Common Tree Diseases in the UK: What to Look For and What to Do
The health of UK trees is under more pressure than at any point in living memory. A combination of changing climate conditions, increased global movement of plant material, and the introduction of new pathogens has created a challenging environment for our native tree populations. Whether you manage a woodland, an estate, or a single tree in your back garden, understanding the most significant disease threats, and knowing what to look for, is increasingly important.
Ash dieback (Chalara fraxinea / Hymenoscyphus fraxineus)
Ash dieback is without question the most significant tree disease threat in the UK today. Caused by a fungal pathogen introduced from Asia, it has already killed or severely damaged tens of millions of ash trees across Europe and is progressing through UK ash populations at a significant rate.
Symptoms include wilting and blackening of leaves (often starting at the margins), dying back of shoots and branches (typically starting from the tips and progressing inward), dark lesions on the bark of young shoots, and a general decline in the health and vigour of the crown. In later stages, the tree may attempt to produce new growth from lower on the trunk, a response known as epicormic growth, as the upper crown dies back.
There is currently no effective treatment for ash dieback. Management focuses on identifying affected trees early, monitoring progression, and planning for safe felling before trees become structurally dangerous. Some individual ash trees show apparent tolerance to the disease, and research into resistant breeding lines continues. If you have ash trees on your property, regular monitoring is essential.
Honey fungus (Armillaria species)
Honey fungus is widely considered the most destructive pathogen in UK gardens. It exists as several related species in the Armillaria genus, all of which cause root rot in a very wide range of woody plants, trees, shrubs, and woody climbers are all susceptible.
The fungus spreads underground via black, bootlace-like structures called rhizomorphs, which can travel significant distances from an infected plant to attack new hosts. The first visible signs are often the appearance of honey-coloured toadstools at the base of affected plants in autumn, alongside white fungal mycelium beneath the bark at the crown and root collar of infected individuals.
Above ground, affected trees and shrubs typically show a general decline in vigour, smaller leaves, reduced growth, yellowing, premature leaf drop, before eventually dying. There is no chemical treatment available in the UK. The most effective management approach is to remove infected material, including stumps and as much of the root system as possible, and to avoid replanting susceptible species in the same location.
Phytophthora root rot and collar rot
Phytophthora is a genus of water moulds, technically not fungi, despite behaving similarly, that affect the roots and base of a wide range of trees and woody plants. Several species are present in the UK, the most significant for trees being Phytophthora cinnamomi, P. ramorum (responsible for sudden oak death and widespread larch death in the west of the UK), and P. kernoviae.
Symptoms depend on the species and the host, but typically include discolouration and bleeding lesions at the bark of the trunk at or near ground level, root death leading to poor anchorage, yellowing and thinning of the crown, and progressive general decline. Waterlogged conditions favour Phytophthora infection, as the mobile spores spread most effectively through water.
Management options are limited. Improving drainage can help to slow progression. Removal of severely affected material and careful disposal are important to prevent spread. Some preventative treatments exist but are expensive and not always effective.
Oak processionary moth (OPM)
Oak processionary moth is an invasive pest rather than a disease, but its impact on oak trees is significant enough to merit inclusion here. Originating in southern Europe, it has become established in parts of London and the South East and is spreading northward.
The caterpillars feed on oak leaves, causing defoliation that weakens trees and, in severe cases, can contribute to their death. The caterpillars are also a significant human health hazard, their hairs contain a protein that causes skin irritation, eye problems, and respiratory issues in humans and animals. Nests found in oak trees should never be approached or disturbed without professional guidance.
If you find OPM nests in your area, the Forestry Commission should be notified, as it is a notifiable pest. Management typically involves specialist treatment by contractors licensed to handle OPM material.
When should you call a professional?
If you notice any of the symptoms described above, or simply a general unexplained decline in the health and vigour of a tree, it's worth getting a professional arborist to take a look. Early identification of disease gives the best chance of management, slows progression to neighbours, and allows for planned, safe intervention rather than emergency response.
At Gem Tree Management, we can assess trees showing signs of ill health, advise on likely causes, and recommend appropriate management, whether that's monitoring, specific works, or in more advanced cases, planned removal before the tree becomes a safety concern.
Ready to book? Contact Gem Tree Management Ltd today for a free, no-obligation site visit and quote. Our team of fully qualified, fully insured arborists is here to help with everything from routine maintenance to complex tree surgery.