FIRST REPORT OF DAMPING OFF OF VACCINIUM BRACTEATUM CAUSED BY CERATOBASIDIUM sp FROM EASTERN CHINA
Oriental blueberry (Vaccinium bracteatum Thunb.) is a source of a traditional Chinese herbal medicine, having anti-fatigue and antioxidant activity. From 2012 to 2013, blueberry seedlings with symptoms of damping off were observed in seven commercial crops in Zhangzhu county, Yixi City, Jiangsu Province, China with a disease incidence of approximately 60%. Infected seedlings showed water soaked lesions on the stem with a brownish halo in the center, which later turned into root rot. Infected seedlings wilted and died prematurely. About 30 infected seedlings were collected, cut into 50 pieces, surface-disinfested with 4% sodium hypochlorite, rinsed 3-times with sterile distilled water, and placed onto potato dextrose agar plates kept at 28°C for 3 days. About 20 fungal isolates were obtained.
This fungus formed white colonies with abundant aerial mycelium, which was septate, branching at right angle, with distinct constriction at the origin of branching. 3-day-old hyphae were 4-6 mm in size. It did not produce conidia. Hyphal cells were stained with DAPI and all the isolates were binucleate. Total genomic DNA was extracted from the mycelial mat using CTAB. Amplification of ITS using primers ITS1/ITS4 yielded a ca. 670 bp product (GenBank Accession No. KP233903). The sequences obtained were 99% similar to Ceratobasidium sp. AG-U isolate (GenBank Accession No. HQ269825.1 from Azalea) (Copes et al., 2011). In pathogenicity tests, blueberry seedlings were inoculated with pure cultures of eight isolates of Ceratobasidium sp. Three 8 mm-disks of mycelia grown on PDA were placed around the roots of each plantlet. There were four replicates of 3-month-old seedlings for each treatment; the experiment was conducted twice. Eight control seedlings were inoculated with three PDA plugs. Both inoculated and non-inoculated seedlings were placed in glasshouse at 28°C. Damping-off symptoms developed in the stems and roots similar to those observed in the field. Control seedlings remained healthy. The fungus was re-isolated from the infected seedlings and confirmed to be Ceratobasidium sp. based on morphological characteristics and rDNA-ITS sequence. This pathogen has a worldwide distribution and a wide host range. To our knowledge, this is the first report of Ceratobasidium sp. causing oriental blueberry damping off in the world. Oriental blueberry production is developing quickly in China; therefore, appropriate and effective disease management strategies of oriental blueberry should include the disease incited by Ceratobasidium sp.