Fungal disease affects maples. What is wrong with the Dutch trees?

It starts insidiously. Airborne spores enter through a weak spot on a maple – an ‘open wound’ where pruning has been done, a piece of gnawed bark. Those spores of the parasitic fungus cryptostroma corticale then attack the wood from the inside. Anyone who pays attention can quickly recognize the first signs of disease: wilting leaves, dying branches. Then the bark swells, the upper part of that bark cracks open, and underlying black spots become visible. As if the wood has been charred. If you rub your finger over it, it will turn black due to the numerous spores.

Here is the work of the sooty bark fungus. In 2014, the disease was first detected in a maple tree in the Netherlands. Since then, managers regularly encounter infested maple trees. Especially after periods of great heat or drought, because the fungus strikes trees that are already weakened. In this respect, this condition differs from other tree diseases in the Netherlands, such as bleeding chestnut disease or Dutch elm disease: they also affect healthy specimens.

Tree infestations come in different shapes and sizes. “We generally distinguish between biotic and abiotic causes,” says independent tree biologist Jitze Kopinga, who was affiliated with Wageningen University until his retirement. “Biotic causes include diseases – caused by fungi, bacteria or viruses – and animal pests.” The typesetter, for example, belongs to the latter category, a species of beetle that targets Norway spruce. “And with abiotic attacks you can think of salt damage, frost damage and air pollution.” Not every infestation is a problem, he adds. “Diseases only become serious when healthy trees die and reach epidemic proportions.”

It is a tree disease that not only affects maples, but also humans

Henry Kuppen Terra Nostra

In that regard, the sooty bark fungus, with its preference for weakened trees, seems to have little cause for concern. “Still, it is important that administrators know about existence,” says Henry Kuppen, director of the Terra Nostra research agency. He conducts research into the sooty bark fungus and recently organized several ‘knowledge days’ on the subject for forest managers. “It is a tree disease that not only affects maples, but also humans. If you inhale the spores frequently, you can become seriously ill and develop chronically inflamed lungs.”

The sooty bark fungus also differs from other tree diseases in this respect. Bacterial burn in walnuts, like the bleeding chestnut disease, is caused by a bacterium that is not harmful to humans. Dutch elm disease is the result of a fungal infection in which the fungal spores spread via beetle feet. There is no chance of inhalation of large spore clouds. And the spores are also not harmful to humans in the case of ash branch death – caused by a fungus known as ‘false ash birch’ – says Jelle Hiemstra, tree disease researcher at Wageningen Research.

Respiratory complaints

Things are different with the sooty bark fungus, at least: if you don’t leave it alone. Passers-by have little to fear from the tracks, says Kuppen. It is mainly arborists and forest workers who cut down an affected tree who are exposed to the microscopic spores. As early as 1945 there were loggers in England with respiratory complaints due to the sooty bark fungus.

If possible, doing nothing is the best advice, according to Kuppen. But the wood of affected maples becomes brittle, causing branches or even entire trees to break off. Because of that fall hazard, removal of some maples is necessary. During the hood, spray mist can prevent the spread of spores, and sawers and processors should wear protective clothing and masks. Cut maple trees can be safely processed in a biomass plant with the right measures.

Tree diseases will never be completely preventable, says Kuppen. “Trees and diseases belong together.” However, he believes there is a possible reason why the sooty bark fungus appears to be occurring more and more: climate change. The spores become active at temperatures above 20 degrees Celsius and, as mentioned, specifically affect trees that have been weakened by, for example, a heat wave or a prolonged dry period. “After three dry summers, in 2018, 2019 and 2020, many maple trees had a hard time.”

Weakness parasite

Kopinga nuances: “A lot is not yet known about the influence of climate change on diseases. The hypotheses are often quite speculative and sometimes based on correlations where cause and effect are sometimes confused. In the case of rust bark disease, we do indeed know that it is a so-called weakness parasite. When trees suffer more from drought stress due to climate change, it is conceivable that the disease will have a greater chance. But then you would first have to determine whether the drought stress on maples has really increased on average in recent decades.” Hiemstra: “The false essen-membrane chalice is also not fond of dry conditions. But the disease occurs as far as Italy, so when ash branch dies, those high temperatures don’t really matter.”

What contributes to all tree diseases, he adds, is the increased international trade and transport. “Many diseases come with imported trees or, for example, with packaging wood. We know from the death of ash trees that the disease came from Asia to the Baltic states. And from there, the disease spread to Western Europe under its own power.” Ash branch mortality has also been known in Dutch trees since 2010.

To combat diseases, it is generally better not to plant a homogeneous forest of one species or one variety, all three researchers say. Kopinga: „The most effective method is the use of insensitive species or varieties. For example, Dutch elm disease – which has killed the majority of Dutch elm trees over the past century – has decreased in frequency due to the increasing use of disease-tolerant varieties.” Hiemstra: “Thanks to that long breeding program, many new varieties have been created, but the ‘real’ Dutch elm has slowly disappeared from the streets.” He says that there is also a vaccine against Dutch elm disease, Dutch Trig. “That has to be administered annually with a syringe.” With regard to the ash trees, Wageningen researchers are now experimenting with cuttings from a few healthy ash trees to see whether they are less sensitive to ash branch death.

Infrared therapy

Treatment methods are also being experimented with for bleeding chestnut disease. For example, the ‘infrared therapy’ with which the company Treehold treats horse chestnuts in Dordrecht seems to be working well. The trees are packed and then brought to a temperature between 42 and 45 degrees Celsius in four hours. “In this way we help the tree to restore the bacterial balance,” says developer Marc Derksen.

There is even international interest in the infrared method. Hiemstra suspects that in practice it will mainly become a method to save special, monumental horse chestnuts. Kopinga says he is curious about the long-term effect. “Infrared therapy on horse chestnuts does not change the chance of reinfection. Sensitive species and genotypes simply remain susceptible to the bleeding disease. In my opinion, the treatment therefore has the character of temporary symptomatic relief.” Derksen: „Trees in ‘weak’ growing places will indeed always require extra attention. In such cases, we recommend additional growth site improvement, for example in the form of fertilization.”

Ultimately, Kuppen concludes, there is only one structural solution against the adverse effects of tree diseases. “That is introducing species diversity and stimulating biodiversity.”