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Hymenoscyphus fraxineus is an ascomycete fungus that causes ash dieback, a chronic fungal disease of ash trees in Europe characterised by leaf loss and crown dieback in infected trees. The fungus was first scientifically described in 2006 under the name Chalara fraxinea .
It is common for the leaves and bark to show signs of disease and brown rot on otherwise healthy plants. After leaves have fallen off the plant in fall and before it begins to leaf out, Oregon ash can be identified by its stout twigs and opposite branching arrangement and opposing buds. Unlike bigleaf maples, ash twigs have woolly hairs.
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Hymenoscyphus fraxineus is a fungal disease that attacks ash trees. The disease causes leaf loss, crown dieback, and lesions in the bark. This fungus is, for the most part, fatal, both directly and indirectly, by weakening the tree's immune system so that it is more susceptible to attacks from pests or pathogens.
The true ash trees are much taller and are typically grown as street trees or shade trees. The smaller mountain ash is a member of the rose family while the true ash belongs to the olive family.
Fraxinus anomala is a deciduous shrub or small tree approaching maximum heights of five to six meters. The leaf may be simple or it may be compound, composed of up to five leaflets which look like individual leaves. Each leaflet is oval-shaped to round and may have teeth along the edges. The nondescript brownish flowers lack petals.
The disease is caused by a fungus, Cryptostroma corticale. Normally, the fungus grows in dead wood as part of the normal decaying process. But the fungus has been increasingly found in living trees.
The leaves are 25–40 cm long, pinnate compound, with 7–13 leaflets, the leaflets 5–20 cm long and 2–5 cm broad, subsessile on the leaf rachis, and with a serrated margin. They turn to a golden-yellow in early autumn, and the tree is usually early to change color.