Dutch elm disease first appeared in north-west Europe around 1910. By the 1940s this first epidemic had died down after causing losses of 10—40% of elms in different European countries. The late 1960s brought the beginning of a second and far more destructive outbreak of the disease. The new outbreak was caused by an entirely different, far more aggressive fungus (Ophiostoma Novo-ulmi) which had been imported into Britain on 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, with predominantly English elm, the new epidemic took rapid hold during the early to mid-1970s, leading to the death of most mature English elm by the early 1980s.
When an English elm (Ulmus procera) is killed by the disease, some roots remain alive and new elms regenerate from these. A single dead elm is often replaced by a thicket of vigorous young elms. Other elm species are prolific seed producers from an early age, and seedling elms grow rapidly. So, while not many big elms remain in Britain there are many millions of young elms growing around the countryside.
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 ‘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. The disease can be confirmed by peeling the bark from symptomatic live twigs which 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 may not be present in all affected branches, especially in lower branches of large trees.
Our main native elms, English elm (Ulmus procera), smooth-leaved elm (Ulmus carpinifolia or Ulmus minor) and wych elm (Ulmus glabra) are all susceptible to Ophiostoma novo-ulmi.
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 area. 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.