Kiwifruit (Actinidia spp.) is a very economically important fruit crop in southern China, namely Fengxin County. In 2011 and 2012, a new anthracnose on the fruit stem of kiwifruit with blight symptoms was observed during the blossom and fruit-set periods. This caused considerable blossom and fruit drop. To isolate the causal pathogen, symptomatic fruit-stem tissue at the junction of diseased and healthy tissue was cut into 30-mm-long segments. The segments were surface-sterilized in 70% ethanol solution for 10 s, followed by 0.1% mercuric chloride solution for 3 min, and a final set of three rinses in sterile distilled water. Segments were plated onto PDA and incubated at 25°C on a 12-h alternating light-dark cycle (1). Colonies emerging from plant tissues were then transferred to new PDA plates to obtain pure cultures. Colonies grew at the rate of 13.75 mm/day, and were initially white, then became dark green. After 25 days incubation, black and subglobose perithecia appeared at the center of the colonies. Through microscopic examination of perithecia at 400× magnification, club-shaped asci, usually containing 8 ascospores, were found. Ascospores were cylindrical to narrowly fusiform, straight or slightly curved, mostly 3-septate, and 54.5 to 107.5 × 5.0 to 7.5 μm in size. Combining cultural and morphological characteristics with reports in the literature (2), the pathogen was preliminarily identified as a Glomerella species. For six isolates, the rDNA internal transcribed spacer (ITS) region was amplified with the primers ITS1 and ITS4, and the sequences were deposited in GenBank (JX885687). A BLASTn search of GenBank showed that the ITS sequence of our Glomeralla isolate (JX885687) had 100% homology with that of an isolate of Glomeralla septospora (GU935911). Twenty representative isolates (G. septospora) obtained from infected fruit stem tissues in the blossom period and the fruit-set period, respectively, were selected for pathogenicity tests on detached young fruit stems of kiwifruit (JinKui). Each isolate was inoculated into three fruit stems, each of which was slightly wounded with a sterile scalpel and inoculated with a 5-mm-diameter colonized potato dextrose agar (PDA) plug. Negative control stems were treated with the sterile PDA plugs. The inoculated stems were incubated in a growth chamber at 25°C and 95% relatively humidity. After 4 days, characteristic symptoms of fruit stem anthracnose were observed on all inoculated stems, whereas the controls remained asymptomatic. Pure cultures of G. septospora were recovered from all inoculated fruit stems, fulfilling Koch's postulates. These results showed G. septospora is the pathogen of fruit stem anthracnose of kiwifruit, and to our knowledge, this is the first report of G. septospora causing fruit stem anthracnose of kiwifruit in Fengxin county, Jiangxi Province of China, as well as in the world.
References: (1) J. Y. Lu. Plant Pathogenic Mycology. China Agriculture Press, Beijing, China, 2004. (2) A. Sivanesan and W. H. Hsieh. Mycological Research, 97:1523, 1993.