How dangerous is soot bark disease?

How dangerous is soot bark disease?

the essentials in brief

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

What is Soot Bark Disease?

soot bark disease

Soot bark disease (according to the old spelling also: soot bark disease) is a disease of trees that is caused by the spores of a weak parasite. The Latin name of this type of mushroom is Cryptostroma corticale. It settles in ailing wood. Infested wood appears charred, which is what led to the German name.

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Development and course of the disease

The fungal spores are considered to be the primary source of infection. They have a great potential to spread and are already deposited in the bark of healthy trees, where they survive until the moment of infection. They infect the tree by entering the organism through wounds or by breaking off exposed wood.

The fungus spreads massively on diseased wood. Its mycelium grows through the fibrous tissue, whereupon the tree seals 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 develop a bald crown
  2. Water shoots arise in the lower trunk area
  3. Slimy spots form on the trunk
  4. The bark swells up like bubbles and peeled off in elongated strips over time
  5. soot-black surfaces appear
  6. Millions of pores form a dust
Five stages of soot bark disease

If a maple suffers from soot bark disease, the dying process can take several years, depending on the health of the tree. Severely weakened trees die completely within one growing season. An infection can remain undetected from the outside for a long time, but inside the fungus spreads increasingly and further weakens the wood.

What favors the disease

Cryptostroma corticale is a heat-loving fungus that is favored by dry and hot climates. Under these conditions it can thrive and produce masses of spores that are optimally spread by winds. Due to the scarcity of water, the trees weaken, which gives the pathogen additional opportunities to grow and spread.

  • The hot summers in recent years have promoted the spread of the disease
  • older trees have grown in well and are 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 the summer months with little rainfall 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

The soot bark disease occurs in Germany on maple trees. An infection of apple trees is not yet known. It has not been clearly established that beeches are also infested. In the past there were only suspicious cases. In Berlin it could be observed that the fungus mainly spreads on sycamore maple and affects Norway and field maple somewhat less often. This observation also applies to the other distribution areas of the mushroom species in Germany.

Brief overview:

  • Fungus also affects linden and hickory nuts in North America
  • Occasional illnesses in birch trees were confirmed
  • Ornamental maples in Germany have so far been spared

Digression

Sycamore maple and its lower resistance

The maple species is less affected by the disease where optimal site conditions prevail. Cryptostroma corticale is dependent on pre-damaged wood, which the fungus uses as a gateway. 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 major role in terms of vitality, because the tree species likes fresh conditions. If there are further 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 soot bark disease

soot bark disease

The fungus can only be clearly identified if the spores are identified under the microscope. There are a number of other fungi that leave blackish deposits on wood. If a tree is affected by soot bark disease, it suffers from leaf wilt and excessive leaf loss. The crown is gradually showing signs of death. If infected trunk wood is cut, greenish, brown or bluish discoloration becomes visible. They are a consequence of the foreclosure reaction.

Clear infection patterns:

  • Mucus flow : viscous plant sap colored reddish to blackish by fungal spores
  • Bark necrosis : local death of the bark, under which soot-like spore dust collects
  • Longitudinal cracks : trunk tears open due to disturbed water balance, so that bark flakes 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 strike the viewer.

classhealth statusSymptoms
0very goodno
1slightly weakenedWater veins, dead wood in the crown
2clearly weakenedBark flakes off selectively, spore beds become visible
3severe loss of vitalitylarger pieces of bark flaked off, a lot of crown deadwood
4thdeadLarge areas of bark flaked off, wood charred

Risk of confusion

For the untrained eye it is almost impossible to recognize the soot bark disease. There are a number of other fungi that cause similar diseases. A reliable identification of the species requires microscopic examination 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. He also benefits from dry conditions and develops black spore beds, so that confusion with the soot 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 indications that allow better disease identification:

  • occurs mainly on young plants
  • sharp transition between living and dead shoot section
  • Spore beds recognizable as black and rounded spots on the shoots
  • locally limited signs of death

Flat corner disc

Behind this species is the Diatrype stigma fungus. This develops a crust-like coating with a black color. The crusts are about a millimeter thick and develop under the bark. In the course of time this loosens so that the spore beds become visible. These have a finely dotted surface and occasionally appear scar-like or cracked with age. The flat corner disc is a common fungus that can be found on dead wood of birch, oak, beech and maple trees.

Burn crust fungus

Burn crust fungus

Kretzschmaria deusta develops crust-shaped spore beds that are predominantly black in color and have a bulging to hump-like surface with a bulging edge. The mushroom is very hard and feels like charcoal as it ages. This creates carbon-like spots that mainly appear over the lower stem area to the roots. This mushroom mainly lives on beeches and linden trees. Occasionally it colonizes maple trees.

  • causes the so-called mold rot in roots
  • often no damage visible from the outside
  • Coal-like crust coatings usually only recognizable after the logs have been broken off

Is there a notification requirement?

Contrary to what is often assumed, there is no obligation to report soot bark disease in Germany. This would make the monitoring of the disease in Germany much easier, but it would involve a lot of effort. If you suspect that it is soot 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
  • Forestry office or the responsible city or municipal administration

Caution: do not take spore samples rashly

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

Take special care when felling trees

Authorities advise special caution if affected trees have to be felled. A spacious barrier is useful so that walkers are not exposed to the risk of spore dust. Ideally, the trees are felled in damp weather, because then the development of dust 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
  • FFP2 class breathing mask

Information for hobby gardeners

The disease predominantly affects sycamores, which rarely grow in private gardens. Anyone who still owns a stately specimen should act quickly in the event of suspicion. So far it is not possible to fight the fungal disease. There is no information about successful fungicide treatment. As soon as the spore beds become visible, the tree dies. It is therefore important to examine affected trees for the disease even if they show the slightest sign of disease.

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Felling by specialist companies necessary

Experts warn against cutting down diseased trees without authorization. This work should be done by tree care companies. The felled wood should not be used as firewood, as large amounts of the fungal spores get into the air when they are cut up. Disposal via hazardous waste is provided for infected wood.

Information on disposal costs:

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

Tips

If infected trees need to be felled in your area, avoid widespread use of 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 get into the lungs when they are inhaled. The first symptoms appear after six to eight hours and can persist for a long time. In rare cases, it takes the body a few days to weeks to recover. Allergy symptoms like dry cough usually go away once the area with spore dust is left. If the fungal spores are highly concentrated and inhaled over a long period of time, this can lead to inflammation of the alveoli. Such cases are known to date from North America.

Symptoms of repeated and intensive contact:

  • dry, irritating cough
  • Fever and chills
  • Difficulty breathing at rest
  • general feeling of illness 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 only appear after a long period of contact.

Usually 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 don't have to worry if they come 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 he chopped up wood stored in the cellar. During this work he noticed that fungal spores were flying around in the room. These developed in the wood of maple trunks that had previously been stored green and healthy. Investigations showed that it was the fungus Cryptostroma corticale.

treatment

Usually, the condition does not require treatment, as symptoms will most often go away on their own. In the event of severe allergic reactions, the emergency number should be dialed. Statements about possible contact with infected trees or staying in areas contaminated with spores are necessary information for the attending physician.

Prevent soot bark disease

soot bark disease

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

Tips

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

Original spread and spread

The German Society for Mycology is of the opinion that the causative agent of this disease originally came from North America and was carried over in the 1940s. At that time, the disease emerged in the UK. As far as is known, in the rest of Europe maple species were only attacked by the fungus after the heat 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 due to the fact that affected trees remain undetected for a long time and cases only become known when they are specifically searched for. Up until 2017 there were only a few cases. After the hot summer of 2018, there were more reports of the disease, which continued into the following year.

  • Baden-Württemberg : first proof for the whole of Germany in 2005 in the Karlsruhe area
  • Hessen : Spread of the fungus since 2009
  • Berlin : first official infection in 2013
  • Bavaria : first confirmed case in 2018, where widespread distribution is assumed

frequently asked Questions

Does the soot bark disease affect apple trees?

No, there is probably a mix-up. The fruit trees are often affected by bark burn. The most important distinguishing feature of this fungal disease are brown spots in the outer cell division layer, which lies under the bark. This browning is sharply demarcated from the 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 that do not heal properly. Pronounced black spots appear over time.

Further course of the disease:

  • Sapwood and heartwood can be affected if it is exposed due to injury
  • Large areas of cambium die off so that sapwood is exposed
  • extensive infection can lead to the death of the tree

When do the spores spread more often?

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 loosens, the spore beds are exposed. Subsequently, winds and precipitation ensure that the spores are blown away or washed out. Even the slightest touch of affected trunk areas can trigger a dust vortex.

Is healthy maple wood suitable as firewood?

Experts suspect that the pathogen causing soot bark disease is an endophyte. Such organisms live in the vegetation of the plant, which does not become ill under optimal growth conditions. Only when the conditions change in favor of spore development does the disease break out. Such theories are based on observations: if healthy wood that was stored without symptoms, an infection with the soot bark disease could later be determined. This raises concerns that supposedly healthy trunk parts should be used as firewood.

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

One assumption lies in the demands on the water supply. The sycamore maple prefers a cool and humid mountain climate. The species does not tolerate water shortages over long periods of time, so that symptoms of weakness occur more quickly than in the related species. The field maple also prefers moist soils. However, this copes well with changing dry conditions. The Norway maple thrives under continental climatic conditions and is somewhat better adapted to more extreme fluctuations.