There is something deeply reassuring about a British woodland in any season, from the first haze of green in spring to the bare, sculptural branches of winter. Yet our native trees face real pressures, and learning to recognise the signs of disease is one of the most useful skills a woodland lover can develop. Whether you own a few acres, volunteer with a conservation group, or simply walk the same paths each week, you can help spot problems early and report them to the right people.

Reading the seasons: when to look and what to note

Timing shapes what you see. In spring, watch for leaf flush that fails to appear evenly, or leaves that emerge small and pale. Summer is the best time for crown symptoms: dieback at the top of a tree, thinning foliage, or early leaf drop stand out against a full canopy. Autumn brings unusual colour changes, such as leaves turning brown or black weeks before they should. In winter, when branches are bare, you can spot lesions, cankers, and dead buds that would otherwise be hidden.

Keep a simple record: tree species, location, date, and photographs. Take both a wide shot showing the whole tree and close-ups of bark, leaves, and any bleeding or lesions. Biosecurity matters too. Clean boots, tools, and bike tyres before and after visiting woods, and avoid moving plant material or soil between sites.

Ash dieback: the most pressing concern

Ash dieback, caused by the fungus Hymenoscyphus fraxineus, has spread across much of the UK. It affects native ash and can be devastating in woodlands. Look for these signs:

• Dark, diamond-shaped lesions on the bark, often where branches meet the trunk or on the trunk itself.
• Leaves that wilt, blacken, and drop in mid-summer, well before autumn.
• Crown dieback that starts at the top and edges of the canopy and moves inward.
• Epicormic growth: bushy shoots sprouting from the trunk and lower branches as the tree tries to compensate.

In winter, the lesions remain visible, and dead buds are clear on affected twigs. Young ash often succumb within a few years, while mature trees may decline slowly over a decade or more. Some ash show tolerance, so not every tree with symptoms will die. If you suspect ash dieback, report it so that its spread can be tracked.

Oak decline: acute and chronic

Oaks are the backbone of many British woodlands, so decline in these trees is always notable. Acute oak decline (AOD) is a rapid condition, often linked to a combination of bacteria and the two-spotted oak buprestid beetle. Look for dark fluid bleeding from cracks in the bark, usually on the trunk, and crown thinning over three to five years. The bleeding may dry to a black, crusty patch.

Chronic oak decline (COD) is slower, unfolding over decades. It often follows drought, soil compaction, or repeated defoliation. Symptoms include sparse foliage, small or yellowing leaves, and dieback in the upper crown. In summer, check mature oaks for stem bleeding. If you see it, report suspected AOD promptly, as early detection helps protect nearby trees.

Other diseases to keep on your radar

Several other problems are worth knowing. Horse chestnut bleeding canker causes dark, weeping patches on the trunk, dying branches, and sometimes leaf blotch. Phytophthora ramorum affects Japanese larch and rhododendron, producing dieback, cankers, and resinous bleeding; it is notifiable in many areas. Sweet chestnut blight shows as orange-brown cankers that crack the bark and lead to crown dieback. Dutch elm disease causes wilting, yellowing, and brown streaking under the bark of elms, spread by bark beetles.

• Horse chestnut: bleeding cankers, early leaf drop, dead branches.
• Larch and rhododendron: dieback, resinous oozing, rapid decline.
• Sweet chestnut: cracked bark with orange-brown fungal growth.
• Elm: sudden wilting and brown streaks beneath the bark.

Disease or distress? Distinguishing the signs

Many symptoms overlap with environmental stress. Drought can cause early leaf drop and crown thinning. Waterlogging, frost, herbicide drift, squirrel damage, and deer browsing all leave their mark. So does simple old age. Look for patterns: is the whole tree affected or just one side? Are there fungal fruiting bodies, insect exit holes, or bleeding? A single dead branch is rarely a crisis.

Note the context: soil type, recent weather, nearby land use, and whether other trees of the same species show similar signs. When in doubt, report and ask. A photograph and a grid reference cost nothing and can be invaluable.

Reporting suspected cases: who to tell and how

In the UK, report suspected tree pests and diseases to the forestry authority for your nation. Tree Alert is the online reporting tool used across the country, and it is the quickest way to get a record to the right experts. You will be asked for the location, preferably a grid reference, the tree species, a description of the symptoms, and photographs. Include the date and your contact details.

Do not move samples or cut branches unless you are instructed to do so, as this can spread disease further. Practise good biosecurity: clean boots and tools, and avoid taking soil or leaves home. Early reporting helps map the spread of ash dieback, oak decline, and other threats, and it gives woodland managers the best chance to act. Your observations, however small, contribute to a national picture that protects our trees for the future.