Ash (Fraxinus excelsior) is an important forest tree species growing in Turkey. This species has been planted particularly along roadsides as ornamental trees. In Autumn 2009, during a survey of ornamental trees diseases, a severe outbreak of powdery mildew on F. excelsior was observed in Samsun province, Turkey. Symptoms of the disease included white superficial mycelium with abundant sporulation on almost all abaxial leaf surfaces. Sporulating fungal structures were dissected from leaves and examined microscopically for morphological characters. Conidia were 57·0–89·7 × 14·3–24·2 μm, single-celled, hyaline and club-shaped. Chasmothecia with acicular appendages and a bulbous base were yellow when young and turned black at maturity. Chasmothecia were 197–268 μm in diameter with appendages 273–379 μm in length. Each chasmothecia contained 9 to 13 unitunicate asci, 71·3–91·2 × 28·5–38·4 μm, broadly oval to ellipsoid, curved, with a flexuous foot cell, containing three ascospores. Ascospores were 25·7–42·7 μm × 14·3–24·8 μm, ellipsoid-ovoid, yellowish-orange and highly guttulate. Based on the asexual and sexual characteristics, the fungus was identified as Phyllactinia fraxini (Pirnia et al., 2007). Specimens were preserved in the Herbarium of the Mycology Laboratory of the Plant Protection Department, Ondokuz Mayis University, Samsun.
Pathogenicity tests were conducted on healthy, mature trees under natural conditions in a location where the disease was not observed, at a temperature between 20–22°C with a humidity of 80–90%. Healthy leaves were inoculated by contact with detached powdery mildew-infected leaves: five young leaves at the tip of three different shoots were inoculated by gently rubbing infected leaves on them. Uninfected leaves with similar features were used as controls. Typical powdery mildew symptoms were observed on inoculated leaves after 10–15 days. No symptoms developed on leaves of control trees. Morphological characteristics of the pathogen and symptoms on the inoculated plants were similar with naturally infected plants. Phyllactinia fraxini has been reported to occur on F. excelsior in Switzerland and Lithuania (Takamatsu et al., 2008), but this is the first report of P. fraxini causing powdery mildew disease on F. excelsior in Turkey.