Tea is the most popular non-alcoholic beverage crop in the world, which originated in China and has been cultivated in over 45 countries. In recent years, a leaf spot disease of unknown etiology has been observed on young leaves of tea trees (Camellia sinensis) grown in Luotian county, Hubei Province, China. Observed symptoms display grayish brown to white spots (about 1 cm in diameter) surrounded by brown edges. Over 20% of the young leaves were affected on surveyed trees. To identify the pathogen, six symptomatic tea leaves were collected from six individual tea trees of unknown variety in August 2012. A thin section (3 to 5 mm) of symptomatic tissue was sterilized in a bleach solution of 3% hypochlorite and placed on potato dextrose agar (PDA) medium at 25°C in darkness for isolation. Six fungal colonies displaying gray-brown and gray-white aerial mycelia were consistently recovered from lesions of the six leaves, termed as T1 to T6, respectively. Conidia produced on the colonies were olive brown, obpyriform, short conical beak at the tip, 0 to 3 vertical and 1 to 6 transverse septa, and length × width of 7.1 to 31.7 (avg. 20.1) × 2.9 to 12.7 (avg. 7.2) μm. T1 to T6 were identified as Alternaria alternata on the basis of morphological characterization, respectively (2). Confirmation of the species identification was obtained by molecular characterization of their internal transcribed spacer (ITS) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) regions amplified from the genomic DNAs using the universal primers (1). The results revealed identical sequences of ITS (GenBank Accession No. KF699530) and GAPDH among the six isolates. BLAST searches showed that they had the highest similarity with A. alternata strains, with 98.3% for ITS (AJ276055) and 96.2% for GAPDH (EF513205), deposited in fungus database (http://www.mycobank.org/). Pathogenicity tests were conducted on the detached leaves expanding for 10 to 20 days of two tea varieties (cvs. Fudingdabai and Taicha No. 12) in triplicate by placing 4 mm diameter discs from 5-day-old PDA plates of T3 and T6, which were incubated in an incubator at 25°C with a 12-h photoperiod for 7 days. All inoculated leaves with or without wound treatment developed brown spots similar to the original ones at 7 days post inoculation (dpi) while the control leaves inoculated with non-colonized PDA plugs remained asymptomatic. Isolates recovered from diseased samples were of the same morphology and ITS sequence as the inoculated ones. Alternaria alternata had been described on C. sinensis in China (3), but it was only reported as a severe foliar fungal pathogen of tea in North Bengal, India (1), and to our knowledge, this is the first report of A. alternata causing leaf spots on tea leaves (C. sinensis) in China. In addition to quantity loss, the species may result in a decrease of quality of tea crop considering that it can produce Alternaria toxins related to animal and public health. The etiologic identification of the disease is expected to provide useful information for its control.
References: (1) B. N. Chakraborty et al. Plant Pathol. 55:303, 2006. (2) E. G. Simmons. Page 1 in: Alternaria Biology, Plant Diseases and Metabolites. J. Chelchowski and A. Visconti, eds. Elsevier, Amsterdam, 1992. (3) F. L. Tai. Page 1527 in: Sylloge Fungorum Sinicorum. eds. Sci. Press Acad. Sin. Beijing, 1979. (4) B. S. Weir et al. Stud. Mycol. 73:115, 2012.
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