Ash Dieback Identification Guide

Two Different Searches for One Fungus

Ash Dieback is unusual among the subjects of a field guide because you look for it in two completely different ways. One is a set of field signs on a living ash tree — shoot death, bark lesions, a thinning crown. The other is the fungus itself, a tiny stalked disc a few millimetres across, fruiting on last year's fallen ash leaf stalks in the litter. Neither on its own is conclusive. Read together, with the right host tree underfoot, they are.

Getting the Setting Right

Everything begins with the tree. This fungus is confined to ash (Fraxinus), so if the host is not an ash you can stop before you start. Confirm the tree first: branches and buds in opposite pairs, pinnately compound leaves with seven to thirteen toothed leaflets, matt black conical winter buds, and grey bark that is smooth on young stems and develops a regular diamond fissure pattern with age.

You will be standing in an ash stand, hedgerow, plantation, or street planting. In summer, the ground beneath those trees is the search area for the fruit bodies: a damp litter layer holding the previous season's leaf rachises — the central stalks left after the leaflets have rotted away.

Reading the Tree

Work from the crown down.

In the canopy, look for shoots whose leaves have wilted, turned dull olive then black, and stayed hanging rather than dropping. Whole leaflets or just their tips can be affected, and the blackening often runs down the rachis. Dead shoot tips scattered through an otherwise leafy crown are the classic pattern, and over successive years the crown thins from the top and outside inward.

On stems, look for lesions. The characteristic one is elongated and diamond-shaped, centred on a dead lateral shoot or a leaf scar, with the long axis running up and down the stem. Fresh lesions are dull orange-brown to dark brown and slightly sunken; older ones dry, crack and become cankerous. Pare the bark away at the edge of a lesion with a knife and the wood beneath is stained brown or grey-brown, with the discolouration extending well above and below the visible mark.

On the trunk, dense tufts of epicormic shoots often break out along the bole and main limbs as the upper crown declines. On its own that is a response to stress of any kind, but alongside diamond lesions and hanging black leaves it fits the picture.

Finding the Fruit Bodies

Kneel down under an affected tree in summer and sift the leaf litter. You are looking for ash rachises from the previous year, and specifically for rachises that have gone hard, blackened and grey-black in patches. That blackening is a pseudosclerotial plate — a crust the fungus lays down at the boundary of the tissue it occupies — and the fruit bodies arise directly from it, often several to a dozen in a row along one rachis.

Damp litter is essential. In a dry summer the rachises shrivel and the crop is sparse; a day or two after rain they swell and the discs reflate.

A Step-by-Step Read of the Fruit Body

The disc (apothecium). One and a half to five millimetres across, occasionally to seven. At first a shallow cup with an inrolled edge, then flattening into a disc, sometimes becoming wavy or saucer-shaped in age. The upper, fertile face is white to cream, sometimes with a faint buff or greenish-cream tint. The underside is smooth, slightly paler, and shows no hairs — check that with a lens, because hairs would take you elsewhere.

The stalk. One to three millimetres tall and very slender, cylindrical, white to cream for most of its length, and often narrowing and darkening at the very base where it emerges from the blackened rachis. Discs seated directly on the substrate without a stalk are not this fungus.

Texture and flesh. Too thin to section usefully in the field. When moist the whole thing is soft, waxy and slightly gelatinous; when dry it shrivels to a horny brown speck that reflates after rain. There is no colour change on handling, and no milk, latex or exuded liquid.

Substrate contact. Always the previous year's ash rachis, occasionally the base of a fallen petiole. Never the living stem, never soil, never wood.

Collecting a Spore Deposit

You cannot make a conventional gilled-mushroom print from a disc fungus, but you can collect spores. Discs of this kind shoot their spores upward from the fertile face, so the practical method is to sit a rachis bearing fresh moist apothecia in a small closed container with a glass microscope slide or a strip of dark card suspended a centimetre above the discs, and leave it in a cool place for a few hours. The deposit is white and, en masse on dark card, looks like a faint chalk dusting.

Under the microscope the spores are hyaline, smooth, cylindrical to club-shaped with one end narrower than the other, usually without a septum when fresh, and roughly 14–20 by 4–5 micrometres. Two ascus characters matter. The apical ring at the tip of the spore sac stains blue in iodine, which places the fungus in its family group. And the sacs arise from croziers — small hooked cells at the base — which is the classic microscopic separator from its native relative.

Season and Conditions

The fruit bodies appear from June to September, peaking in July and August, and require humid litter and moderate temperatures. Field signs on the tree, by contrast, build through the growing season: shoot wilting from midsummer, lesions visible year-round once formed, and the crown pattern clearest in full leaf.

Separating It from Look-alikes

Hymenoscyphus albidus. The one that matters. It fruits on the same host, on the same substrate, at the same time of year. It is generally smaller — discs usually under two millimetres — the rachis it grows from is far less strongly blackened and rarely carries a well-developed pseudosclerotial plate, and the stalk base is not darkened to the same degree. Where the two overlap in size, the reliable separator is microscopic: the spore sacs of H. albidus arise without croziers.

Other pale stalked discs in litter. Hymenoscyphus scutula and relatives sit on herbaceous stems rather than ash rachises. Lachnum species carry a fringe of fine white hairs around the rim, obvious with a lens. Mollisia species are sessile grey to blue-grey discs with no stalk at all. Rutstroemia species are brown to tan and often larger. Cudoniella prefers wet wood and plant debris in seeps.

Look-alike signs on the tree. Ash bacterial canker produces rough, cracked, sunken and often wet-looking bark rather than a crisp diamond lesion. Neonectria cankers develop concentric target-shaped ridges around a central wound. Honey fungus at the root collar reveals white sheets of mycelium beneath the bark and black bootlace strands in the soil. Wilt caused by Verticillium streaks the sapwood without producing a bark lesion. Drought scorch browns leaf margins evenly across the crown without killing shoot tips. Ash flower gall mite deforms the inflorescences into brown woody clusters that persist through winter.

Field Checklist

Ash host confirmed; blackened rachises in last year's litter; white to cream discs one to five millimetres across on slender pale stalks rising from the black crust; no rim hairs; white spore deposit; summer season; and, on the tree overhead, hanging black shoots and diamond-shaped stem lesions.