Ash dieback

Ash Dieback: A Comprehensive Guide to Understanding and Managing Hymenoscyphus fraxineus

1. Introduction:

Ash dieback, caused by the fungus Hymenoscyphus fraxineus (formerly Chalara fraxinea), is a devastating disease affecting ash trees ( Fraxinus species) across Europe and beyond. Its prevalence has surged since its initial detection in Poland in the 1990s, causing widespread mortality in ash populations and significant ecological and economic impacts. The disease primarily targets European ash (Fraxinus excelsior) but also affects other ash species, though with varying degrees of susceptibility. The impact on plant health ranges from leaf blemishes and dieback to complete tree mortality.

2. Causes and Development:

Ash dieback is caused by the ascomycete fungus H. fraxineus. The disease cycle begins with the fungus infecting leaves in the summer, producing spores that spread readily by wind, rain splash, and possibly through infected plant material. These spores germinate on the leaves and petioles, colonizing the tissues and eventually leading to leaf spotting and premature leaf abscission.

The fungus overwinters in the infected leaf petioles still attached to the twigs, forming characteristic fruiting bodies (apothecia) in the spring. These release abundant ascospores the following spring, initiating a new cycle of infection. Warm, humid conditions favor spore germination and fungal growth. Cool, wet springs are particularly conducive to widespread infections. The fungus invades the vascular system of the tree, disrupting water and nutrient transport, leading to dieback of branches and ultimately, the death of the tree.

3. Symptoms and Identification:

Symptoms of ash dieback vary depending on the severity of infection and the age of the tree. Early symptoms include:

• Leaflet lesions: Dark brown or black lesions appearing on leaflets, often starting at the leaf margins.
• Leaflet wilting and premature leaf drop: Affected leaflets wilt and prematurely drop from the tree.
• Dieback of shoots and branches: Progressive dieback of shoots and branches, starting from the top of the tree.
• Dark discolouration of bark: Dark streaking or blotches in the bark of affected branches.
• Crown thinning: A noticeable thinning and loss of foliage in the tree’s crown. In later stages, the crown may be almost entirely defoliated.
• Necrotic lesions on twigs: Dark brown and sunken necrotic lesions on the twigs.
• Black and stubby shoots: Short, stubby, darkened, and blackened shoots, failing to grow properly

(Include high-quality images here showing each of the listed symptoms. These images should be sourced from reputable sources such as forestry research institutes or universities.)

Differentiating ash dieback from other plant problems requires careful observation. Similar symptoms can be caused by other fungal pathogens or environmental stress, but the combination of leaf lesions, dieback, and characteristic fruiting bodies on the leaf petioles points towards H. fraxineus. Consult with a plant pathologist or arborist for confirmation if unsure.

4. Prevention:

Preventing ash dieback involves a multi-pronged approach:

• Source control: Avoid planting ash seedlings or trees from infected areas.
• Plant selection: Choose ash cultivars known to possess some level of resistance if available.
• Proper planting: Plant ash trees in appropriate locations with well-drained soil and adequate sunlight.
• Hygiene: Regularly remove and destroy infected leaves and branches.
• Sanitation: Clean tools after pruning infected trees to prevent transmission.

5. Organic Treatment Methods:

While no completely effective organic treatment eradicates H. fraxineus, certain methods can help mitigate the disease’s impact:

• Biofungicides: Research into biofungicides effective against H. fraxineus is ongoing. Some promising options may emerge in the future but currently, there’s no widely available, highly effective organic treatment.
• Promoting tree health: Healthy trees are better equipped to withstand infection. Focus on proper fertilization, watering, and soil management.
• Beneficial microorganisms: Exploring the use of beneficial fungi or bacteria that may compete with H. fraxineus or enhance tree resistance.

6. Chemical Treatment Methods:

Chemical control of ash dieback is challenging because the fungus invades the tree’s vascular system. Fungicides may offer some protection for young trees or in early stages of infection, but systemic control is difficult to achieve. Fungicides with potential activity against this diseases (efficacy varies) include systemic fungicides containing propiconazole, tebuconazole, or pyraclostrobin. Always follow label instructions carefully. Frequent applications may be required.

Crucially, the development of fungicide resistance is a major concern. Therefore, a rotational strategy with different classes of fungicides is crucial to delay resistance development. Chemical application should ideally be part of an integrated management strategy, not used in isolation.

Safety Precautions: When handling fungicides, wear appropriate protective clothing, including gloves, goggles, and a mask. Ensure proper disposal of empty containers according to local regulations. Be aware of potential environmental impacts (to non target organisms, soil health and water resources) and avoid drift onto non-target plants.

7. Integrated Pest Management (IPM) Strategies:

IPM for ash dieback combines various approaches:

1. Monitoring: Regularly inspect trees for symptoms. Early detection is crucial.
2. Prevention: Implement preventative measures as described above, focusing on cultural practices and plant health.
3. Targeted treatments: If infection occurs, consider the application of chemical fungicides only where deemed necessary and effective based on severity and risk assessment to avoid escalating resistance development. Prioritise trees of high value.
4. Resistance selection: Support research and development of resistant ash cultivars for long-term management.
5. Sanitation and hygiene: Remove and destroy infected plant material to interrupt the disease cycle.

8. Conclusion:

Ash dieback poses a significant threat to ash populations. Effective management requires a multi-faceted approach combining preventative measures, early detection, and strategic, responsible use of control methods tailored at reducing overall reliance on chemicals. Developing ash cultivars with increased resistance is critical for long-term sustainability. Collaboration between researchers, forest managers, and landowners is essential to mitigate the ongoing impact of this devastating disease. While complete eradication might prove challenging, conscientious management strategies can help minimize losses and conserve healthy ash populations for the future.