First reported outbreak of charcoal disease caused by Biscogniauxia mediterranea on Turkey oak in Slovenia

In August 2003, the leaves of about 50% of coppice-grown Turkey oak (Quercus cerris), covering about 180 ha, turned brown. In spring 2004, whole trees were dead and the bark of the others died off in strips; there were shoots only from parts of the tree crowns. The same phenomenon also occurred patchily over approximately 16,000 ha of mixed forests in the Karst region of Slovenia. The bark began to crack and fall off in June 2004 (Fig. 1) and in August the first perithecial stromata of Biscogniauxia mediterranea were observed in bark cracks on tree trunks and branches (Fig. 2). At the beginning of March 2005 there were a large number of stromata on the bark of dead trees; more than hundred of them could be found on a single tree with a diameter of 27 cm at chest height. Stromata measured (3-) 19.6 (-36) × (1.5-) 3.4 (-7.5) cm, perithecia were tubular (0.43-) 0.62 (-0.81) × (0.08-) 0.14 (-0.22) mm, ascospores measured (13-) 16 (-19.5) × (6-) 7.5 (-9) µm, asci (114-) 145 (-175) × (8.5-) 10.5 (-14) µm, the outer dehiscing layer was (0.07-) 0.16 (-0.24) mm thick (Fig. 3). Measurements indicate that the fungus could be classified as B. mediterranea var. microspora (Ju et al., 1998). The perithecial stromata and pure cultures were deposited in the Herbarium of the Slovenian Forestry institute with accession nos. 1505, 1506 and 1507.

The disease symptoms appeared after severe drought and unusually hot weather. Total rainfall before the onset of symptoms in June, July, and August 2003 was 27, 12 and 66% respectively of the 30-year average, while average monthly temperatures for the same months were 5.8, 3.6 and 5.6°C higher than the 30-year average. The development of charcoal disease on oaks after drought is well documented (Vannini & Valentini, 1994). The endophytic presence of this fungus in living bark enables it to quickly overgrow the stressed tissues of the host and destroy them (Vannini, 1998).

Charcoal disease is a serious problem in cork oak (Quercus suber) and Turkey oak in the Mediterranean area but it had never been detected further north than southern Tuscany (Vettraino et al., 2002). The appearance of this new disease in Slovenia, approximately 350 km north-east of Tuscany, indicates that the predicted climate change could lead to outbreaks of this disease further north.