Ash dieback fungus

Ash dieback, caused by the fungus Hymenoscyphus albidus, is a devastating disease affecting ash tree populations worldwide. While historically considered a harmless saprophyte living on decaying ash petioles, it has evolved into a highly aggressive pathogen that threatens the future of European ash species (Fraxinus excelsior) and other related varieties.

The disease primarily impacts ash trees, causing systemic necrosis that affects the foliage, branches, and stems. Younger trees are particularly vulnerable, often succumbing to the infection within a few growing seasons, while older trees may persist for longer periods but eventually decline due to the repetitive nature of the infection and secondary pest attacks.

Symptoms typically manifest as wilting of leaves, lesions on the bark, and the death of terminal shoots. As the fungus spreads, it causes deep cankers and prevents the tree from transporting nutrients effectively. The presence of tiny, white, disc-shaped fruiting bodies on the rachises of fallen leaves in summer is a key diagnostic feature of the fungus.

The development of the disease is heavily influenced by environmental factors such as high humidity and rainfall. The fungus completes its life cycle on fallen ash leaves from the previous year, releasing airborne spores that infect healthy trees during the summer. Once the canopy becomes infected, the rapid production of spores can lead to an epidemic within a forest stand.

Effective management and prevention strategies are crucial to mitigating the spread:

• Removal of leaf litter in high-value areas to reduce the local spore load.
• Selecting and breeding for genetically resistant ash genotypes.
• Felling of heavily infected trees to prevent further spread in nursery settings.
• Promoting diverse forest plantings to prevent the monoculture-driven rapid spread of the pathogen.