Hymenoscyphus albidus is the teleomorph of Chalara fraxinea
In 2008, studies were carried out in affected ash (Fraxinus spp.) stands in Poland (near Kraków) to try to identify the teleomorph of Chalara fraxinea (EPPO Alert List), the causal agent of ash dieback. In Poland, a discomycete was observed in a nursery and in a forest affected by ash dieback. The apothecia of this fungus occurred preferentially on ash petioles from the previous year in the litter, but occasionally also on the shoots of 1-3 year-old dead ash seedlings. Apothecia were flat discs of 1.5-3 mm diameter, first whitish or creamy, and then turning into cinnamon brown. This fungus was identified as Hymenoscyphus albidus and was shown to be the teleomorph of C. fraxinea (ascospore cultures, comparison of morphology and ITS sequences). H. albidus is considered to be widespread in Europe (it was already described in 1850 as Peziza albida), but it has rarely been collected and herbarium material is scarce. According to the literature, H. albidus occurs exclusively on Fraxinus petioles in the leaf litter; although in Poland it has occasionally been observed on shoots of dead seedlings (see above). It is also noted that in contrast to the sticky conidia of C. fraxinea, the ascospores of H. albidus are wind-dispersed which might explain the rapid spread of the disease. As H. albidus is apparently native and widespread in Europe, the emergence of a new disease caused by this species is difficult to explain. It is considered that the taxonomy of the H. albidus/C. fraxinea complex should be further studied to better understand the possible causes of the emergence of ash dieback in Europe.
Sources
Kowalski T, Holdenrieder O (2009) The teleomorph of Chalara fraxinea, the causal agent of ash dieback. Forest Pathology (in press).