In recent years, this infectious disease has spread increasingly in Germany. Nationwide, more and more cases were reported in which maple trees showed the typical signs of disease. The disease is favored by certain conditions and is usually not recognized until late.

Sooty bark disease primarily affects maple trees

Table of Contents

Show all
  1. the essentials in brief
  2. Explanation of terms
  3. Affected Trees
  4. recognize the clinical picture
  5. Sooty bark disease - reporting requirement
  6. Information for hobby gardeners
  7. dissemination and spread
  8. frequently asked Questions

    9. the essentials in brief

    • Sooty bark disease is a fungal infection that affects maple trees. The pathogen is considered a weak fungus that leads to the slow death of the trees. An outbreak of the disease is favored by hot and dry summer months.
    • In Germany, the sycamore maple is predominantly affected. Norway and field maples can also show the typical bark necrosis, trunk cracks and slime flows.
    • There is a risk of confusion with the non-notifiable disease. A clear Artinfection is possible by mycologists. If there is any suspicion, the responsible authorities should be informed.
    • The spores can be inhaled and cause various symptoms. At risk are lumberjacks and people in areas with infected trees.

    What is sooty bark disease?

    Sooty bark disease is caused by a fungus

    Sooty bark disease (according to old spelling also: sooty bark disease) is a disease of trees that is caused by spores of a weak parasite. The Latin name of this species of fungus is Cryptostroma corticale. It settles in weakening wood. Infested wood appears as if charred, which has 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 for spreading and already accumulate in the bark of healthy trees, where they survive until the moment of infection. They infect the tree by penetrating the organism through wounds or open wood that has been broken off.

    The fungus spreads massively on diseased wood. Its mycelium grows through the fibrous tissue, after which the tree closes off these affected areas from the healthy wood. If the fungus penetrates into the cambium, black-brown spore beds are formed.

    Typical course of the disease:

    1. infected trees get a bare crown
    2. Water shoots develop in the lower trunk area
    3. Slimy spots form in the trunk
    4. Bark swells like blisters and peels off in long strips over time
    5. soot-black areas appear
    6. Millions of pores form a dust

    If a maple suffers from sooty bark disease, the dying process can take several years, depending on the state of health of the tree. Strongly weakened trees die off completely within one growing season. An infection can go undetected from the outside for a long time, but the fungus spreads more and more inside and additionally weakens the wood.

    What causes 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 winds. The trees weaken due to water scarcity, which offers the pathogen additional opportunities for growth and spread.

    • Hot summers in recent years have promoted the spread of the disease
    • older trees are well established and therefore better supplied with water
    • young trees are more susceptible because of the less developed root system

    The fungus benefits significantly from climate change, which brings with it low-precipitation summer months and high temperatures. Under laboratory conditions, the species showed optimum growth when the thermometer was at 25 degrees. This result confirms the fact that
    Cryptostroma corticale has a warmth-loving character.

    Affected Trees

    Sooty bark disease occurs on maple trees in Germany. An infection of apple trees is not known so far. It has not been clearly clarified that beeches are also affected. In the past there were only suspected cases. In Berlin, it was observed that the fungus spreads mainly on sycamore maples and affects Norway and field maples somewhat less frequently. This observation also applies to the other distribution areas of the fungus species in Germany.

    Short overview:

    • Fungus also affects basswood and hickory in North America
    • diseases in birch trees have been confirmed in isolated cases
    • Ornamental maples in Germany have so far been spared

    digression

    Sycamore and its lower resilience

    The maple species is less affected by the disease where optimal site conditions prevail. Cryptostroma corticale depends on pre-damaged wood, which the fungus uses as a portal of entry. If the sycamore 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 major role in terms of vitality, as the tree species likes fresh conditions. If there are more years with long periods of drought and heat during the summer, the infestation situation at such optimal locations may 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. When a tree is affected by sooty bark disease, it suffers from leaf wilt and excessive leaf loss. The crown is gradually showing signs of dying off. If infested trunk wood is cut, greenish, brown or bluish discolorations become visible. They are a consequence of the lockdown response.

    Clear infection patterns:

    • slime flow: viscous plant sap colored reddish to blackish by fungal spores
    • bark necrosis: local dying 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 (short: LFW) has developed a "credit rating key for the assessment of sycamore" with which the disease stage can be evaluated. This is categorized into five classes and shows the typical symptoms that first catch the eye of the beholder.

    class health status symptoms
    0 very good none
    1 slightly weakened Water veins, deadwood in the crown
    2 significantly weakened Bark flakes off at certain points, spores become visible
    3 severe loss of vitality larger pieces of bark flaked off, a lot of dead wood in the crown
    4 dead Large area of bark flaked off, wood charred

    risk of confusion

    For the untrained eye, spotting sooty bark disease is nearly impossible. There are a number of other fungi that cause similar symptoms. Reliable identification of the species requires microscopy of the fungal spores. Samples can be sent to mycologists for examination.

    Stegonsporium maple dieback

    The fungus Stegonsporium pyriforme is responsible for this disease. It also benefits from dry conditions and develops black spore deposits, so confusion with sooty bark disease is not uncommon. This fungus infects weakened and previously diseased trees through wounds and broken branches. The infected branch then dies. There are some clues that allow better disease identification:

    • mainly occurs on young plants
    • sharp transition between living and dead shoot section
    • Spore beds recognizable as black and rounded spots on the shoots
    • localized die-offs

    Flat corner disc

    This species hides the fungus Diatrype stigma. This develops a crust-like coating with a black colour. The crusts are about a millimeter thick and are developed under the bark. Over time, this peels off so that the spore bearings become visible. These have a finely punctured surface and occasionally appear pitted 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.

    burn crust fungus

    The burn crust fungus forms black crusts that appear burnt

    Kretzschmaria deusta develops crusty spore deposits, which are predominantly black in color and have a bulging to bumpy surface with a convex edge. The fungus is very hard and feels like charcoal when aged. This creates charcoal-like patches that appear mainly across the lower trunk area to the roots. This fungus lives mainly on beech and linden trees. Occasionally he 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 a reporting requirement?

    Contrary to what is often assumed, there is no obligation to report sooty bark disease in Germany. This would make it significantly easier to monitor the disease in Germany, but would involve a great deal 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 space office or lower nature conservation authority in your region
    • local tree care company
    • forest office or the responsible city or municipal administration

    Caution: Do not take spore samples thoughtlessly

    A suspected infestation should be confirmed by a competent authority in your federal state, even if sooty bark disease is not notifiable. You can send spore samples of the fungi to the appropriate authorities, but you should contact the staff before sending the samples. They will tell you how to proceed. Taking a sample is not without risk, as the spores enter the human respiratory tract and pose health risks.

    Take special care when felling trees

    Authorities advise caution when affected trees have to be felled. A spacious barrier makes sense so that walkers are not exposed to the risk of spore dust. Ideally, the trees are felled in damp weather, because then the dust development is comparatively low. Forest workers must equip themselves with protective clothing and wear respiratory masks. The cleared wood should be stored under tarps until it is transported to a waste incineration plant.

    Recommended protective equipment:

    • Full body protective suit
    • hat and goggles
    • Class FFP2 respirator mask

    Information for hobby gardeners

    The disease mainly affects sycamore maples, which rarely grow in private gardens. If you still own a stately specimen, you should act quickly in suspicious cases. So far it has not been 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 is already dying. It is therefore important to check affected trees for the disease even if there are even the slightest signs of disease.

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    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 quantities of fungal spores are released into the air when it is crushed. Infested wood is to be disposed of with hazardous waste.

    Information on disposal costs:

    • Disposal is complex and can be expensive
    • Acceptance points must be able to properly burn contaminated wood
    • Prices up to 400 euros per ton of wood are possible

    tips

    If your area requires felling of infected trees, you should avoid the area extensively. If you suffer from previous illnesses, you can also protect yourself by wearing an FFP2 fine dust mask with an exhalation valve.

    Sooty bark disease: Humans 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 containing spore dust is left. If the fungal spores are highly concentrated and inhaled over a longer period of time, the alveoli can become inflamed. Such cases are known from North America.

    Symptoms of repeated and intensive contact:

    • dry tickly cough
    • fever and chills
    • shortness of breath at rest
    • General malaise with headache and body aches

    vulnerable people

    A health hazard can exist for people who have intensive contact with the infested tree or who are in areas with diseased trees. These include forest workers or arborists who have been tasked with felling diseased trees. Symptoms appear only after a long period of contact.

    Normally people don't have to worry. A health risk exists in areas with infected trees.

    People with respiratory problems should avoid affected areas. Mushroom pickers and healthy walkers needn't worry about getting near diseased trees. Since there is hardly any information about cases of illness, the risk can only be estimated.

    digression

    First known case of illness in 1964

    A master gardener who worked for the Berlin Horticultural Office complained of severe irritation of the respiratory tract, diarrhea and vomiting after chopping up wood stored in the basement. While doing this he noticed that fungal spores were flying around the room. These developed in the wood of maple trunks that were previously stored green and healthy. Investigations showed that it was the fungus Cryptostroma corticale.

    treatment

    Usually, a condition does not need to be treated because, in most cases, the symptoms go away on their own. In the case of severe allergic reactions, emergency services should be called. Statements about possible contact with infected trees or a stay in areas contaminated with spores are necessary information for the attending physician.

    Prevent sooty bark disease

    Young sycamores need a lot of water to thrive

    In order to protect trees from infection with the weak parasite, optimal care is necessary. The predominantly affected sycamore maples should be watered sufficiently at a young age so that the water balance does not come to a standstill and the trees grow healthily. In hot months, all endangered trees need additional watering to minimize the risk of drought stress.

    tips

    A vital tree that is allowed to enjoy optimal care can defend itself against the penetration of spores with active defense mechanisms. For example, it produces resin and thus flushes out spores. Maintaining the water supply is essential for this.

    Original distribution and spread

    The German Society for Mycology believes that the causative agent of this disease originally came from North America and was introduced in the 1940s. At that 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 undiscovered for a long time and cases only become known when they are specifically searched for. By 2022, there were only isolated cases. After the hot summer of 2022, there were more reports of the disease, which lasted into the following year.

    • Baden-Wuerttemberg: first record for the whole 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 2022, with widespread distribution suspected

    frequently asked Questions

    Does sooty bark disease affect apple trees?

    No, it's probably a mix-up. The fruit trees are often afflicted by bark blight. The most important distinguishing feature for this fungal disease are brown spots in the outer cell division layer, which lies under the bark. This browning is sharply demarcated from healthy tissue.Apple trees suffer from this infectious disease mainly on the trunk and strong branches. Increasingly, cracks in the bark can be observed in these areas, which do not heal properly. Over time, pronounced black spots appear.

    Further course of the disease:

    • The sapwood and heartwood can be affected if they are exposed due to injuries
    • The cambium dies off over a large area, leaving the sapwood exposed
    • widespread infection can lead to the death of the tree

    When do the spores multiply?

    Cryptostroma corticale spores are developed in a layer several millimeters thick under the tree bark. This layer appears powdery. As soon as the dead bark comes off, the spore stores are exposed. Winds and precipitation then ensure that the spores are blown away or washed away. Even the slightest touch to affected trunk areas can trigger a dust whirl.

    Is healthy maple wood suitable as firewood?

    Experts suspect that the causative agent of sooty bark disease is an endophyte. Such organisms live in the vegetative body of the plant, which under optimal growth conditions does not become diseased. The disease only breaks out when the conditions change in favor of spore development. Such theories are based on observations: an infection with sooty bark disease could later be detected in healthy wood that was stored without symptoms. This raises concerns that supposedly healthy parts of the trunk should be used as firewood.

    Why are sycamore maples attacked more often than Norway and field maples?

    One assumption is the demand on the water supply. The sycamore prefers a cool and humid mountain climate. The species does not tolerate a lack of water over a long period of time, so that signs of weakness occur more quickly than in the related species. The field maple also prefers moist soil. However, this one copes well with alternating dry conditions. The Norway maple thrives in continental climate conditions and is slightly better adapted to more extreme fluctuations.

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