First report of Botryosphaeria dothidea causing bark dieback of European hop hornbeam in Slovenia

Extensive mortality of European hop hornbeam (Ostrya carpinifolia) has occurred in the western part of Slovenia, where this tree species is invading abandoned grassland. The Slovenian Forest Service first reported damage to hop hornbeam in 1997. The extent and intensity of the phenomenon depends on weather conditions. Thus, in a drought year (2003) the affected area covered 6,800 ha with a 50% mortality of hop hornbeam; while in a wetter year (2004) mortality dropped to 35% with 133 ha affected. Bark necrosis is characteristic of the disease. It can develop in healing cankers or the lesions expand to include the whole trunk or branches of the tree, which consequently die (Fig. 1).

Mass occurrence of Botryosphaeria dothidea ascomata in winter and Fusicoccum aesculi conidiomata in the growing season have been regularly observed on the dead bark of hop hornbeam. Asci measured 80-98.5 (85.5) × 17-20 (18) µm, ascospores 14-34 (22) × 6-9.5 (8) µm and conidia from culture 20.5-30 (25.5) × 4.5-7 (6) µm (Fig. 2). Samples of conidiomata, ascomata and isolations from dead bark on malt extract agar (MEA) were deposited at the Herbarium of the Slovenian Forestry Institute (Acc. Nos 1508-1510). The pathogen had been previously reported as Botryosphaeria ribis (Jurc et al., 2003), but with new delineations in the taxonomy of the pathogen (Slippers et al., 2004) the true identity of the fungus has been unravelled. The sequences of the rDNA ITS region from pure cultures (GenBank accession numbers AJ938004 & AJ938005) share 99-100 % similarity with several deposited sequences of B. dothidea isolates, hence indicating a clear identification.

Inoculations of two isolates taken from the necrotic bark of hop hornbeam and isolated on MEA were performed in nature in six replications on hop hornbeam coppice shoots. The bark was removed with a cork borer (diameter 6 mm) and an agar plug was inserted (with mycelium facing towards the stem). The wound was covered with micropore tape. Control stems were inoculated with sterile agar plugs. After one year the average necrosis of inoculated stems was 6.7 cm long, while the controls had healed (Fig. 3). A Fusicoccum anamorph was reisolated from the edge of the necrosis thus fulfilling Koch's postulates.

B. dothidea has a world-wide distribution and is capable of infecting numerous plant species. Its host range comprises mostly trees and shrubs and even 70 years ago it was reported from 68 genera (Smith, 1934); the host range was later increased by an additional 17 genera (Hepting 1971). In North America it occurs on Ostrya virginiana with saprobic activity (Hepting 1971). It has been found on a fallen O. carpinifolia branch in Europe with no indication of its pathogenicity (Slippers et al., 2004). This is the first report of Botryosphaeria dothidea causing dieback of Ostrya carpinifolia in epiphytotic dimensions in Slovenia.