Ecology And Environment

About Dutch Elm Disease



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Dutch elm disease (sometimes known by its rather morbid acronym, DED) is a fatal elm tree disease caused by the fungus Ophiostoma. Its name refers to its discovery in Holland during the 1920s, not to the particular types of elm diseases which can be affected by the fungus. Currently, all elms are susceptible. In some cases the fungus may also spread via interconnecting root systems, and by saws which have been used to prune or cut down an infected tree and then a healthy tree without proper cleaning.

- Signs and Effects of the Disease

Three fungal species within the genus Ophiostoma are now known to cause Dutch elm disease. In all cases, they are carried by various species of bark beetle from infected trees into new, susceptible hosts. In an attempt to check the infection, trees respond by growing gum and other "plugs" in the tissue used to transport water and nutrients up the tree, called the xylem.

In an interesting analogy to an allergic reaction, these plugs eventually prove fatal, blocking the passage of nutrients necessary to the tree's health. First, upper branches will show the effects by withering early, during summer rather than autumn, as a result of lack of nutrients. Eventually, this will spread so that the full tree is afflicted and dies. The affliction is fatal, although decades of research have led to promising results in terms of breeding trees which are resistant to the fungus.

- Discovery, Tracking, and Treatment -

There were reports of large elm tree die-offs in Europe during the Renaissance period, and paleobotanists have found evidence of similar die-offs in Europe thousands of years ago. In the modern era, however, the disease spread in Europe during the 1910s, and then Britain after 1927. It took its name after the first country to actually identify the responsible fungus, the Netherlands, where Bea Schwarz did so in 1921. This first fungus, later traced to the Himalayas, then spread to America via European lumber, and subsequently made its way north into Canada, wreaking havoc in elm populations as it went.

Beginning in the 1940s and then particularly following serious outbreaks in the 1960s, this was followed by the spread of a technically separate species of fungus which caused the same reaction in elm trees. The second outbreak continues to cause severe problems today, particularly in the British Isles, where surviving large elm stands are uncommon.

Barring the breeding of resistant trees, since the 1970s Dutch elm disease has been checked to an extent through pesticides injected into trees. These can be effective in preventing an infection, but in general can only delay the progress of fungus through an affected tree by several years' time.

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