First Report of Chalara fraxinea on Common Ash in Italy

Abstract

In many European countries, the anamorphic Chalara fraxinea Kowalski (teleomorph Hymenoscyphus albidus [Roberge ex Desm.] Phillips; 1–3) is responsible for a severe and rapidly spreading dieback of common ash (Fraxinus excelsior L.) since it was first reported in Poland. Recently, this disease was added to the EPPO Alert List and the NAPPO Phytosanitary Alert System. Symptomatic trees were observed in a 1.8-ha ash-maple forest in northeastern Italy (Fusine, UD; 46°30′N, 13°37′E; 782 m above sea level) along the Italo-Slovenian border in July 2009. Symptoms were found on approximately 10% of mature common ash and 70% of seedlings. Main symptoms were shoot, twig, and branch dieback, wilting, and bark cankers (1). Fungal fruiting bodies were not found on or near the canker surface. Furthermore, longitudinal and radial sections through the cankers revealed gray-to-brown xylem discoloration. One symptomatic 3-year-old plant was randomly selected and from the necrotic margin of one canker previously surface-sterilized with 3% sodium hypochlorite and rinsed, four 2-mm-wide chips were placed on malt extract agar (MEA) and incubated at 21 ± 1°C in the dark. Among a variety of microorganisms, after 19 days, slow-growing colonies (mean radius of 12 mm) appeared that were effuse, cottony, and often fulvous brown but sometimes dull white with occasional gray-to-dark gray patches. The purified isolate was then transferred to the same medium at 4 ± 1°C in the dark, and after 11 days, hyaline-to-dark gray phialides were observed producing numerous conidia in slimy droplets and sometimes in chains. Phialophores measured 8.6 to 21.0 (15.1) μm long (n = 20), 4.2 to 13.4 (8.8) × 3.6 to 5.5 (4.7) μm at the base, and 5.2 to 8.7 (6.5) × 2.5 to 3.1 (2.8) μm at the collarette; conidia measured 2.8 to 4.2 (3.4) × 1.9 to 2.5 (2.2) μm (n = 40); and first formed conidia measured 5.5 to 6.5 (5.9) × 1.8 to 2.5 (2.1) μm (n = 20). These morphological characteristics matched Kowalski's (1) description of C. fraxinea. In August of 2009, the fungal isolate was used to test pathogenicity with current year shoots of 25 6-year-old (150 to 210 cm high) asymptomatic common ash trees under quarantine conditions (Slovenian Forestry Institute's experimental plots). For every plant, the bark of the main shoot (10 to 13 mm in diameter) was wounded with a 6-mm-diameter cork borer. Twenty saplings were inoculated with one 6-mm-diameter mycelial plug obtained from the margin of a 26-day-old culture (MEA), while five saplings were inoculated with sterile MEA plugs. All wounds were sealed with Parafilm and aluminum foil. After 28 days, all plants inoculated with the C. fraxinea showed bark lesions (2 to 39 mm long, mean 7 mm) and wood discoloration (6 to 85 mm long, mean 22 mm) from which the pathogen was reisolated. These symptoms were absent from controls and the pathogen was never reisolated. To our knowledge, this is the first report of C. fraxinea in Italy. Investigations on its presence in all Fraxinus species naturally growing in the investigated area and in the nearest regions are in progress. The obtained isolate is preserved in both Padova and Ljubljana herbaria as CFIT01. References: (1) T. Kowalski. For. Pathol. 36:264, 2006. (2) T. Kowalski and O. Holdenrieder. For. Pathol. 39:1, 2009. (3) T. Kowalski and O. Holdenrieder. For. Pathol. 39:304, 2009.