During September 2006, disease symptoms were observed on mature highbush blueberry (Vaccinium corymbosum L.) cvs. Bluecrop and Covoille in a blueberry commercial field in Dalian, China. The maximum and minimum rainfalls in June to September are 3,111.9 and 1,745.6 ml, respectively. The highest temperature during the summer is 35.3°C and relative humidity may achieve 90%. Circular to irregular, light brown-to-gray leaf spots with brownish red borders, initially 3 to 7 mm in diameter, enlarged and coalesced. Reddish, circular spots appeared on stems, developing small, insignificant cankers. A fungus was recovered on potato dextrose agar (PDA, pH nature) from the margin of necrotic leaf spots. Morphological traits of the strain that developed from a single-spore culture were as follows: colonies were regular and flat, with a rough upper surface that peripherally was olive-green with a black center and dull white spots; short conidiophores arising singly and measuring 81.6 to 163.2 × 4.1 to 8.2 μm; conidia was abundant, ovoid, and obclavate muriformly septate, which horizontal and vertical septations varied from 1 to 6 and 0 to 2, respectively, and its size varied from 26 to 48.8 × 9.7 to 16.3 μm with an average beak length of 9.6 μm, and sporulation pattern is budding. Conidia derived from conidiophores. Koch's postulates were fulfilled for the isolates by spray inoculating two healthy mature plants with 2 × 105 conidia per ml homogenized in sterile water. As a control, two plants were sprayed with sterile water. Plants were placed inside plastic bags to maintain humidity and incubated in a growth chamber at 26°C under fluorescent light for 14 h and 20°C in darkness for 10 h. After 2 days, the plastic bags were removed and plants were maintained under the same conditions for 30 days. Symptoms on inoculated plants were similar to those previously observed. Symptoms were not observed on control plants. Cultures isolated from inoculated plants had the same morphological traits as those that were isolated previously from the field plants. The morphological descriptions and measurements were similar to Alternaria tenuissima (2). The 5.8S subunit and flanking internal transcribed spacers (ITS1 and ITS2) of rDNA and partial cds histone gene were amplified from DNA extracted from single-spore cultures using the ITS1/ITS4 and H3-1a/H3-1b primers, respectively, and sequenced (GenBank Accession No. EF031053) (1,3). The ITS sequence was identical to the ITS regions of A. tenuissima strain EGS34-015 (100%; GenBank Accession No. AY751455), the partial cds histone gene sequence was similar to A. tenuissima isolate MA6 (99%; GenBank Accession No. AF404634). The morphology, secondary conidiation, and sequences of ITS and partial cds histone gene identify the causal fungus as A. tenuissima. To our knowledge, this is the first report on the presence of A. tenuissima affecting blueberry plants in China.
References: (1) J. C. Kang et al. Mycol. Res. 106:1151, 2002. (2) E. G. Simmons. Mycotaxon 70:325, 1999. (3) T. J. White et al. Pages 315--322 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.
