Particularly following the recent years heavily impacted by COVID, there is no question that emerging infectious diseases have the potential to endanger the entire globe. However, it's not just humans who face these risks! Due to extensive worldwide trade, numerous tree parasites are inadvertently brought into Europe through packaging or directly on merchandise. While traversing in the timber, on flora, or within the soil of their containers, they can persist unnoticed for extended durations.
Dr. Miloň Dvořák, from Mendel University's Department of Forest Protection and Wildlife Management in Brno, Czechia, emphasizes the vast and inconspicuous variety of parasitic fungi. "The forms of life exhibited by these fungi are highly diverse and often nearly imperceptible," he explains. "An afflicted tree can maintain an outward appearance of perfect health for a considerable period, significantly complicating disease control efforts. It brings to mind the analogy of the ancient Trojan Horse, where European trees are caught off guard, defenseless, and ultimately succumb, much like the Trojan warriors."
According to Dr. Dvořák, just as in the human body, stress can serve as a catalyst for the transformation of a seemingly healthy tree into an ailing one. "Under the influence of changing climatic conditions, trees become less tolerant to pathogenic fungi, leading to their eventual demise," he explains. The tree's resistance to the fungus weakens, and as a result, it succumbs to the disease and begins to deteriorate.
An example of such a disease is Sooty Bark Disease (SBD), which commonly affects maples and is caused by a minuscule fungus known as Cryptostroma corticale. Dr. Dvořák notes, "The introduction of this fungus to Europe likely occurred during the Second World War, and throughout the remainder of the 20th century, it remained relatively obscure."
However, the situation has since evolved, and in the past two decades, reports of the fungus have become increasingly prevalent. Following periods of dryness and high temperatures, infected trees begin to succumb to the infection, accompanied by the formation of dark brown to black masses resembling "soot" beneath the peeling bark of the affected maples.
Therefore, the organism became a focal point for a consortium of plant disease experts assembled under a European HORIZON 2020 initiative named "Holistic approach for emerging woodland pests and diseases (HOMED)." Researchers from six nations (Czechia, France, Italy, Portugal, Sweden, and Switzerland), led by Dr. Dvořák, resolved to create an accurate, DNA-oriented (real-time PCR) diagnostic technique for identifying and tracking the pathogen in atmospheric samples. They released their methodology, the results of its implementation, and their novel discoveries concerning SBD epidemiology in the freely accessible journal NeoBiota.
How can one extract DNA from air samples? Volumetric air samplers, simple devices, can draw air through a sticky tape, effectively trapping every particle for subsequent analysis. Dr. Dvořák explains, "While these devices may not be inexpensive and require regular maintenance, they are widely used throughout Europe, as evident in the familiar weather forecast's 'pollen report' catering to allergic individuals. The European Aeroallergen Network (EAN) comprises over 600 stations, each equipped with a volumetric air sampler that consistently collects samples and maintains an archive."
The HOMED team got in contact with their national EAN collaborators and processed their samples with molecular techniques (real-time PCR).
The implementation of this highly sensitive detection method yielded remarkable results during the sample survey. The "sooty" fungus was detected in air samples collected from countries where the disease had previously been identified. Furthermore, a more comprehensive investigation conducted in France unveiled the presence of the pathogen in the air as far as 310km away from the presently affected trees. This finding strongly indicates that the fungus has the ability to disperse over long distances through wind transmission.
"In our study, we have observed an exponential growth phase of the SBD disease in France and Switzerland, coinciding with a significant water deficit, leading to a surge in the number of reported cases," state the researchers in their study. They express their optimism that the early detection of C. corticale through aerial means in countries currently unaffected by the disease could facilitate the implementation of more effective strategies for detecting and eradicating SBD in affected areas.
“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.
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