Chalara fraxinea associated with dieback of narrow-leafed ash (Fraxinus angustifolia)

Narrow-leafed ash, Fraxinus angustifolia is an ecologically and economically important tree species in floodplain forests in southern Europe and some parts of central Europe. In recent years, afforestations of this species along the river March near Hohenau/March in eastern Austria (province Lower Austria) have been severely affected by dieback and tree mortality. Symptoms included shoot and twig dieback, necrotic lesions and cankers in the bark (Fig. 1A-C) as well as discolouration of the wood (Fig. 1D). In 2008 Chalara fraxinea (Fig. 2), the anamorphic stage of Hymenoscyphus albidus (Kowalski & Holdenrieder, 2009) was consistently isolated from small necrotic lesions on shoots of diseased F. angustifolia saplings in this area and from diseased seedlings from a nursery near Kapuvár in northwest Hungary. In spring 2009 the fungus was also commonly detected on symptomatic narrow-leafed ash seedlings in a nursery in Lower Austria.

Colonies on malt extract agar (MEA) were cottony, white, orange brown or fulvous brown, with grey sectors in areas associated with phialophore production. Pseudoparenchymatous stromata formed occasionally after prolonged incubation. Micromorphological characteristics of two Austrian isolates (CBS Accession Nos. 123139, 123140) were as follows: phialides 16.3 (13.7-18.3) μm x 4.7 (3.9-6.1) μm at the base and 2.6 (2.2-3.0) μm at the collarette; conidia 3.3 (2.7-3.9) x 2.2 (1.5-2.7) μm; first-formed conidia 6.3 (5.5-7.7) x 2.2 (1.8-2.8) μm.

In May 2008, 20 potted, two-year-old F. angustifolia seedlings were wound-inoculated with C. fraxinea isolate CBS 123140. Inoculum consisted of autoclaved F. excelsior phloem (approximately 10 x 4 x 2-3 mm) that had been placed for 15 days on C. fraxinea cultures on MEA. Within three months 55% of the plants showed wilting of leaves (Fig. 3A) and dieback. Necrotic phloem lesions (mean length = 7.7 cm) and wood discolouration developed on all seedlings inoculated with C. fraxinea (Fig. 3B-D), but none on any of the 20 control seedlings. C. fraxinea was re-isolated from 60% of the fungus-inoculated seedlings but not obtained from any of the control plants. This is the first definitive report of C. fraxinea from a host other than F. excelsior. On the latter ash species this fungus has been causing severe dieback in Europe (Kowalski, 2006; Halmschlager & Kirisits, 2008; Szabó, 2008). The detection of C. fraxinea in forest nurseries may suggest that diseased plants for planting are an important pathway for accelerating the spread of this emerging pathogen.