Ash Dieback

Ash Dieback is caused by Hymenoscyphus fraxineus, a small discomycete in the Helotiaceae — the same broad group as the many little white and yellow cup fungi found on twigs and litter. The mushroom-forming stage is easy to overlook: white to cream apothecia only 2-6 mm across, each on a slim stalk, growing in numbers from last year's blackened ash leaf stalks on the woodland floor.

On the living tree the fungus is known instead by its effects: black-brown lesions on leaves and shoots, diamond-shaped sunken patches of discoloured bark where a leaf stalk joined the twig, and progressive thinning of the crown as branch tips die back and epicormic shoots break out along the trunk.

Originally described from East Asia, where it lives inconspicuously on Manchurian ash leaves, the fungus reached Europe in the late twentieth century and spread rapidly westward, and it is now one of the most closely watched tree-associated fungi in the region.

Fruiting bodies

Apothecia are shallow, saucer- to goblet-shaped white or cream cups 2-6 mm across, narrowing into a slender stalk 3-15 mm long. They are thin-fleshed, slightly translucent when fresh, and appear in dense scattered troops on darkened ash rachises half-buried in the litter, often with a small basal swelling on the stalk.

Substrate signs

The host leaf stalks are blackened and often bear a fine dark line (a pseudosclerotial zone) running along them — a useful marker even when no cups are present.

Signs on the tree

Leaflets develop brown-black blotches and wilt while still attached; the infection travels into the twig, producing an elongated diamond-shaped lesion centred on the leaf scar. Beneath the lesion the wood is stained grey-brown. Crowns thin from the top down and dense clusters of side shoots appear on the trunk and main limbs.

Flesh and spore print

Flesh is thin, white and brittle; spore deposit is white.

The fungus fruits on the previous year's ash leaf rachises lying in leaf litter, usually in damp shaded woodland with a closed canopy, and colonises the leaves, shoots and inner bark of living Fraxinus trees. European ash and narrow-leaved ash are heavily colonised; Manchurian ash, its original host, shows only minor leaf spotting.

It occurs across most of Europe, from Scandinavia to the Balkans and westward to Britain and Ireland, and in East Asia. The white cups appear mainly from June to September, releasing wind-dispersed ascospores that infect ash leaves during the summer, while shoot lesions become clearest from late summer into autumn.

Hymenoscyphus fraxineus has a two-part life cycle that combines saprotrophic and parasitic phases. Ascospores released from litter apothecia land on ash leaflets in summer, germinate and grow into the leaf, then advance down the petiole into the twig, where the mycelium kills patches of bark and cambium and stains the underlying wood.

Infected leaves fall in autumn, and the fungus continues in the dead rachis as a saprotroph, forming a black protective zone that reserves the substrate for itself over winter. The following summer it fruits from that rachis, completing the cycle. Repeated annual infection of new shoots produces the cumulative crown thinning that gives the disease its name, and the fungus does not spread from tree to tree through roots or bark contact.

Hymenoscyphus albidus, the native European ash cup, fruits on the same substrate and is nearly identical to the eye; it lacks the swollen stalk base and blackened pseudosclerotial line typical of H. fraxineus and is separated reliably only by microscopy. Other small white Helotiales such as Hymenoscyphus scutula grow on herbaceous stems rather than ash rachises.

On the tree, these signs can be confused with ash bud moth damage, which leaves chewed tissue and frass, or with drought scorch, which browns leaf margins uniformly and produces no bark lesion. Bark damage from Armillaria is distinguished by white mycelial sheets and black bootlace rhizomorphs at the root collar rather than diamond-shaped shoot lesions.