The fungus was introduced to Europe from the Great Lakes region of North America after World War II. Credit: MENDELU.
Brno, Aug 31 (BD) – The sooty bark disease, to which maple trees are particularly susceptible, is caused by the fungus Cryptostroma corticale. The fungus was apparently introduced to Europe after World War II from the Great Lakes region of North America. For many decades, cytopathologists were more or less unaware of the disease, but in the last 20 years,it has spread throughout Europe. Climate change is believed to be to blame, as the disease thrives especially in hot, dry summers. Scientists from the Institute of Forest Protection and Hunting at the Mendel University Faculty of Forestry and Wood Technology (LDF MENDELU), together with European colleagues, used pollen monitoring samples and detected the movement of spores of this pathogen across Europe. Spores are capable of spreading hundreds of kilometres through the air.
The fungus that causes sooty bark disease in maples thrives best in dry months with a lack of precipitation, and at average temperatures of around 25 °C. In such a situation, the trees are stressed and weakened, and ideal conditions are created for the fungus to arise.
“The mushroom will begin to spawn and also be seen. But it lives in the tree for many years before we can see it,“ explained Miloň Dvořák from the Institute of Forest Protection and Hunting. “It grows through the middle part of the trunk, and when the time is right, it grows up to its edge into the layers of the bark. Inside the bark, the fungus creates a fruiting body, a black, several decimeter to metre-wide mass called the stroma, which is revealed when the outer bark is torn off due to the growth of the stroma itself. And at that moment we can observe the most typical symptom of the presence of this fungus, a layer of black-brown soot – spores, asexual spores, which, as we found out, travel even hundreds of kilometres thanks to air currents.”
Airborne spores can easily infect other trees if they have any exposed wood. But inhaling them can be dangerous for humans as well. “It probably won’t harm a healthy person, but it can cause hypersensitivity pneumonitis in sensitive people,” said Dvořák. If left untreated, they can even cause death. These cases were recorded as early as the 1940s in North America, where the fungus was first described. To this day, this disease is called maple bark peeler’s lung, and is also listed in the International Classification of Diseases.
The researchers, along with colleagues from France, Switzerland, Italy, Portugal and Sweden, established cooperation in their countries with state pollen services, united under the banner of the European Aeroallergen Network (EAN), to find out if it is possible to monitor the spread of spores of the fungus by analysing routinely collected pollen samples.
“We managed to obtain samples of daily pollen captures from June to September 2018. We decided not to look for spores with a microscope like EAN, but by isolating all DNA and then using the real-time PCR method. But it was not easy to isolate enough DNA, the samples were several years old, fixed with a special solution between microscope slides and stained with chemicals that can interfere with the course of the PCR reaction,” explained Dvořák.
Despite these problems, laboratory diagnostics confirmed the occurrence of C. corticale across the European continent, specifically in the countries where the disease is currently observed, i.e. in the Czech Republic, France, Switzerland and Italy.
However, a number of questions remain unanswered, and will be the subject of further research. “Recognizing the pathogen is all the more difficult because the similar black mass of spores that we find under the bark of maples is also produced by some other fungi, but they colonise purely dead maples. Perhaps this is also the reason why the mushroom was overlooked by the scientific community for some time. We thought that the black mass was something we’ve known for a long time, that it’s a so-called saprophyte that takes over when the maple tree dies. Although there is one difference, the spore mass of sooty maple bark disease is not pure black, but black-brown, forms thicker layers of stroma than other fungi, and is released into the air upon contact. All it takes is a gust of wind and a dark cloud rises from the tree,” said Dvořák.
However, sooty maple bark disease is not only a European problem. A study trip to the west coast of the USA confirmed the occurrence of the fungus, in newly reported occurrences in the states of Washington and California, as well as in Oregon. Therefore, Miloň Dvořák is currently working on the development of a dispute catcher.
“My colleagues in Oregon and I agreed that I would try to build a cheap, small, but custom-tailored spore trap that would allow us to monitor the spread of the fungus. For the monitoring to make sense, the traps must be placed in a large area, and since one standard volumetric trap that EAN uses costs over CZK 100,000, we agreed on this procedure,” he explained.
The only way to protect maples from the fungal pathogen C. corticale is to collect more information. “Those who are in daily contact with trees, foresters and managers of green areas in cities, must be instructed to prevent damage to maple trunks, because the fungus enters the tree through mechanical damage to the bark. The disease is detected mainly in maples growing in cities. That is where the trees are damaged the most, for example as a result of insensitive arboricultural interventions, or in completely abandoned ruminants, where maples spontaneously rejuvenate and compete hard for available moisture in overgrown stands,” concluded Dvořák.
