Hymenoscyphus pseudoalbidus sp. nov.: teleomorph of Chalara fraxinea
The first studies on ash dieback revealed the presence of a pathogenic fungus which was described as a new species: Chalara fraxinea (EPPO Alert List). Further investigations then showed that C. fraxinea was the anamorph of an already described fungal species: Hymenoscyphus albidus. However, H. albidus has been known to occur in Europe since 1851, being found in leaf litter but without causing any particular damage to ash trees. Therefore, the emergence of a new disease of ash trees in Europe associated with H. albidus was difficult to explain. But recent molecular studies carried out on several specimens assigned to H. albidus and collected from areas affected by ash dieback (e.g. Czech Republic, Germany, Poland, Switzerland), as well as from areas still free from the disease, could separate these specimens into two distinct groups. These groups were very difficult to distinguish by morphological traits but were clearly distinguished by their molecular characteristics. No evidence for hybridization between these two groups could be found. Therefore, it was proposed that H. albidus was in fact composed of two cryptic species: H. albidus and a new species called H. pseudoalbidus. In addition, it was suggested that ash dieback was caused by H. pseudoalbidus (teleomorph of C. fraxinea) and that the non-pathogenic species was H. albidus. This hypothesis was supported by observations made in Switzerland where H. albidus is the only species detected in regions where ash dieback is absent. During these molecular studies, it was also discovered that 2 herbarium specimens collected in Switzerland in 1978 and 1987 should now be assigned to H. pseudoalbidus, although ash dieback outbreaks were not noticed before 2007 in Switzerland (has the pathogen been introduced before and failed to establish?). More studies are still needed to improve understanding of the invasion patterns of ash dieback in Europe and its possible causes (introduction of an exotic species or emergence of a new variant favoured by subtle changes in the environment?).
Sources
Queloz V, Grünig CR, Berndt R, Kowalski T, Sieber TN, Holdenrieder O (2011) Cryptic speciation in Hymenoscyphus albidus. Forest Pathology 41(2), 85-168.
INTERNET
ETH (Swiss Federal Institute of Technology Zürich). New fungus strikes the ash at its core by Peter Rüegg (2010-04-08). http://www.ethlife.ethz.ch/archive_articles/100408_eschenpilz_per/index_EN