Erysiphe elevata (syn. Microsphaera elevata), a new North American powdery mildew fungus in Europe infecting Catalpa bignonioides trees

Since September 2002, Catalpa bignonioides trees heavily infected with powdery mildew have been observed in five localities in Hungary. Infections appeared and spread rapidly on both young and older Catalpa trees planted as ornamentals in parks and along the streets (Fig. 1).

White mycelia principally covered the upper surfaces of the broad Catalpa leaves but were also found on the lower surfaces. Both young and older leaves were infected. Sporulating powdery mildew colonies were also found on the fruits. Conidia were produced singly on the conidiophores and measured 22-35 x 8-15 μm. Appressoria were simple or lobed, opposite or spread along the hyphae. Ascomata were produced in abundance on some leaves, mostly on the upper leaf surfaces (Fig. 2).

These measured 96-128 μm in diameter and bore 5-9, 115-420 μm long appendages. The appendages terminated in dichotomously branched, knob-like or slightly recurved tips (Fig. 2A-C). The ascomata contained 4-7, stalked (Fig. 2E) or sessile asci. Based on these characteristics, the pathogen was identified as Erysiphe elevata (syn. Microsphaera elevata). It clearly differed from E. catalpae, a species described from Armenia (Simonian, 1984) and also reported from some European countries (Braun, 1995). Pathogenicity was confirmed in a test conducted as described in Szentiványi et al. (2003) for snowberry powdery mildew. E. elevata is a common powdery mildew species infecting Catalpa spp. trees in North America (Braun, 1987), but its occurrence has not been reported outside the USA and Canada to date. To our knowledge, this is the first report of E. elevata in Europe, suggesting that this North American species has increased its area of distribution only recently. Herbarium specimens were deposited in the herbarium of Martin Luther University, Halle, Germany (HAL).