Hymenoscyphus fraxineus

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Fraxinus excelsior on Gotland (Sweden) infected with Hymenoscyphus fraxineus - on the left a presumably disease-tolerant tree among many dead and/or dying trees (click on image to enlarge it)
Author(s): Audrius Menkis et al.
Source: Forest Pathology (2020), 50 (1), art. e12572
Hymenoscyphus fraxineus symptoms on ash branch (click on image to enlarge it)
Author(s): Andrej Kunca, National Forest Centre - Slovakia
Source: IPM Images

Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz & Hosoya 2014

This fungus causes wilting and dying of ash trees, Fraxinus spp. (ash dieback) in Europe. The disease affects mainly European ash (Fraxinus excelsior). It was first noticed in the mid 1990's in Poland and subsequently spread to most parts of Europe. Between 2000 and 2010 the disease incidence increased dramatically in Europe. There are indications that it is also found in Japan, where it was named Lambertella albida.

The infected trees show wilting of the leaves as well as necrosis and cankers on the shoots, branches and stems, resulting in dieback. The fungus grows slowly on agar and also on the trees. Infections start on the leaves where symptoms appear after an incubation period of about 2 weeks. While many infected leaves fall off, the fungus can also reach the stem (and even the roots) through the petioles. White fruiting bodies, around 3 mm large, are formed in the summer. The ascospores are aseptate and about 15 x 4 µm large. The conidia are elongated, single-celled and cylindrical, 2.5-6 x 2-2.5 µm. Identification should be confirmed with DNA analysis. Dispersal takes place through the air via the ascospores.

Taxonomic note: This species has been known under the names Chalara fraxinea (anamorph) and Hymenoscyphus albidus (teleomorph). The teleomorph name was changed to Hymenoscyphus pseudoalbidus in 2010 after the discovery that Hymenoscyphus albidus is not pathogenic to ash and may be only a non-pathogenic endophyte or a saprophyte. H. albidus has been present in Europe long before the 1990's. With the introduction of the new taxonomic system (one-fungus-one-name) it became clear that the genus Hymenoscyphus is the correct genus for this species but that Chalara fraxinea is the older name, resulting in the new combination Hymenoscyphus fraxineus.

Vernacular names
• Deutsch: Eschentriebsterben
• English: ash dieback
• Français: dépérissement du frêne
maladie du flétrissement du frêne

Chalara fraxinea
Hymenoscyphus pseudoalbidus