Ash dieback in Cornwall: what it looks like and what to do

Ash dieback is caused by a fungus, Hymenoscyphus fraxineus, which arrived in Britain from the continent and was first confirmed in 2012. Its spores blow in on the wind in summer, land on ash leaves, and grow down the leaf stalk into the twig, then the branch, then the stem. The tree loses leaves and twigs from the tips inward, and over several years the crown dies back until the tree is a skeleton with tufts of leaves on new shoots from the trunk. Once the stem is affected other fungi, particularly honey fungus, move into the weakened tree and rot the base.

Not every ash dies. A small proportion show real tolerance, and in a wood or a hedge those are the trees worth keeping and letting seed. Young trees and trees in damp, sheltered, crowded spots go fastest. Big, open grown ash with plenty of air around them often decline more slowly, and some old trees are still standing fifteen years on.

Look in July and August, when a healthy ash should be in full leaf.

• Dead, leafless twigs at the tips of the branches, with the leaves that remain clustered further in.
• Blackened, wilted leaves and leaf stalks in summer, hanging on rather than dropping cleanly.
• Diamond or lens shaped dark lesions on the bark of young stems and branches, often around a dead side shoot.
• A crown that is thin at the top and full of dead wood, with bushy growth sprouting from the trunk and main limbs as the tree tries to compensate.
• In later stages, brackets or honey fungus at the base.

Most dead trees stay reasonably sound for a few years. Ash killed by dieback does not. The wood becomes brittle unusually quickly, the branches snap rather than bend, and where honey fungus has followed, the base can be soft while the crown still looks as though it has a year or two left. That makes a dead or dying ash risky to climb, and it is why we deal with them from a crane or a platform wherever we can rather than putting a climber into the tree. It also means a dead ash over a road, a lane, a footpath or a garden should not be left to fall down on its own schedule.

An infected ash in a wood or a wide field margin, away from anything it can land on, can be left. It is habitat, it may turn out to be one of the tolerant ones, and the Forestry Commission's advice is not to fell healthy or lightly affected ash simply because dieback is about.

An ash with more than roughly half its crown dead, or with any decay at the base, that stands over a road, a path, a building, a car park or a neighbour's garden should come down, and sooner rather than later while it can still be worked on safely. Between the two, a lightly affected tree over a target can be watched year by year, and dead wooded in the meantime.

Landowners with ash along roads and public paths have a duty of care and should have their ash looked at regularly. Cornwall Council and the National Trust have been removing roadside ash across the county for several years for this reason. Felling ash in any quantity in woodland can need a felling licence, and diseased trees are not automatically exempt, so we check before quoting on a hedge or a wood.

Where an ash comes out of a hedge or a garden, something should go back. Ash cannot be replanted with any confidence, so we suggest the trees that do the same job in the same ground: sessile oak, field maple, wild cherry, hornbeam, small leaved lime, alder on wet ground, and rowan, hawthorn and hazel in a hedge. The free tree health check covers ash along with everything else, and it is the easiest way to find out where the trees on your land stand before the next winter.