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First Report of Corynespora Leaf Spot Caused by C. cassiicola on Golden Vicary Privet (Ligustrum × vicaryi) in China

Golden vicary privet (Ligustrum × vicaryi Rehd.), a hybrid between L. ovalifolium ‘Aureum’ Rehd. and L. vulgare L., is widely used as a landscape shrub for horticultural ornamentation. From 2009 to 2011, a leaf spot disease of L. × vicaryi was observed in the parks in Luoyang, Henan Province, China. Lesions were initially brown and punctiform, and with age the lesions turned into elliptic, subcircular to irregular and pale brown, faintly zonate, and depressed. Fully mature lesions were mostly irregular to circular, 5 to 15 mm in diameter, centers tan to dark brown, with reddish brown to reddish purple margins of varying width. The disease eventually caused substantial premature defoliation. After infected leaves were collected from parks and maintained in a humid atmosphere, a layer of black mold developed on the surface of the lesions. Conidiophores were cylindrical, straight to slightly curved, brown, unbranched (2 to 7 septa), and were 98.5 to 403.9 μm in length, 4.5 to 6.6 μm in width. Conidia were solitary or in short chains containing two to five spores, and were cylindrical or obclavate, straight or flexuose (3 to 19 pseudosepta), pale olivaceous brown or brown when mature, and were 78.8 to 315.2 μm in length by 5.3 to 12.3 μm in width. Morphological characteristics of the fungus were similar to those of Corynespora cassiicola (Berk. & Curt.) Wei (1). Lesions on leaves were excised, surface sterilized, and plated on potato dextrose agar (PDA) and incubated at 25°C for 2 to 3 days. A fungus was isolated from the lesions, and pure isolates that were obtained after from single spored isolates were cultured on PDA. Colonies on PDA formed concentric growth rings, abundant aerial mycelia, and were grey or dark brown in color. The isolate ST1 was selected as a representative for molecular identification. The ITS1-5.8S-ITS2 region of the isolate was amplified by PCR with primers ITS1/ITS4 (3) and sequenced. BLAST analysis of the 559 bp amplicon (GenBank Accession No. KC138855) indicated 100% sequence similarity with C. cassiicola (GU138988) (2). To validate Koch's postulates, pathogenicity tests were performed by spraying leaves of five healthy potted L. vicaryi with a 10⁶ conidia per ml aqueous suspension. Control plants were inoculated with sterile water. Plants were covered with plastic bags for 24 h after inoculation and maintained at 25°C. After 3 days, all inoculated plants showed typical symptoms, whereas water sprayed controls remained healthy. C. cassiicola was consistently reisolated from these lesions. The reisolated conidia showed the same morphological characteristics as described above. To our knowledge, this is the first report of leaf spot caused by C. cassiicola on L. × vicaryi in China. Its confirmation is a significant step toward management recommendations for growers.

References: (1) M. B. Ellis et al. Corynespora cassiicola. CMI Descriptions of Pathogenic Fungi and Bacteria, no. 303. Commonwealth Mycological Institute, Kew, UK, 1971. (2) X. B. Liu et al. Plant Dis. 94:916, 2010. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.