Chalara fraxinea causing common ash dieback newly reported in Slovenia

Dieback of common ash (Fraxinus excelsior) was first observed in Northeastern Slovenia in 2006. The symptoms were shoot, twig and branch dieback (Fig. 1), wilting (Fig. 2), leaf and bark lesions, and grey to brown discoloration of the wood. In 2007, the incidence of ash dieback became widespread throughout Slovenia. The fungus Chalara fraxinea has previously been recorded as causing similar symptoms on ash in other parts of Europe (Kowalski, 2006; Halmschlager & Kirisits, 2008; Kowalski & Holdenrieder, 2008).

C. fraxinea was most frequently isolated from the leading edge of discoloured wood and necrotic leaf petioles, but rarely from necrotic bark (Fig. 3). Colonies on malt extract agar (MEA) were effuse, cottony, most often fulvous brown, sometimes dull white, with occasional patches becoming grey to dark grey. The colonies were slow growing, 25–45 mm diameter after 21 days at 20°C in the dark. Phialophores were 19.8 (13.9–25.9) μm long (n = 22); 12.2 (6.2–18.3) x 4.0 (3.4–4.9) μm at the base and 7.5 (6.3–8.6) x 2.6 (2.1–3.1) μm at the collarette; conidia 3.2 (2.6–3.6) x 2.3 (1.9–2.6) μm (n = 20), first formed conidia 6.7 (6.2–7.5) x 1.9 (1.8–2.1) μm (n = 20). (Fig. 4.) These morphological characteristics matched Kowalski's (2006) description of C. fraxinea.

Koch’s postulates were performed with an isolate of C. fraxinea from F. excelsior (Slovenia, 2007, SFI, Accession No. 176). A pathogenicity test was performed in July 2008 on current year shoots of common ash. The shoots were wounded with an 8-mm punch. Mycelial plugs 8 mm in diameter from a 40 day old culture (MEA) were placed on the wounds and covered with Parafilm and aluminium foil. After 27 days, control wounds inoculated with sterile MEA plugs had healed, but the C. fraxinea- inoculated wounds showed lesions in the bark (mean length 18.8 mm) and wood discoloration (mean length 25.4 mm). C. fraxinea was successfully re-isolated from the infected bark and wood. The pathogenicity of C. fraxinea was thus proved and the observations of Kowalski & Holdenrieder (2008) confirmed. Little is known about C. fraxinea biology as the pathogen has only recently been described affecting ash trees in Europe (Bakys et al., 2008). However, its aggressiveness and rapid spread in Slovenia support the hypothesis of an introduced pathogen that is becoming widespread across parts of Europe (Bakys et al., 2008; Halmschlager & Kirisits, 2008).