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First Report of Septoria Spot of Clementine Caused by Septoria citri in Italy

In January 2010, a fruit spot of “Clementine” (Citrus clementina Hort. ex Tan.) was observed on Italian-grown fruit in a market in northern Italy. The surface of the peel of infected fruits had black, irregular, necrotic areas that were slightly depressed, 10 mm in diameter, and surrounded by chlorotic halos. No pycnidia were observed on the necrotic spots. Tissues beneath the necrotic spots, which included the albedo (white pith), appeared dark and a black rot affected the external part of the juice vesicles. Small sections (approximately 3 mm²) of infected internal tissues were cultured on potato dextrose agar (PDA) amended with 25 ppm of streptomycin and maintained at 22 to 24°C. A slow-growing fungus with dark colored mycelium that produced pycnidia was consistently isolated. Conidia were hyaline, elongate, straight or slightly curved, unicellular, but sometimes with one septum, and measured 9.0 to 25.4 × 1.0 to 2.7 (average 17.7 × 1.8) μm. Preliminary morphological identification of the fungal isolates resembling Septoria spp. was confirmed by PCR using genomic DNA extracted from the mycelia of pure cultures. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS4/ITS6 and sequenced. BLAST analysis of the 508-bp segment showed a 99% homology with the sequence of Septoria citri (GenBank Accession No. DQ897650). The nucleotide sequence has been assigned the GenBank Accession No. HQ176410. Pathogenicity of one isolate was tested by inoculating 10 fruits. These were wounded at the equatorial level (three wounds per fruit, 5 mm depth) and dipped for 10 s in a conidial suspension (1.2 × 10⁷ conidia/ml). Ten wounded noninoculated fruits were dipped in sterilized water and served as control. Fruits were kept at 10 ± 1°C. After 50 days, dark, sunken necrosis appeared around the wounds of inoculated fruits and the same symptoms first observed were present into the tissues beneath the wounds. S. citri was consistently reisolated from the inoculated fruits. Noninoculated fruits remained healthy. The pathogenicity test was carried out twice. The same disease was observed on other fruits belonging to the Rutaceae family, such as lemon in Greece (3) and on orange and lemon in Australia (1). In Italy, S. citri has been reported on lemon (2) fruit. To our knowledge, this is the first report of the presence of this pathogen on clementine in Italy as well in the world. The presence of Septoria spot on clementine fruits is currently sporadic in Italy; however it is necessary to monitor the incidence of this disease with field and postharvest surveys.

References: (1) T. G. B. Osborn and G. Samuel. Trans. R. Soc. Aust. 46:166, 1922. (2) P. Petri. Boll. Stn. Patol. Veg. Roma N.S. 16:1, 1936. (3) D. G. Zachos. FAO Plant Prot. Bull. 6:41, 1957.