First report of pomegranate stem scab caused by Botryosphaeria dothidea in China

Pomegranate trees (Punica granatum) are commercially important and widely planted throughout China. Stem scab symptoms were first noticed in Zaozhuang, Shandong Province in 1996 and recently in Xi'an, Shaanxi Province in China. Symptoms mainly occur in the trunk of trees that are at least two years old, as well as in shoots and twigs of young and old trees. Typical symptoms include swollen circular, ellipsoid or irregular spots (0.2-5.5 x 0.2-3.0 cm) which often occur around lenticels and may enlarge and merge. The diseased bark will become cracked and coarse. The phloem and xylem may also be affected, and the tree vigor will be drastically reduced.

From 1996 to 2007, 20 pomegranate scab samples were collected from phloem, xylem and bark of trees. Fungal isolates with identical cultural characteristics were isolated from all the samples. Pathogenicity tests were carried out using mycelium discs of isolates placed on pomegranate stems, while control stems were inoculated with sterile potato dextrose agar (PDA) discs. Spots resembling natural infections occurred after 20-30 days on stems. Colonies of the fungus on PDA medium were white, and gradually became grey to dark grey. Abundant aerial mycelium was produced. Reverse sides of the plates were initially white, and then became dark to olive green. Pycnidia were circular or irregular, and aggregated mono- to multi-locularly in one stroma. Conidiophores were hyaline, with smooth cylindrical branches at the base. Conidia were 3.9-7.8 x 18.2-26.0 µm (average 5.8 x 22.10 µm) in size, one-celled, hyaline, thin-walled, smooth, fusiform to fusiform-elliptical. The ITS region was amplified from total DNA of a typical isolate using universal primer pairs (White et al., 1990). The resulting 583 bp sequence (GenBank Accession. No. EU441944) showed 99-100% identity with the ITS sequences of Botryosphaeria dothidea available in the GenBank (AY640254, AY786320, AY786321, AY786322). All these results indicated that the pathogen of pomegranate stem scab in China is Botryosphaeria dothidea (Slippers et al., 2004). B. dothidea can induce canker and bark dieback of other tree species (Jurc et al., 2006). In China, B. dothidea has been reported to infect poplar and other trees (Xiang et al., 1979). However, to our knowledge, this is the first report of B. dothidea causing disease on pomegranate in China or elsewhere in the world.