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Ash dieback is on the rise and is threatening the existence of the ash tree. We explain the disease and show what to do about it.
Since 2000, ash trees (Fraxinus) have been dying off their shoots. At the beginning, people were puzzled about what triggers the shoot dieback in the ash trees. Now it is certain that it is caused by a fungus that is threatening the ash population not only in Germany but throughout Europe.
Ash dieback: Occurrence and causes
False white stem cup (Hymenoscyphus fraxineus) is the name of the harmful fungus that causes ash dieback. A synonym for this harmful fungus is also Hymenoscyphus pseudoalbidus. The secondary fruit form of the fungus is called Chalara fraxinea.
This fungus causes serious shoot dieback on ash trees and occurs on trees of all ages. The fungus penetrates the shoots of the ash trees via the leaf scars or dead side branches. Normally, ash dieback leads to the death of younger trees, but older trees are now also affected. The disease can even be observed on one-year-old seedlings and plants from the nursery.
This fungus is thought to have been introduced to Europe by infected plants from northeast Asia. This assumption comes from the fact that in 2012 the fungus on the Manchurian ash (Fraxinus mandschurica) and the beaked ash (Fraxinus rynchophylla) was found in Japan and China could prove. There it is a harmless fungus that does not cause any symptoms of disease on the ash leaves
As early as 1992, ash dieback occurred in Poland, and in 1996 cases were also observed in Lithuania. From there, the disease then spread rapidly across Europe. In 2005, ash dieback was detected for the first time in Austria. In 2015, 29 European countries were already affected, from Norway, Ireland and Croatia to Russia.
Especially the European or common ash(Fraxinus excelsior) and narrow-leaved ash (Fraxinus angustifolia) are highly susceptible to ash dieback. The leaves of the flowering ash (Fraxinus ornus) are also affected, but shoot death has not yet been observed. The Pennsylvania ash (Fraxinus pennsylvanica) shows a higher resilience, but ultimately it is also affected by dieback.
Ash dieback: biology of the pathogen
Hymenoscyphus fraxineus belongs to the sac fungi also called Ascomycota. Especially in summer you can often find this fungus in the ground. There, 2 to 7 mm, cup-shaped, white fruiting bodies form on the rotting foliage from the previous year. More precisely, these cups form on the ash leaf stems, fruiting bodies can only rarely be found on the woody parts. Spores are formed in these fruiting bodies, which are then released and spread in the air by the wind.
When these spores land on ash leaves, the spores germinate and infect the next ash tree. The fungus then grows from the leaves into the shoots and branches, causing the bark and wood to die off. In principle, the fungus penetrates through the leaf scars or dead side branches, but if symptoms (bark necrosis, discoloration) also appear at the base of the trunk, the fungus has probably penetrated through the undamaged bark. It is even suspected that the fungus can also affect the roots.
Ash dieback: symptoms of damage
The most noticeable symptoms of ash dieback are of course the dying shoots, twigs and branches. However, the disease causes an unusually large number of different symptoms, which can appear on the bark, wood, shoots and leaves of our ash trees. Usually, the leaves begin to wilt and turn brown first, while the young, green shoots gradually wilt, then turn cinnamon brown and eventually die. Trees of all ages can be affected.
If you take a closer look at the ash trees, you should be able to see necrosis on the bark (dead parts of the bark) on affected trees. These are elongated, demarcated, sometimes sunken, and turn brown. These necroses are often tongue-shaped and expanding. Often there is also a dead side branch in the middle of the necrosis. The trees try to overwhelm the necrosesand limit the spread like this.
With older ash trees, the most noticeable thing is of course the sparse crowns, which are caused by the early leaf drop and the dying of the shoots. The crowns continue to die from the outside inwards. The leaves turn brown to black, dry up and remain on the tree for a long time.
Because older trees have a large trunk diameter, it takes longer for the necrosis to cover the entire trunk. This gives the ash trees more time to sprout again and bunches of leaves are formed. The trees are also weakened by infection with ash dieback and are vulnerable to other pathogens such as secondary bark fungi or wood rot pathogens.
Young trees die easily because the bark necrosis quickly covers the entire trunk. If the leading shoot is attacked, the young ash trees try to sprout again - but these shoots usually also die the next year. This new shoot creates so-called bifurcations, the shoots divide more, so that one shoot of a branch forks becomes two shoots.
The wood of the ash trees also shows symptoms under the necrosis: Diffuse grey-brown discolouration occurs which does not follow the course of the growth rings. The discoloration extends in the longitudinal direction far beyond the area of necrosis, the discoloration of the wood is even recognizable in cross section.
Ash dieback causes the following symptoms:
- Death of the tree
- Dying off shoots, branches, twigs
- Grey-brown discoloration of the wood
- Premature shedding of leaves
- Bark Necrosis
- Barren treetops with tufts of foliage
- Withered
Ash dieback: What can help?
Infested tree nursery plants are a major threat to the spread of ash dieback. Therefore, when buying, make sure that your tree plants are he althy and have no dead shoots.
Unfortunately there is currently no experience with curative measures against ash dieback. Since the infection starts from fallen leaves, you should try to reduce the infection pressure with the following measures:
- Foliageburn, bury or store away from ash trees
- Incorporate leaf residue into the soil or cover with soil
In forestry, it is generally discouraged to plant new ash trees at the moment, however, individual ash trees that are resistant or at least tolerant to ash dieback have been observed in infested regions. Therefore, there are projects to track down resistant ash trees and set up seed orchards - with this we could have the first seeds for ash trees with a higher disease tolerance in an estimated 20 years. All hope lies in this resistance breed.
Such research in relation to resistant ash trees is available, for example, from the Federal Research Center for Forests and the University of Natural Resources and Life Sciences Vienna. For this purpose, the project "Esche in Not" was launched to find ash individuals with a higher disease resistance and bring them together in seed orchards. In such plantations, the resistant ash trees are planted so that seeds can then be obtained from these trees. If you want to know more about this project, you can find more information here.
Ash dieback: do not confuse
There are always symptoms on plants that are very similar but are triggered by different things.
When female ash trees begin to flower, which does not happen every year, the sight can be reminiscent of ash dieback. And even if ash trees fruit heavily, crown thinning can occur. However, no necrosis occurs on the bark and only the female ash trees are affected.
Perennial tree canker (Nectria galligena) causes open wounds on ash trees and deformation of the trunk. Red fruiting bodies, about the size of a pinhead, can be seen at the edges of the canker sores.
If the ash from bacterial canker (Pseudomonas syringae subsp. savastanoi pv. fraxini) If infected, the branches and trunk will first swell, which will then burst lengthwise. Irregular dark growths then grow out of these wounds.
Late frosts in spring can also cause wilted leaves and dead shoots, but the bark necrosis and discolouration in the wood are missing.
Abiotic causes such as drought or damage caused by road s alt are also possibleCrown thinning shows, but bark necrosis and wood discolouration are missing.
Possible confusion with ash dieback:
- Crown defoliation in flowering female ash trees; strong fruiting
- Perennial tree canker (Nectria galligena)
- Ash bacterial canker (Pseudomonas syringae savastanoi pv. fraxini)
- Withered and dead leaves and shoots due to late frost
- Abiotic causes (drought, s alt damage)