Vietnam annually exports about 300,000 t of pomelo (Citrus maxima, syn. C. grandis) fruits to the EU. Production mainly takes place in Vinh Long, Hau Giang, and Can Tho provinces. Fruits are harvested before fully mature, rinsed in 5% chlorine (v/v), and covered with plastic before postharvest storage at 15°C. The ripening takes place during storage and transportation when losses may occur due to fungal infection. Infected fruits may totally decay in 2 to 3 days. Losses amount to about 5% in the dry and 20% in the wet season when 60% and 40% of the annual crops are produced, respectively. Decay starts from the pedicel and the tissue turns water-soaked and soft. Fruits generally keep their shape until the late stages of decay. Symptoms become more severe if the pedicel is completely lost at harvest and therefore, a 5-mm pedicel is retained. Fungal isolations from symptomatic fruits were made on V8-juice agar and single spores were later grown on potato dextrose agar. Single-spore colonies displayed white, dense aerial mycelium with honey-colored drops of exudate. Reverse sides of the cultures were pale brown after 5 to 7 days, turning darker by 10 to 14 days. Macroconidia were 21.7 ± 3.7 µm long and 6.4 ± 0.9 µm wide (n = 200) and matched descriptions of the plant-root and mammalian pathogen Fusarium (formerly Cylindrocarpon) lichenicola C. Massal. This species characteristically does not produce asymmetric foot cells as normally seen in Fusarium spp. and colony growth, morphology of hyphae, conidia, and chlamydospores, as well as absence of microconidia also corresponded with past descriptions (Summerbell and Schroers 2002). No sexual stage was observed despite long incubation times. The fungal pathogen was also identified by amplifying and sequencing the ITS region of rDNA and EF-1α as described by O’Donnell et al. (1998). The amplicons were trimmed as isolates from O’Donnell et al. (2008) and deposited to Genbank (Accession Nos. KJ768839 and KP903345). BLASTn searches indicated >99% similarity with Fusarium lichenicola, a member of the F. solani species complex (Summerbell and Schroers 2002; O’Donnell et al. 2008). Pathogenicity tests were made using pomelo (C. maxima), mandarin (C. deliciosa), Persian lime (Citrus × latifolia), Key lime (C. aurantifolia), king orange (C. nobilis), and sweet orange (C. sinensis) fruits. Spore suspensions (106 spores/ml) were either sprayed to run-off onto fruits injured by pinpricking, or one ml was injected into each fruit at the pedicel. Controls were treated with sterile distilled water. Five fruits were used for each Citrus sp. and treatment. Fruits were placed separately in polyethylene bags and kept at 20°C in the dark for 3 days, followed by incubation at 28°C under low light conditions. All inoculated fruits became infected using both inoculation methods and showed the same fruit rot symptoms 4 to 6 days after inoculation as seen for naturally infected fruits. Control fruits remained healthy. Koch’s postulates were fulfilled by reisolation of F. lichenicola from all inoculated fruits. F. lichenicola, identified by morphological and molecular tools, was hitherto considered a human and animal pathogen, but this report broadens its host range to plants because it can induce pomelo fruit rot with a potential to cross-infect other citrus fruits.
Tạp chí khoa học Trường Đại học Cần Thơ
Lầu 4, Nhà Điều Hành, Khu II, đường 3/2, P. Xuân Khánh, Q. Ninh Kiều, TP. Cần Thơ
Điện thoại: (0292) 3 872 157; Email: tapchidhct@ctu.edu.vn
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