In recent years, this infectious disease has increasingly spread in Germany. Nationwide, more and more cases were becoming known in which maple trees showed the typical signs of disease. The disease is favored by certain conditions and is usually only recognized late.
What is sooty bark disease?
Sooty bark disease is caused by a fungus
Soot bark disease (according to old spelling also: soot bark disease) is a disease of trees that is caused by spores of a weakness parasite. The Latin name of this type of fungus is Cryptostroma corticale. It settles in weak wood. Infected wood appears as if it has been charred, which led to the German name.
Development and course of the disease
The fungal spores are considered the primary source of infection. They have a great potential to spread and accumulate in the bark of he althy trees, where they survive until the moment of infection. They infect the tree by entering the organism through wounds or through broken wood.
The fungus spreads massively on diseased wood. Its mycelium grows through the fibrous tissue, whereupon the tree seals off these affected areas from he althy wood. If the fungus penetrates into the cambium, black-brown spore deposits are formed.
Typical course of the disease:
- infected trees develop a bare crown
- Water shoots arise in the lower trunk area
- Slimy spots form on the trunk
- Bark swells like bubbles and over time peels off in elongated strips
- soot-black areas appear
- Millions of pores form a dust
If a maple tree suffers from sooty bark disease, the dying process can take several years depending on the he alth of the tree. Severely weakened trees die completely within one growing season. An infection can go undetected from the outside for a long time, but inside the fungus increasingly spreads and further weakens the tree.
What promotes the disease
Cryptostroma corticale is a thermophilic fungus favored by dry and hot climates. It can thrive under these conditions and produce masses of spores that are optimally spread by wind. Due to water shortages, the trees weaken, which offers the pathogen additional opportunities to grow and spread.
- Hot summers in recent years promote the spread of the disease
- older trees are well established and therefore better supplied with water
- young trees are more vulnerable because of their less developed root system
The fungus benefits significantly from climate change, which brings low-precipitation summer months with high temperatures. Under laboratory conditions, the species showed optimum growth when the thermometer was at 25 degrees. This result confirms the fact thatCryptostroma corticale has a thermophilic character.
Affected trees
Sooty bark disease occurs on maple trees in Germany. An infection of apple trees is not yet known. It is not clear that beech trees are also affected. There have only been suspected cases in the past. In Berlin it was observed that the fungus spreads predominantly on sycamore maple and, somewhat less frequently, affects Norway maple and field maple. This observation also applies to the other distribution areas of the mushroom species in Germany.
Quick overview:
- Fungus also attacks lime trees and hickory nuts in North America
- Individual diseases have been confirmed in birch trees
- Ornamental maples in Germany have so far been spared
Excursus
Sycamore maple and its lower resistance
The maple species is less affected by the disease where optimal site conditions prevail. Cryptostroma corticale relies on previously damaged wood, which the fungus uses as a portal of entry. If the sycamore maple thrives on a forest floor with an optimal pH value of 6.0, the absorption of phosphorus can take place optimally.
Moisture also plays a big role in vitality because the tree species likes fresh conditions. If further years with long-lasting drought and heat periods occur during the summer, the infestation situation in such optimal locations may also change in the future.
How to recognize sooty bark disease
The bark dies off completely and separates from the trunk
A clear identification of the fungus is only possible if the spores are identified under the microscope. There are a number of other fungi that leave blackish deposits on the wood. If a tree is affected by sooty bark disease, it will suffer from leaf wilt and excessive leaf loss. The crown is gradually showing signs of dying. If infected trunk wood is cut, greenish, brown or bluish discolorations become visible. They are a consequence of the isolation reaction.
Distinct infection patterns:
- Mucus flow: viscous plant sap colored reddish to black by fungal spores
- Bark necrosis: local death of the bark, under which soot-like spore dust accumulates
- Longitudinal cracks: Trunk tears open due to disturbed water balance, causing bark to flake off
Evaluation key for the course of the disease
The Bavarian State Office for Agriculture (LFW for short) has developed a “credit rating key for assessing sycamore maples” with which the stage of the disease can be assessed. This is categorized into five classes and shows the typical symptoms that first catch the viewer's eye.
| Class | He alth status | Symptoms |
|---|---|---|
| 0 | very good | none |
| 1 | slightly weakened | Water ricer, dead wood in the crown |
| 2 | significantly weakened | Bark flakes off in spots, spore deposits become visible |
| 3 | severe loss of vitality | Larger pieces of bark chipped off, lots of dead crown wood |
| 4 | deceased | Bark chipped off over a large area, wood charred |
Likelihood of confusion
It is almost impossible for the untrained eye to recognize sooty bark disease. There are a number of other fungi that cause similar symptoms. A reliable species identification requires microscopy of the fungal spores. Samples can be sent to mycologists for examination.
Stegonsporium maple shoot dieback
The fungus Stegonsporium pyriforme is responsible for this disease. It also benefits from dry conditions and develops black spore deposits, so it is not uncommon to be confused with sooty bark disease. This fungus infects weakened and previously diseased trees through wounds and branch breaks. The infected branch then dies. There are some clues that allow better disease identification:
- occurs mainly on young plants
- sharp transition between living and dead shoot section
- Spore deposits visible as black and roundish spots on the shoots
- locally limited die-off phenomena
Flat corner disc
Behind this species is the fungus Diatrype stigma. This develops a crust-like coating with a black color. The crusts are about a millimeter thick and are developed beneath the bark. Over time, this peels off so that the spore deposits become visible. These have a finely dotted surface and occasionally appear scar-like or cracked with age. The flat corner disk is a common fungus that can be found on dead wood of birch, oak, beech and maple trees.
Burst crust mushroom
The burnt crust fungus forms black, burnt-looking crusts
Kretzschmaria deusta develops crust-shaped spore beds that are predominantly black in color and have a bulging to bump-like surface with a bulging edge. The fungus is very hard and feels like charcoal when it ages. This creates charcoal-like spots that appear mainly along the lower trunk area down to the roots. This fungus mainly lives on beech and linden trees. It occasionally colonizes maple trees.
- causes the so-called soft rot in roots
- often no damage visible from the outside
- Coal-like crustal coatings usually only visible after the trunks have been broken off
Is there an obligation to report?
Contrary to what is often assumed, there is no obligation to report sooty bark disease in Germany. This would make monitoring the disease in Germany much easier, but would involve a lot of effort. If you suspect that it is sooty bark disease, you should urgently contact one of the following:
- Official information centers for plant protection of the federal states (plant protection services)
- Green spaces office or lower nature conservation authority in your region
- local tree care company
- Forestry office or the responsible city or municipal administration
Caution: Do not take spore samples carelessly
A suspected infestation should be confirmed by a responsible authority in your federal state, even if sooty bark disease is not reportable. You can send mushroom spore samples to the appropriate locations, but you should contact staff before sending samples. They will tell you how to proceed. Taking a sample is not without risk as the spores penetrate the human respiratory tract and pose he alth risks.
Extra caution when felling trees
Authorities advise to be particularly careful if affected trees have to be felled. It makes sense to have a wide barrier so that walkers are not exposed to the risk of spore dust. Ideally, the trees are felled when the weather is damp, as then the amount of dust produced is comparatively low. Forest workers must equip themselves with protective clothing and wear respiratory masks. The cleared wood should be stored under tarpaulins until it is transported to a waste incineration plant.
Recommended protective equipment:
- Full body protective suit
- Hat and goggles
- Respirator mask class FFP2
Information for hobby gardeners
The disease predominantly affects sycamore maples, which rarely grow in private gardens. Anyone who still owns a stately specimen should act quickly if there are any suspicions. So far it is not possible to combat the fungal disease. There is no information about successful treatment with fungicides. As soon as the spore deposits become visible, the tree dies. It is therefore important to examine affected trees for the disease, even with the slightest signs of disease.
Felling by specialist companies necessary
Experts warn against felling diseased trees on your own. This work should be carried out by tree care companies. The felled wood should not be used as firewood, as large amounts of fungal spores are released into the air when chopped. Infested wood is intended to be disposed of as hazardous waste.
Information on disposal costs:
- Disposal is complex and can be expensive
- Receiving points must be able to burn contaminated wood properly
- Prices up to 400 euros per ton of wood are possible
Tip
If felling of infected trees is necessary in your area, you should avoid the area. If you suffer from previous illnesses, you can also protect yourself by wearing an FFP2 fine dust mask with an exhalation valve.
Soot bark disease: People can get sick
The fungal spores are only a few micrometers in size and enter the lungs when inhaled. The first symptoms appear after six to eight hours and can last for a long time. In rare cases, the body needs a few days to weeks to recover. Allergy symptoms such as a dry cough usually disappear once the area with spore dust is left. If the fungal spores are highly concentrated and inhaled over a longer period of time, inflammation of the alveoli can occur. Such cases are known from North America.
Symptoms of repeated and intensive contact:
- dry cough
- Fever and chills
- Difficulty breathing at rest
- general feeling of illness with headaches and body aches
People at risk
There may be a he alth risk for people who have intensive contact with the infected tree or who are in areas with diseased trees. These include forest workers or arborists who have been commissioned to fell diseased trees. Symptoms only appear after a long period of contact.
Usually people don't have to worry. A he alth risk exists in areas with infected trees.
People with respiratory problems should avoid affected areas. Mushroom pickers and he althy walkers need not worry when they come near diseased trees. Since there is hardly any information about cases of illness, the risk can only be estimated.
Excursus
First known case of illness in 1964
A master gardener who was employed by the Berlin Horticultural Department complained of severe respiratory irritation, diarrhea and vomiting after chopping up wood stored in the basement. While doing this work, he noticed that fungal spores were flying around the room. These developed in the wood of maple trunks that had previously been stored green and he althy. Investigations revealed that it was the fungus Cryptostroma corticale.
Treatment
Usually, a disease does not require treatment because in most cases the symptoms go away on their own. If you have a severe allergic reaction, you should call emergency services. Statements about possible contact with infected trees or a stay in areas contaminated with spores are necessary information for the treating doctor.
Prevent sooty bark disease
Young sycamore trees need a lot of water to thrive
Optimal care is necessary to protect trees from infection with the weakness parasite. The predominantly affected sycamore trees should be watered sufficiently at a young age so that the water balance does not come to a standstill and the trees grow he althily. In hot months, additional irrigation is necessary for all endangered trees in order to minimize the risk of drought stress.
Tip
A vital tree that enjoys optimal care can defend itself against the penetration of spores with active defense mechanisms. For example, it produces resin and flushes out spores. Maintaining the water supply is essential for this.
Original distribution and dispersal
The German Society for Mycology is of the opinion that the pathogen causing this disease originally comes from North America and was introduced in the 1940s. At this time the disease appeared in Great Britain. As far as is known, maple species in the rest of Europe were only attacked by the fungus after the hot year of 2003.
Situation in Germany
So far there is not enough data to create a meaningful picture of the spread of the fungus. This is because affected trees remain undetected for a long time and cases only become known when a targeted search is made for them. Until 2017 there were only isolated cases. After the hot summer of 2018, there were increasing reports of the disease, which continued into the following year.
- Baden-Württemberg: first evidence for all of Germany in 2005 in the Karlsruhe area
- Hesse: Spread of the fungus since 2009
- Berlin: first official infection in 2013
- Bavaria: first confirmed case in 2018, although widespread spread is suspected
Frequently asked questions
Does sooty bark disease affect apple trees?
No, it's probably a case of mix-up. Fruit trees are often affected by bark burn. The most important identifying feature for this fungal disease are brown spots in the outer cell division layer, which lies under the bark. These brownings are sharply demarcated from he althy tissue. Apple trees suffer from this infectious disease mainly on the trunk and strong branches. Cracks in the bark that do not heal properly can increasingly be observed in these areas. Over time, pronounced black spots appear.
Further course of the disease:
- Sapwood and heartwood can be affected if they are exposed due to injuries
- Cambium dies over a large area, leaving sapwood exposed
- Severe infection can lead to the death of the tree
When do the spores spread more widely?
The spores of Cryptostroma corticale develop in a layer several millimeters thick under the tree bark. This layer appears powdery. As soon as the dead bark comes off, the spore beds are exposed. Winds and rainfall then ensure that the spores are blown away or washed out. Even the slightest touch to affected trunk areas can trigger a whirlwind of dust.
Is he althy maple wood suitable as firewood?
Experts suspect that the causative agent of sooty bark disease is an endophyte. Such organisms live in the vegetation body of the plant and, under optimal growth conditions, the plant does not become ill. Only when the conditions change in favor of spore development does the disease break out. Such theories are based on observations: he althy wood that was stored without symptoms was later found to be infected with sooty bark disease. This raises concerns that supposedly he althy trunk parts should be used as firewood.
Why are sycamore maples more frequently attacked than Norway and field maples?
One assumption lies in the demands on the water supply. The sycamore maple prefers a cool and moist mountain climate. The species does not tolerate a lack of water over long periods of time well, so symptoms of weakness appear more quickly than in related species. The field maple also prefers moist soils. However, it copes well with variable dry conditions. The Norway maple thrives in continental climate conditions and is somewhat better adapted to more extreme fluctuations.