Dutch Elm Disease
Dutch elm disease (DED) is a fungal attack which primarily affects members of the Ulmus family (Elm trees). The disease probably arose in the far East and was introduced to Europe in about 1910, arriving in the USA a few years later. European and American elms trees which had no natural resistance to the condition were much reduced in the early years of the 20th century. DED re-appeared in the late 1960's in a more virulent form and by the mid-1970's 25 million elm trees have died in the UK alone where full-sized elms are now virtually unknown. Only smaller elms such as Autumn Gold seem to have a chance of surviving at present.
The disease is spread by the female elm bark beetle. She flies at about 30 feet (9-10 metres) when egg laying, presumably looking for a more mature tree on which her young can feed. Her eggs carry the fungus which is carried into the tree when her eggs hatch and the larvae begin to bore into the bark to reach the cambium layer. In response to the fungal attack, an elm tree attempts to isolate the fungus to stop its spread. It does this by effectively blocking its own cambium layer which stops the flow of sap to affected parts of the tree. As a result, they die. The elm effectively kills itself above ground. Many varieties of elm sucker (produce new growth from existing roots) and these suckers can often appear after an elm has "died". They grow to about 30 feet (9-10 metres) in about 12- 15 years, reach the necessary height, are found by a female elm bark beetle and the process repeats.
DED is not restricted to dutch elms. The word Dutch refers to the fact that it was first identified in the Netherlands species may have arisen as a hybrid between O. ulmi and O. himal-ulmi. The new species was widely believed to have originated in China, but a comprehensive survey there in 1986 found no trace of it, although elm bark beetles were very common.
The classic sign of infection is that the leaves on one branch, more than 30 feet above ground level begin to yellow and fade well before leaves normally fall in autumn. This will spread to the remainder of the elm over a period of years depending on its size and age.