Ash Dieback

Ash dieback, caused by the Hymenoscyphus fraxineus fungus, poses a significant threat to ash trees across Europe. This devastating disease has had a profound impact on tree management and control practices. In this comprehensive guide, we will explore the management strategies, legal requirements, and research efforts related to ash dieback. We will also delve into the biology and life cycle of the fungus, the origins of the disease, and its distribution across various countries.

When it comes to managing infected ash trees, there are no specific legal requirements in place unless a Statutory Plant Health Notice (SPHN) is issued by the country’s forestry or plant health authority. However, it is generally advised against felling living ash trees, infected or not, unless there are concerns for public safety or timber production. This is because some ash trees have shown tolerance to H. fraxineus infection and can recover to good health. By preserving as many ash trees as possible, we can identify those that exhibit resistance and use them for breeding tolerant ash trees in the future.

While preserving the trees is important, public safety should always be prioritized. Regular monitoring of the trees’ safety is crucial, especially as the disease progresses. If any branches or the entire tree pose a threat of injury or damage, pruning or felling may be necessary. Basal lesions, which weaken the trunk, should be closely monitored as they increase the risk of tree failure.

Although there is no known cure for ash dieback, some fungicides may help suppress the disease and save valuable ash trees, particularly those with high amenity, heritage, or cultural value. However, these treatments often require periodic re-application and can be costly.

To assist woodland managers in minimizing the impact of ash dieback, a Chalara manual has been developed. This manual provides detailed advice and guidance on maintaining woodlands and minimizing damage to ash timber crops.

In October 2012, upon the discovery of infected ash trees in the wider environment, a Great Britain-wide survey was conducted to assess the distribution of the disease. The survey focused on sites that had received saplings from nurseries where the disease was found. This initiative aimed to establish an understanding of the disease’s extent and impact.

In 2012, national measures were introduced in the UK to combat ash dieback, requiring that all imports of ash trees (Fraxinus species) originate from pest-free areas for ash dieback. These measures effectively prohibited the import and movement of ash plants.

Plant Health Regulations implemented on December 14, 2019, marked a shift in the legal framework. The current national measures against ash dieback have not been in effect since January 13, 2020. The UK continues to adhere to World Trade Organization (WTO) obligations and operates under the EU Plant Health Regulation until the end of the Exit from the EU Transition Period on December 31, 2020.

While the UK no longer maintains national measures against ash dieback due to its wide distribution, the focus now shifts to minimizing the risk of introducing new strains of the disease through imports. Importation of ash plants from third (non-EU) countries is prohibited to reduce the risk of new strains of H. fraxineus and other pests, such as the emerald ash borer, entering the country.

The Hymenoscyphus fraxineus fungus has a two-phase life cycle: sexual and asexual. The asexual stage primarily affects the leaves, bark, twigs, and branches of infected trees. In contrast, the sexual reproductive stage manifests as tiny, white, mushroom-like fruiting bodies on the stalks of fallen leaves from the previous year. These fruiting bodies release infective spores during the summer, which can be spread by the wind to healthy trees, triggering the asexual phase and subsequent infection.

Government scientists have conducted extensive research on the disease, leading to several key findings:

• Spores are unlikely to survive for more than a few days.
• Wind dispersal of spores from mainland Europe is possible.
• Trees require a high dose of spores to become infected.
• Spores are produced from infected dead leaves between June and September.
• The probability of dispersal on clothing, animals, and birds is low.
• The disease can affect any species of ash.
• Symptoms become noticeable within months rather than years.
• Properly treated wood products do not contribute to the spread of the disease.
• Once infected, trees cannot be cured.
• Some trees may possess genetic factors that make them tolerant or resistant to the disease.

Plant Health Regulations implemented on December 14, 2019, marked a shift in the legal framework. The current national measures against ash dieback have not been in effect since January 13, 2020. The UK continues to adhere to World Trade Organization (WTO) obligations and operates under the EU Plant Health Regulation until the end of the Exit from the EU Transition Period on December 31, 2020.

While the UK no longer maintains national measures against ash dieback due to its wide distribution, the focus now shifts to minimizing the risk of introducing new strains of the disease through imports. Importation of ash plants from third (non-EU) countries is prohibited to reduce the risk of new strains of H. fraxineus and other pests, such as the emerald ash borer, entering the country.

The Hymenoscyphus fraxineus fungus is believed to have originated in eastern Asia, where it co-evolved with Asian ash species and caused minimal damage. The disease’s presence in European ash species, particularly F. excelsior, was first reported in Poland in 1992. Since then, ash dieback has been discovered throughout Europe.

Initially, the fungus responsible for the disease was named Chalara fraxinea in 2006. Subsequent research suggested that Chalara fraxinea was the asexual stage of the fungus Hymenoscyphus albidus, a non-pathogenic species known in Europe since 1851. However, further investigation in 2010 led to the recognition of the sexual stage as a distinct species, named Hymenoscyphus pseudoalbidus due to its close resemblance to H. albidus. In 2014, the International Botanical Congress standardized the name for both stages as Hymenoscyphus fraxineus.

Ash dieback was first confirmed in the UK in February 2012 when infected ash plants were imported from a nursery in the Netherlands to a nursery in Buckinghamshire. Subsequent outbreaks were found in established woodlands, indicating that the disease had spread beyond the imported plants. While the exact entry point of ash dieback into Europe remains unknown, it is suspected that infected ash plants imported from Asia played a role. However, the discovery of infected trees unrelated to recent nursery plantings suggests other potential natural entry pathways, such as wind dispersal or transportation on clothing, animals, or vehicles.

According to the European Plant Protection Organization (EPPO), ash dieback is present in numerous European countries, including Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, the Netherlands, Norway, Poland, Slovenia, Sweden, and Switzerland.

Ash dieback has had a significant impact on tree management and control practices across Europe. Preserving ash trees is crucial for identifying tolerant individuals that can be used for breeding resistant trees. While there is no cure for ash dieback, fungicides may help suppress the disease in valuable ash trees. Legal measures and regulations aim to minimize the risk of introducing new strains of the disease through imports.

Understanding the biology and life cycle of the Hymenoscyphus fraxineus fungus is essential for effective management strategies. Ongoing research efforts are focused on identifying genetic factors that contribute to tolerance or resistance to the disease. By combining scientific knowledge and practical measures, we can work towards mitigating the impact of ash dieback and ensuring the long-term health of our ash tree populations.

*This article has been created with the support of Big Heart Tree Care, a leading provider of tree services committed to preserving the health and beauty of trees.