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First Report of Alternaria alternata Causing Leaf Spot on Cotinus coggygria in China

Zhang, S. L., Liang, W. X., Yang, Q. Q.
Plant disease 2018 v.102 no.12 pp. 2644
Alternaria alternata, Cotinus coggygria, DNA-directed RNA polymerase, actin, adenosinetriphosphatase, antimicrobial properties, antiseptics, calmodulin, conidia, culture media, dipping, ethanol, fungi, genes, internal transcribed spacers, leaf spot, leaves, mycelium, ornamental plants, pathogen identification, pathogenicity, plasma membrane, ribosomal DNA, solar radiation, tissues, trees, China
Cotinus coggygria Scop., a member of Anacardiaceae, is an ornamental plant native to southern Europe, central Asia, and northern China, now widely cultivated in China. C. coggygria possesses antiseptic, antioxidative, anti-inflammatory, and antimicrobial activities and thus is used for medicinal purposes. In October 2016, ∼30% of C. coggygria Scop. leaves from four trees nearby were observed to have brown and irregular or circular spots with or without yellow haloes in Qingdao, China. Small pieces of tissue taken from the margins of the disease spots were sterilized by dipping in 70% ethanol for 1 min, rinsed in sterile distilled water, placed on potato dextrose agar, and incubated at 25°C in the dark. Eleven out of 13 strains isolated from infected leaves produced dark brown mycelia. Conidia were produced in long chains, typically obclavate to obpyriform or ellipsoid, with a short conical beak at the tip. Conidia of the fungus were dark brown, septate, 16 to 48 × 7 to 16 μm (n = 100) in size, and had from two to seven horizontal septa and from zero to five vertical septa. For molecular identification, the internal transcribed spacer region of rDNA, plasma membrane ATPase, actin, RNA polymerase II beta subunit, and calmodulin gene were amplified and sequenced (GenBank accession nos. MG722823, MG736305, MG736306, MG736307, and MG736308) (Glass and Donaldson 1995; Lawrence et al. 2013). These sequences had over 99% nucleotide identity with the corresponding sequences of the reference strain of Alternaria alternata in GenBank (MG722823: 99% to MF072619.1, MF422130.1, MF422132.1, and MF422133.1; MG736305: 99% to KY996474.1, KJ908221.1, KJ908223.1, and JQ811979.1; MG736306: 99% to KY131956.1; MG736307: 100% to KY131957.1 and KY996471.1; MG736308: 99% to KJ920958.1, KY996473.1, KJ920968.1, and J920957.1). Based on morphological and molecular characteristics, the pathogen was identified as A. alternata (Fr.:Fr.) Keissl (Simmons 2007; Solel 1991). Pathogenicity tests were performed by placing 20 μl of conidia suspension (10⁷ conidia/ml in sterile distilled water) on the left side of detached C. coggygria leaves (20 per isolate), whereas 20 μl of sterile distilled water was placed on the right side as a control treatment. After 4 days of incubation at 25°C with 16 h of daylight, necrotic spots similar to those observed on the original plants were found, whereas the control treatment remained symptomless. The experiment was repeated two additional times. A. alternata was reisolated from the necrotic tissues, fulfilling Koch’s postulates. To our knowledge, this is the first report of A. alternata affecting C. coggygria in China. Because C. coggygria is widely cultivated for economic and medicinal purposes, leaf spots caused by A. alternata would not only affect its appearance but also its medical value.