Dutch elm disease

from Wikipedia, the free encyclopedia

The Dutch elm disease is a through tube fungi of the genus Ophiostoma ( Ophiostoma novo-ulmi / Ophiostoma ulmi syn. Ceratocystis ulmi caused) and elm bark beetle disease spread, which most European elms infects and predominantly the mountain elm (Ulmus glabra) on the brink of extinction brings.

origin

The first detectable Dutch elm disease occurred in the middle Holocene . Further incisions confirmed by pollen analyzes were e.g. B. in the British Isles around 4000 and 2500 BC Chr. ( Pollen analysis in Lough Sheeauns ). The mid-Holocene cause debate has been going on for decades. In the last few years several researchers have strongly argued for the inclusion of an elm-specific disease, similar or identical to today's "Dutch elm disease". The problem with this thesis is that it is difficult to prove or disprove.

Since 1918, a fungus that lives in East Asian elms, Ophiostoma ulmi , was brought to Europe by humans and spread from the Netherlands to Europe. In contrast to the East Asian elms, which were able to adapt to the fungus over evolutionary periods, their European relatives fell victim to the "Dutch elm disease" (hence: "Dutch elm disease") to a large extent. The fungus was brought from Europe to North America in 1928 - probably via veneer trunks - where it destroyed a large part of the elms there. The intensity of the disease decreased in the middle of the 20th century. Dutch programs for breeding resistant clones contributed to this.

Towards the end of the 1960s - again through the import of veneer trunks - a more aggressive strain of the fungus was re-imported from America, which also attacked the plants considered resistant. This strain is now regarded as a separate species ( Ophiostoma novo-ulmi ). Another aggressive variant of the fungus was able to enter Europe via Asia at about the same time. In this second wave of Dutch elm disease, Europe had to accept severe cuts in the elm population. In England alone there was a reduction of around 70% of the elm population (or 20 million trees) between 1971 and 1978 due to the fungus. The elm has not yet become extinct, but the prognosis for the surviving elms is very serious.

Symptoms

Infested elms can be recognized by sudden wilting of the crown - usually one-sided, rarely the entire crown. The leaves occasionally turn yellow, sometimes brown, curl up and finally dry up. Most of the time, the leaves stick to the branches. This results in the characteristic hanging, dry twig tips with dried up leaves ("flags"), which are a clear indicator of the disease in trees that are infected late even in winter.

Water veins are another characteristic of the disease. If the disease progresses slowly, the foliage is thin and leaves fall early. In addition, dark vascular discolorations in the youngest annual rings can be seen in the cross-section of infected branches, which are expressed by brownish stripes when the branch bark is peeled off.

The infection usually occurs in May by elm bark beetle (see below), the first symptoms can then be observed in June. In contrast, if an infection occurs after July, the outbreak usually only occurs in the next year. Young plants can succumb to the disease after a few months, otherwise the disease can drag on for several years.

distribution

O. ulmi and O. novo-ulmi are mainly spread by the small and large elm beetle , which belong to the bark beetles . The young beetles ( Imagines ) contaminate themselves with the sticky spores ( Koremien ) in breeding programs, where these are particularly hard formed. After the young beetles have escaped, new trees are infected when they mature or when they drill new mother tunnels. The spores germinate and pass into the xylem , from where they can be transported to the topmost shoots and leaves. The fungus can also spread through the growth of the roots, with the microconidia of the pathogen passing from one tree to another.

Injury and cause of death

An infected elm ultimately dies from a lack of water. As a defense reaction to the infection, the vessels become vellified (and turn brown) and gum deposits in living cells. There is also a mechanical closure of the vessels due to the ingrown fungus mycelium (narrowing) and yeast-like spreading bodies (clogging). Ultimately, the water transport in the vessels is prevented. Elms are among the ring-pored woods and have relatively few, but all the larger vessels. As a result, significant transport losses can occur even with a few infected cells. The disease is a typical tracheomycosis .

Due to the vertylation , the vascular bundles in the trunk, branches and twigs of diseased trees can be seen in cross-section as brown to brown-black points, mainly in the early wood . In this way, a overcome illness can also be recognized and dated.

While O. ulmi only stays in the tree for one year and it requires a new infection to kill the tree, O. novo-ulmi can overcome the tree ring boundary and continue the damage without re-infection.

The involvement of low molecular weight wilt toxins produced by fungi has recently been discussed.

Control and Prevention

The vector is mainly combated ; Infested trees are felled early and then the bark beetle is destroyed. The antagonists of the elm bark beetle are wood wasps and predatory bugs . These are attracted by chemical signals that the elms release shortly after infestation and attack the brood of beetles. There are also approaches for the use of the bacterium Pseudomonas syringae , which forms antifungal metabolites and is therefore considered to be an antagonist of the fungus.

In terms of breeding, cross-breeding of resistant Asian species has been tried out for many decades and is now on the market. For various reasons, however, they can at best replace native elm species in gardens and parks, but not in the open countryside. The European white elm ( Ulmus laevis ), which is native to large parts of Europe, is largely insensitive to Dutch elm disease. Therefore, where it is possible to plant it, it is a better measure of conservation than crossbreeding and resistance breeding.

See also

literature

  • Fritz Schwerdtfeger : The forest diseases. A textbook of forest pathology and forest protection. Paul Parey, Hamburg et al. 1981, ISBN 3-490-09116-7 , p. 88.
  • Karen Molloy, Michael O'Connel: The nature of the vegetational changes at about 5000 BP with particular reference to the Elm decline: Fresh evidence from Connemara, Western Ireland. In: The New Phytologist. Vol. 107, No. 1, 1987, ISSN  0028-646X , pp. 203-220, JSTOR 2434892 .
  • Heinz Butin : Diseases of the forest and park trees. Diagnosis, biology, control. 2 spore boards. 3rd, revised and expanded edition. Thieme, Stuttgart 1996, ISBN 3-13-639003-2 .
  • Ceratocystis ulmi. In: Peter Schütt , Hans J. Schuck, Bernd Stimm (eds.): Lexicon of tree and shrub species. License issue. Nikol, Hamburg, 2011. ISBN 978-386820-123-9 , p. 109.

Web links