When the bark of a sycamore maple splits open and turns black, it’s too late. Then the tree suffers from sooty bark disease and is irretrievably lost. Its causative agent is the fungus Cryptostroma corticale. It attacks maple trees through small wounds in the bark. Below her, its spores then form a blackish coating up to one centimeter thick. When the disease breaks out, the trees drop their leaves, and shortly thereafter the first branches die. At a later stage, large pieces of bark flake off the trunks, revealing the dark spore coating. The tree dies.
“Now in the summer, the disease will also appear on trees that previously appeared completely healthy,” says Andreas Hahn from the State Institute for Forestry and Forestry (LWF) in Munich-Freising. “Cryptostroma corticale develops a particularly large number of spores in a hot July like this.” Many sycamore maples have been carrying the fungus for a long time without the sooty bark disease breaking out in them. This only happens when it is hot and dry for a long time. In some regions of Bavaria, for example in Lower Franconia, sooty bark disease could become a serious threat to the sycamore trees there.
Cryptostroma corticale originally comes from North America and was probably introduced to Europe through the global plant trade. The fungus was first detected here in England in 1945. From there he worked his way to the mainland. In the 1950s it was documented in parks in Paris and in 1964 in the urban area of Berlin. Bayern was spared for a comparatively long time. The first detection in the Free State took place in 2018 – in Lower Franconia, in the Würzburg area. Since then, sooty bark disease has spread massively in Bavaria. In the meantime she has arrived in the district of Landsberg am Lech.
Hahn and his colleague Nicole Burgdorf, both of whom work on sooty bark disease at the LWF, assume that Cryptostroma corticale is already widespread across the board. From their point of view, the sooty bark disease can no longer be contained, the vast majority of affected sycamore maples only show no symptoms. In their research, they are therefore concentrating on the factors that lead to the outbreak of sooty bark disease. One thing is certain: the fungus is a beneficiary of global warming. In dry years, the infestation literally explodes.
Spores also dangerous for humans
Cryptostroma corticale spores can also be dangerous for humans. Breathing them in can cause inflammation of the air sacs in the lungs. The symptoms are a dry cough and fever, chills and shortness of breath. The LWF therefore advises wearing a breathing mask in addition to the usual protective clothing when felling and transporting infested trees and cleaning saws and other equipment thoroughly after the work is complete. The wood of a maple with sooty bark disease should be disposed of in an incinerator.