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The latest research results from scientists at the Institute of Forest Ecology of the Slovak Academy of Sciences bring a worrying update on the state of ash trees: the invasive fungal pathogen Hymenoscyphus fraxineus no longer threatens only native species, but has also expanded its host range to include non-native ash trees. This shift significantly increases the risk of further disease spread and ecological damage.

The massive dieback of ash trees has been observed in Europe for more than a decade, with Fraxinus excelsior being the most affected species. However, new research from Slovakia shows that the situation is worsening. Research conducted in four arboretums – Borová Hora Arboretum, Kysihýbel Arboretum, Liptovský Hrádok Arboretum and Mlyňany Arboretum – confirmed the presence of the pathogen in 23 out of 34 taxa of the genus Fraxinus examined. Almost 25% of all assessed trees showed symptoms of the disease, with ash trees of all ages affected.

"Our results clearly show that the pathogen is expanding its host range. It is no longer just a problem of native species, but a broader ecological threat," said Miriam Kádasi Horáková, an independent researcher at the Institute of Forest Ecology of the Slovak Academy of Sciences.

For the first time in Slovakia, the research confirmed the occurrence of the fungus on several non-native ash tree species, including North American species (Fraxinus cinerea, Fraxinus latifolia, Fraxinus pennsylvanica, Fraxinus quadrangulata) and Asian species (Fraxinus bungeana, Fraxinus chinensis, Fraxinus mandshurica). Of particular importance is the first recorded occurrence of the species Fraxinus bungeana, which also represents the first global finding, thereby expanding the pathogen's host range.

"The detection of infection in new species, especially of Asian origin, indicates that the pathogen has a greater adaptive potential than we expected," added Miriam Kádasi Horáková.

The most severe damage was recorded in the native European ash (Fraxinus excelsior) and its ornamental cultivars. However, symptoms also appeared in the less susceptible species, Fraxinus ornus, confirming the gradual weakening of the natural resistance of various species. Ash dieback is one of the most serious forestry and ecological problems in Europe. The disease causes the gradual death of crown tops, premature browning, and leaf fall, leading to the gradual weakening of trees and their subsequent death.

In some countries, most of the ash population has already decreased. New findings from Slovakia confirm a trend that may have major impacts on biodiversity, landscape character and forestry.

The research results point to the need for:

• intensive monitoring of ash trees,
• research into resistant genotypes,
• taking measures to limit the spread of the pathogen.

"If we want to preserve ash trees in our country, we must act quickly and in a coordinated manner. The situation is evolving and requires new approaches," concludes Miriam Kádasi Horáková.

Edited by: Jozef Bednár

Photo: canva.com