First Report of Leaf Spot on Onion Caused by Fusarium acuminatum in Georgia

An unknown disease was observed on shallot onions (Allium cepa L.) in Tattnall County, Georgia, in September of 2012. Disease symptoms included leaf tip die back and scattered light brown lesions on leaves, oval to round in shape, with a size ranging from 0.5 to 2.0 cm. The disease occurred in approximately 1.2 ha of commercial onion fields, and disease incidence ranged from 50 to 70%. Infected leaf tissues from 10 plants were surface sterilized with 0.5% NaOCl and plated on quarter-strength potato dextrose agar (QPDA). The fungus grew on all plates, producing bright orange to rose pigmentation in the medium after incubation at 25°C for 5 days. The fungus produced sickle shaped macroconidia with 3 to 6 septa, 4 to 5 μm wide, and 30 to 54 μm in length. Chlamydospores were formed in chains and averaged 20.4 × 16.8 μm. The fungus was identified as Fusarium sp. based on morphological characteristics (3). Genomic DNA was extracted from single conidial cultures of three representative isolates and the internal transcribed spacer regions of rDNA (ITS1-5.8S-ITS2) were amplified and sequenced with primers ITS1 and ITS4 (2). MegaBLAST analysis of the sequences showed that they were 100% identical to a Fusarium acuminatum isolate (Accession No. JN624894). ITS sequence of an isolate FAON-1 was deposited in GenBank (KC477845). Pathogenicity tests were performed with the three isolates grown on QPDA at 25°C for 7 days. Eight-week-old shallot onion seedlings were inoculated by foliar spray with conidial suspensions at 2 × 10⁶ spores/ml (5 ml per plant). Ten plants were inoculated with each isolate and 10 plants were sprayed with water as a control. The plants were incubated in a humidified chamber at (25 ± 3°C) with >95% relative humidity and 12-h photoperiod for 48 h after inoculation, and then kept in a greenhouse at 22 ± 2°C. Inoculated plants started to show symptoms identical to those observed in the field 7 days after inoculation, and disease incidence reached 100% within 14 days. No disease occurred on the control plants. The fungus was reisolated from the diseased plants and confirmed to be F. acuminatum based on morphological characteristics and molecular identification. Onion was previously reported to be a host of F. acuminatum in Montana (4). To our knowledge, this study is the first report confirming F. acuminatum causing disease on onion in Georgia. Onion is a major vegetable crop in Georgia with an annual production of approximately 5,000 ha (1). Occurrence of leaf spot caused by F. acuminatum and the impact of the disease needs to be considered in developing and implementing disease management programs in onion production. References: (1) S. R. Boatright and J. C. Mckissick, Univ. of Georgia CAES Publication AR-10-02, 2010. (2) M. A. Innes et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. 1990. (3) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006. (4) C. G. Shaw. Washington State Univ. Agric. Exp. Sta. Bull. 765:12, 1973.