Understanding honey fungus and how it spreads

Honey fungus (Armillaria species) is a group of fungi that live in soil and on dead or dying wood. In British gardens and small woodlands, it is one of the most common causes of premature death in ornamental trees. The fungus spreads underground by two main methods: by growing along the roots of susceptible trees, and by producing black, root-like strands called rhizomorphs, often nicknamed 'bootlaces'. These bootlaces can travel through soil for several metres, seeking out new roots. Spores from mushrooms can also spread the fungus, but this is less important in gardens. Once it reaches a tree, it colonises the root system, decaying the wood and cutting off the flow of water and nutrients. The tree may decline slowly over several years, or collapse suddenly in summer. Because the fungus persists in infected roots and stumps for many years, ignoring it rarely works.

Recognising the symptoms in your ornamental trees

Symptoms vary with tree species and the vigour of the tree. Early signs include thinning of the canopy, smaller leaves than normal, and premature autumn colour. You may see dieback in the upper branches, and the tree may produce an unusually heavy crop of flowers or fruit as a stress response. In summer, a tree may wilt and die within weeks, especially in dry weather. At the base of the trunk, look for:

• Cracks in the bark or a swelling at ground level
• Bleeding of dark fluid from the trunk
• Bark that lifts away easily
• A white, fan-shaped layer of fungal tissue (mycelium) under the bark, smelling strongly of mushrooms
• Black bootlaces under the bark or in the soil around the roots
• Clusters of honey-coloured mushrooms on or near dead stumps in autumn

Confirming the diagnosis before you act

Not every white mycelium is honey fungus. Other fungi, such as those causing root rot, can look similar. To be sure, look for the combination of symptoms: bootlaces, white mycelial fans, and the characteristic mushroom smell. A hand lens helps. If you are unsure, a professional tree surgeon or mycologist can identify the species. Correct diagnosis matters because honey fungus is not the only problem that kills trees. Waterlogging, drought, and other root diseases can produce similar decline. If you act on a wrong diagnosis, you may remove a healthy tree unnecessarily. If you do find honey fungus, note that it often attacks already stressed trees. So improving growing conditions can help other trees resist. But once a tree is heavily infected, it cannot be cured.

Cultural and preventative measures that make a difference

There is no chemical cure for honey fungus in the soil. The most effective control is to remove infected material. If a tree has died, do not just cut it to ground level. The stump and as much of the root system as possible should be dug out or ground out. For large roots, a stump grinder can remove the main mass, but lateral roots may remain. In a small garden, hand digging with a fork and spade is feasible for smaller trees. Where removal is impractical, you can create a physical barrier. Bury a vertical sheet of butyl rubber or heavy-duty plastic to a depth of at least 60 centimetres, leaving a small gap above soil level to stop rhizomorphs crossing. This is not a perfect barrier, but it slows spread. Also, avoid moving infected soil or roots to other parts of the garden. Clean tools after use.

Managing soil and replanting after infection

After removing infected roots, leave the area fallow for at least a year, keeping it free of susceptible plants. You can sow a green manure like mustard or clover, which may help reduce fungal populations, though evidence is mixed. Before replanting, improve soil structure with well-rotted manure or compost, and ensure good drainage. Choose species that show greater resistance. Yew, holly, hawthorn, and box are often recommended, but no tree is completely immune. Highly susceptible species include lilac, privet, willow, birch, cherry, apple, and forsythia. If you want to grow these, plant them in raised beds with clean soil and a barrier, or in large containers. Keep new trees well-watered during dry spells and mulch annually to reduce stress. Healthy, vigorous trees are better able to withstand attack.

Long-term woodland management and living with honey fungus

In small woodlands, honey fungus is a natural part of the ecosystem. It breaks down dead wood and recycles nutrients. Complete eradication is neither possible nor desirable. Instead, aim to manage it. Thin overcrowded stands to reduce competition and stress. Remove dead or dying trees promptly, but leave some dead wood for wildlife if it is not infected. Monitor remaining trees each year, looking for early symptoms. If you lose a tree, consider replanting with a diverse mix of resistant species rather than a single species. Avoid planting large groups of highly susceptible trees. Keep records of where infection has occurred so you can avoid replanting the same spot with vulnerable species. With careful management, you can limit losses and maintain a healthy, varied garden or woodland.