The “Sooty Bark Disease”, harmful for maples and humans, can be monitored by pollen sampling stations

The hyper-allergenic spores of the fungus causing Sooty bark disease in maples were detected in six European countries.

Sycamore maples destroyed by the Sooty bark disease. Photo by Dr Miloň Dvořák

Especially after the last few COVID-affected years, nobody doubts that emerging infectious diseases can threaten the whole world. But humans are not the only ones at risk! With intensive global trade, many tree parasites are accidently introduced to Europe in packaging or directly on goods. Traveling in the wood, on plants or in the soil of their pots, they can remain undetected for a long time.

“Forms of life of parasitic fungi are extremely diverse and very often practically invisible,” says Dr Miloň Dvořák of the Department of Forest Protection and Wildlife Management at Mendel University in Brno, Czechia. “An infected tree may look completely healthy for some time, which complicates the control of the disease enormously. It reminds me of the ancient Trojan Horse, where European trees are so surprised, defenceless, and later defeated, like the Trojan warriors.”

How can an infected tree look healthy and then suddenly get sick? “Like in the human body, in trees too, the trigger can be stress,” explains Dr Dvořák. The tolerance of trees to a pathogenic fungus turns lower under the conditions of changing climate and so the tree starts to die of the disease.

One typical example of such a disease is the Sooty Bark Disease (SBD) on maples, caused by a microscopic fungus called Cryptostroma corticale. “The fungus was probably introduced to Europe during the Second World War and for the rest of the 20th century we did not hear much about it,” says Dr Dvořák.  

Sooty Bark Disease (Cryptostroma corticale) on Sycamore. Photo by gailhampshire used under a CC BY 2.0 license

The situation has changed and over the last twenty years the fungus has been reported more and more often. After dry and hot periods, the trees start to die of the infection, which is accompanied by the creation of brown-black masses of “soot” under the peeling bark of the maples.

The “soot” is in fact spores, which help the fungus spread and infect other trees. It is harmful for wounded trees, but it can also cause hypersensitivity pneumonitis in humans.

So, the species became a target for a group of phytopathologists gathered by an European HORIZON 2020 project entitled “Holistic management for emerging forest pests and diseases (HOMED)”. Scientists from six countries (Czechia, France, Italy, Portugal, Sweden and Switzerland), including Dr Dvořák, decided to develop a precise, DNA based (real-time PCR) diagnostic method to detect and monitor the pathogen in air samples. They published their method, the outcomes of its use, and their new findings about SBD epidemiology in the open-access journal NeoBiota.

Volumetric air sampler installed in Brno, Czech Republic, sampling pollen for allergen forecast. Photo by Aneta Lukačevičová

How to look for DNA in air samples? Simple devices called volumetric air samplers can suck the air against a piece of sticky tape, where every particle gets stuck and can be analyzed. “These devices are not really cheap, moreover, they demand regular maintenance,” explains Dr Dvořák. “But, actually, they are in common and regular use in the whole of Europe – remember the weather forecast, particularly that part about the “pollen report” for allergic people. This forecast is based on data of more than 600 stations united by the European Aeroallergen Network (EAN). Every station permanently maintains one volumetric air sampler and keeps an archive of the samples.”

The HOMED team got in contact with their national EAN collaborators and processed their samples with molecular techniques (real-time PCR).

Thanks to this sensitive detection method, the survey among samples was very successful. The “sooty” fungus was found in air samples from countries where the disease has been reported, and, in a more detailed study in France, the pathogen was found in the air 310km from currently diseased trees! This result suggests that the fungus can disperse long distances by wind.

Black stromata – source of billions of hyper-allergenic spores. Photo by Dr Miloň Dvořák

“Our results show that the SBD disease is at an exponentially increasing phase in France and Switzerland with an increase in the magnitude of the number of disease cases that peaks following a marked water deficit,” the researchers write in their study. They hope that early aerial detection of C. corticale in disease-free countries could help implement more efficient measures for SBD detection and eradication in the field.

“This European experiment fully confirmed the potential of this approach to monitor the pathogen’s outbreaks in early stages of its spread,” concludes Dr Dvořák. 

Research article:

Muller E, Dvořák M, Marçais B, Caeiro E, Clot B, Desprez-Loustau M-L, Gedda B, Lundén K, Migliorini D, Oliver G, Ramos AP, Rigling D, Rybníček O, Santini A, Schneider S, Stenlid J, Tedeschini E, Aguayo J, Gomez-Gallego M (2023) Conditions of emergence of the Sooty Bark Disease and aerobiology of Cryptostroma corticale in Europe. In: Jactel H, Orazio C, Robinet C, Douma JC, Santini A, Battisti A, Branco M, Seehausen L, Kenis M (Eds) Conceptual and technical innovations to better manage invasions of alien pests and pathogens in forests. NeoBiota 84: 319-347.

Follow NeoBiota on Facebook and Twitter. Follow the HOMED project on Twitter.

How non-native tree species affect biodiversity

Non-native forest tree species can reduce native species diversity if they are planted in uniform stands, finds an international review study.

Non-native forest tree species can reduce native species diversity if they are planted in uniform stands. In contrast, the effects of introduced species on soil properties are small. This was found by an international review study with the participation of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.

Curse or blessing? Opinions are divided on non-native tree species. In addition to native species, many foresters also plant non-native species that can withstand the increasing summer drought. In various parts of Europe, the latter are already important suppliers of timber. However, conservationists fear ecological damage, for example if native species are displaced or tree pathogens and insect pests are introduced.

In Switzerland, Douglas fir is partly used for afforestation. However, large pure stands, such as those found in Germany, are prohibited there. Photo by Thomas Reich

Now a team of European researchers, led by Thomas Wohlgemuth of WSL, has looked at the state of knowledge on the ecological consequences of alien tree species in Europe. They analysed the results of 103 studies on seven such species. All of these studies had investigated how stands dominated by non-native tree species affected biodiversity or soil condition under the trees compared to stands of native tree species. The organisms studied included plants, mosses, microorganisms and insects from the forest floor to the treetops.

Of the seven alien species studied, only the Douglas fir is currently planted in larger numbers in the Swiss forests. While foresters used to value its fast, straight growth and its versatile wood, today they appreciate its higher drought tolerance compared to spruce. Other species are problematic because they can spread uncontrollably. The North American Robinia, for example, is invasive and can displace native species. It was already introduced in Europe 400 years ago and used in Switzerland, among other things, to stabilise soils.

Robinia can spread rapidly and form stands as here in Valais. Photo by Thomas Reich

Negative effects on biodiversity predominate

Across the 103 studies, the consequences of non-native species for biodiversity were negative. Comparisons from 20 studies show, for example, that on average fewer insect species live on and in Douglas fir than in spruce or beech stands. Robinia also reduces the diversity of insects, eucalyptus that of birds. This is hardly surprising, says Wohlgemuth, head of the WSL Forest Dynamics Research Unit. Because: “These results apply to comparisons between pure stands.” In continuous, uniform plantations, many alien species clearly have worse impacts than native species.

Proportion of cases with increasing (green), decreasing (red) or non-significant (grey) effects of tree species non-native to Europe on diversity attributes (abundance, species richness or diversity) of different taxonomic groups in comparison to native vegetation. Numbers of cases are shown next to the NNTs names, below the diversity attributes and above the bars.

But alien species do not only have negative impacts. Most of them do not affect soil properties. The easily degradable needles of Douglas firs can even make more nutrients available than the poorly degradable spruce needles. “When it comes only to soil properties, the Douglas fir has no negative impact,” Wohlgemuth says. In general, an equal number of studies found positive and negative effects of the seven non-native species on the soil.

Douglas firs are attractive for forestry because of their fast growth, good wood properties and – in regard to climate change – their drought resistance. Photo by Thomas Reich

Furthermore, it makes a difference whether the alien species are more closely or more distantly related to European tree species. “Tree species without closer relatives, such as eucalyptus and acacia from Australia, reduce species diversity more strongly across all studies than closely related species, such as Douglas fir and wild black cherry from North America,” adds Martin Gossner, head of the WSL Forest Entomology Group and second author of the study.

A Douglas fir. Photo by Neptuul under a CC BY-SA 4.0 license

It all depends on the management

Management has a significant influence on whether Douglas fir or other tree species are good or bad for a forest overall. Uniform and dense Douglas fir stands are unsuitable habitats for many organisms. However, the same is true for spruces, which have been planted extensively for timber production in lowland areas of Central Europe over the last 100 years. On the other hand, Douglas firs in stands of native forest trees, individually or in small groups, would hardly disturb the ecosystem, Wohlgemuth says: “We conclude that the impact on native biodiversity is low with mixed-in Douglas firs.”

Should foresters plant non-native tree species or not? Despite certain negative aspects, Wohlgemuth does not recommend total renunciation. “Particularly in the case of Douglas fir, the facts show that moderate admixture in stands has little impact on native biodiversity, while at the same time preserving ecosystem services such as the production of construction timber. This is especially true when other, less drought-resistant conifers are increasingly lacking with regard to unchecked climate change.”

Research article:

Wohlgemuth T, Gossner MM, Campagnaro T, Marchante H, van Loo M, Vacchiano G, Castro-Díez P, Dobrowolska D, Gazda A, Keren S, Keserű Z, Koprowski M, La Porta N, Marozas V, Nygaard PH, Podrázský V, Puchałka R, Reisman-Berman O, Straigytė L, Ylioja T, Pötzelsberger E, Silva JS (2022) Impact of non-native tree species in Europe on soil properties and biodiversity: a review. NeoBiota 78: 45-69.

Web news piece originally published by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. Republished with permission.

Follow NeoBiota on Twitter and Facebook.