First report of the ash dieback pathogen Chalara fraxinea on Fraxinus excelsior in Austria

Common ash (Fraxinus excelsior) is an ecologically and economically important tree species in many parts of Europe. Ash dieback was first observed in Austria in 2005 (Cech, 2006) and by 2006-2007 this phenomenon (Fig. 1) was widespread and serious. Symptoms included wilting and blackish discoloration of leaves (Fig. 2), shoot, twig and branch dieback (Figs. 1 & 2), necrosis of bark tissue, discrete necrotic lesions in the bark (Fig. 2) and brownish to greyish discolouration of the wood that often extended in a longitudinal direction beyond necrotic areas in the bark (Fig. 3). Affected trees showed prolific formation of epicormic shoots on twigs, branches and the stem (Fig. 1). Ash trees of all ages are affected, but mortality is particularly common amongst saplings. Symptoms resemble those of ash dieback in other European countries, where the recently described hyphomycete Chalara fraxinea is thought to be involved (Kowalski, 2006; Thomsen et al., 2007).

In June 2007 C. fraxinea (Figs. 4 & 5) was isolated for the first time from young symptomatic F. excelsior trees at two localities in Austria (Edt bei Lambach, Upper Austria and Altaussee, Styria, CBS accession nos. 122191 and 122192, respectively). Between July 2007 and March 2008 the fungus was also found at 11 other localities, two in the province Upper Austria, seven in the province Lower Austria and two in the province Vienna. Methods of fungal isolation were those described by Kowalski (2006).

Isolates of C. fraxinea were generally slow growing on malt extract agar (MEA), covering 5 cm diameter Petri dishes within 3 to 8 weeks at 23-25°C. Colonies on MEA were woolly, dull white or more often fulvous brown, sometimes partly or entirely turning light grey (Fig. 4). Pseudoparenchymatous stromata formed in some cultures when incubated for at least two weeks (Fig. 4 B). Prolonged incubation at +4°C greatly enhanced phialophore production (Fig. 4 D). Micromorphological characteristics of Austrian isolates were similar to those described by Kowalski (2006): phialides 17.6-28 x 3-5 µm (n = 20), conidia 2.5-4.2 x 2.0-2.8 µm (n = 60), first-formed conidia 6.5-9.0 x 2.0-2.8 µm (n = 60).

C. fraxinea has first been recorded in Poland (Kowalski, 2006) and subsequently in Germany (Schumacher et al., 2007), Sweden (Thomsen et al., 2007), Lithuania (R. Vasaitis, pers. comm.) and now in Austria. Proof of pathogenicity has thus-far not been published for this fungus, but T. Kowalski (pers. comm., 2007) consistently obtained typical symptoms of ash dieback using Polish isolates of C. fraxinea in inoculation trials on F. excelsior. He also consistently re-isolated the fungus from symptomatic tissue of artificially infected trees. The biology of C. fraxinea remains enigmatic, however, and further investigations are required to assess its precise role in the severe and widespread dieback phenomenon of ash in Europe. As it may be an introduced organism and due to its potential damaging effect to ash, C. fraxinea has recently been added to the EPPO alert list (http://www.eppo.org/QUARANTINE/Alert_List/fungi/Chalara_fraxinea.htm).