolerance of, the disease, including tolerant hybrid cultivars. The main British organisation working in this area is the Conservation Foundation .

In an international research project centred on New Zealand, scientists have been exploring the potential for introducing a virus into the fungus population to reduce its virulence.

Symptoms

Symptoms of the disease first appear in early summer as clusters of wilting or yellowing leaves which then turn brown and fall. Affected shoots die back from the tip, and the twigs sometimes turn down to form ‘shepherds' crooks’. Because the disease is progressive, an affected tree can have a mixture of healthy foliage, yellow or brown foliage, and defoliated shoots, showing infection in different branch systems. The disease can be confirmed by peeling the bark away from symptomatic live twigs, which will show dark brown or purple longitudinal streaks in the outer wood. Cutting across the twig should reveal a ring of dark brown staining in the outer wood. This brown streaking might not be present in all affected branches, especially in the lower branches of large trees.

Symptoms guide

Susceptible species

Our main native elm spcies, English elm (Ulmus procera), smooth-leaved elm (U. carpinifolia or U. minor) and wych elm (U. glabra), are all susceptible to Ophiostoma novo-ulmi.

Wych elm, which is the dominant elm species in northern Britain, tends to be the least affected, for four main reasons:

  • it does not reproduce from suckers, as the other do, so it suffers less from transmission of the disease through root grafts;
  • it is much less favoured by elm bark beetles for feeding;
  • the Phomopsis fungus readily invades the bark of newly dying wych elm trees, making them less suitable for elm bark beetles to breed in; and
  • the cooler northern climate might create fewer opportunities for beetle-originated infections in summer, and lead to lower bark beetle populations and fewer generations.

North American species of elm are also suscepible to O. novo-ulmi.

Management

Certain local authorities have legal powers [Dutch Elm Disease (Local Authorities) Order 1984] to take steps to prevent the spread of the disease. Local authorities may exercise the powers only in their own areas. Officers who suspect the presence on any premises of elm trees infected by this disease may, on production of their authority, enter land to inspect trees and to take samples. Where the disease is present the officer may either take action himself, or he may require the owner or occupier to do so, to prevent the spread of the disease by destroying the tree, usually by burning on site.  Exceptionally, he may authorise the removal of the tree to another place for destruction.   

Reporting suspect trees

There is no requirement to report sightings of Dutch elm disease.













Last updated: 30th January 2018