First report of Botryosphaeria dothidea associated with dieback of aspen (Populus tremula) in Italy

In summer 2007, a dieback was observed on European aspen (Populus tremula, Salicaceae) growing in a nature reserve in Catania province, Italy. Trees showed decline symptoms of yellowing, wilting and browning of the leaves, followed by leaf drop. The dieback was associated with cankers on the branches or trunk (Fig. 1). More than 30% of trees were affected; most were about 20 years old, with a few trees over 100 years old that were showing symptoms (Fig. 2). A fungus was constantly isolated from affected bark on to potato dextrose agar (PDA). The colonies were fast growing, with immersed light-brown mycelium bearing aggregated dark-brown to black pycnidia, up to 300 µm in diameter. The conidia were aseptate, smooth, fusiform, 19-38 x 2-6 µm (Fig. 3). The fungus was identified as Fusicoccum aesculi, the anamorphic state of Botryosphaeria dothidea (Sutton, 1980). A representative isolate (IMI 395176) was deposited at CABI Bioscience, UK, who confirmed the identity of the isolate as B. populi (syn. B. dothidea).

The rDNA regions ITS1-5.8S-ITS2 of the isolate were amplified by PCR using universal primers ITS1 and ITS4. The sequence (GenBank Accession No.GU119957) revealed 100% homology with 21 isolates of Botryosphaeria dothidea in GenBank. The pathogenicity was tested on ten, two-year-old seedlings of European aspen. A 5 mm patch of bark was removed from the stems and replaced with PDA plugs colonized by the fungus. Ten control plants were inoculated with sterile agar plugs. All the trees were kept in the open air. After ten months, all inoculated plants showed cankers at the inoculation point and canopy wilting. Darkening of wood and bark was observed 3-6 cm above and below the inoculation point. Control plants remained healthy. Koch’s postulates were met by the re-isolation of the fungus from symptom-bearing tissues. B. dothidea is reported as a damaging pathogen of poplarin China (Wang et al., 2007). In Serbia it causes cankers on the poplar clone I-214(Populus deltoides × Populus nigra) (Karadžić et al., 2000), and in Portugal B. populi causes damage to P. nigra (Phillips, 2000). However, this is the first finding of this organism on European aspen in Italy.