Hymenoscyphus pseudoalbidus (chalara ash dieback)

Hymenoscyphus pseudoalbidus was originally discovered in Poland in 2006, although there are suggestions that ash dieback witnessed in northern Europe in the 1990s was caused by this fungus. It is now known from most countries in northern and central Europe.

‘Chalara ash dieback’ was first found in the UK in February 2012 and is currently spreading from infection sites in East Anglia and Kent through the movement of ash saplings in the forestry trade. There are fears that H. pseudoalbidus will cause widespread loss of ash trees in the UK and result in landscape-level changes in some parts of the country.

Disc-shaped fruiting bodies of Hymenoscyphus pseudoalbidus (Image: V. Queloz / ETH Zurich, Switzerland)

Fruiting bodies: White to cream, disc-shaped fruiting bodies 1.5–3.0 mm in diameter on short stalks 0.5–2.0 mm tall and 0.2–0.5 mm in diameter. Fruiting bodies become brownish with age and when dry. The basal part of the stalk may be blackened; fruiting bodies arise from blackened areas of fallen petioles (leaf stalks) or dead shoots.

Sexual spores (ascospores): Formed within specialized cells (asci). Asci measure 80–110 µm in length and 6–12 µm in diameter and are cylindrical to narrowly club-shaped with a distinct stalk. Each produces eight colourless, non-septate ascospores. These measure 13.0–17.0 (–21.0) × 3.5–5.0 µm in size and are ellipsoidal to fusiform in shape, broadly rounded above and narrowed below.

Asexual spores (conidia): Produced when the fungus is grown in pure culture from specialized fertile hyphae, presumably also in the natural environment. Their function may be related to genetic exchange.

The threat that Hymenoscyphus pseudoalbidus poses to valued timber and landscape trees means that its conservation in Europe will be of low public priority.

There are concerns that death of ash trees on which it grows will also affect other groups of organisms. For example, 220 of the 582 species of lichens and related fungi known to grow on ash bark in Great Britain and Ireland are nationally rare or scarce, and 84 of these have a conservation status of Critically Endangered, Endangered, Vulnerable or Near-threatened.

Fraxinus excelsior (European ash) is under threat from 'chalara ash dieback'

Control of Hymenoscyphus pseudoalbidus is difficult, although removal and incineration of infected tissues may be beneficial during early stages of disease outbreak.

There is evidence that genetic diversity in its host trees, Fraxinus species, is relatively large and that a proportion of trees are resistant. In the long-term, therefore, selection for resistance may be effective. However, Hymenoscyphus pseudoalbidus seems to spread primarily via sexual spores, mixing the gene pool and potentially circumventing host resistance.

Little is known about the native European species, H. albidus, and it is possible that other cryptic species might be discovered that are morphologically similar to this species and H. pseudoalbidus. The potential for hybridisation within the species complex cannot be discounted.

Hymenoscyphus pseudoalbidus has not yet been recorded at Kew or Wakehurst Place.

More about fungi

Kew’s Mycology team

Kew’s profile on European ash (Fraxinus excelsior)

CABI Invasive Species Compendium

European and Mediterranean Plant Protection Organisation (EPPO) (as Chalara fraxinea)

Forestry Commission – chalara dieback of ash

ETH Life – article on outbreaks in Switzerland

British Lichen Society – lichens on ash trees