First Report of Fusarium Wilt Caused by Fusarium oxysporum on Roselle in the United States

Roselle, Hibiscus sabdariffa var. sabdariffa, is an annual that is grown primarily for its inflated calyx, which is used for drinks and jellies. It is native from India to Malaysia, but was taken at an early date to Africa and is now widely grown in the tropics and subtropics (2). In late 2005, dying plants were noted by a producer in South Florida. Plants wilted, became chlorotic, and developed generally unthrifty, sparse canopies. Internally, conspicuous vascular discoloration was evident in these plants from the roots into the canopy. After 5 days on one-half-strength potato dextrose agar (PDA), salmon-colored fungal colonies grew almost exclusively from surface-disinfested 5 mm2 pieces of vascular tissue. On banana leaf agar, single-spored strains produced the following microscopic characters of Fusarium oxysporum: copious microconidia on monophialides, infrequent falcate macroconidia, and terminal and intercalary chlamydospores. Partial, elongation factor 1-α (EF1-α) sequences were generated for two of the strains, O-2424 and O-2425, and compared with previously reported sequences for the gene (3). Maximum parsimony analysis of sequences showed that both strains fell in a large, previously described clade of the F. oxysporum complex (FOC) that contained strains from agricultural hosts, as well as human clinical specimens (2; clade 3 in Fig. 4); many of the strains in this clade have identical EF1-α sequences. Strains of F. oxysporum recovered from wilted roselle in Egypt, O-647 and O-648 in the Fusarium Research Center collection, were distantly related to the Florida strains. We are not aware of other strains of F. oxysporum from roselle in other international culture collections. Roselle seedlings were inoculated with O-2424 and O-2425 by placing a mycelial plug (5 mm2, PDA) over a small incision 5 cm above the soil line and then covering the site with Parafilm. Parafilm was removed after 1 week, and plants were incubated under ambient temperatures (20 to 32°C) in full sun for an additional 5 weeks (experiment 1) or 7 weeks (experiment 2). Compared with mock-inoculated (wound + Parafilm) control plants, both O-2424 and O-2425 caused significant (P < 0.05) vascular disease (linear extension of discolored xylem above and below wound site) and wilting (subjective 1 to 5 scale); both isolates were recovered from affected plants. F. oxysporum-induced wilt of roselle has been reported in Nigeria (1) and Malaysia (4) where the subspecific epithet f. sp. rosellae was used for the pathogen. We are not aware of reports of this disease elsewhere. To our knowledge, this is the first report of F. oxysporum-induced wilt of roselle in the United States. Research to determine whether the closely related strains in clade 3 of the FOC are generalist plant pathogens (i.e., not formae speciales) is warranted. References: (1) N. A. Amusa et al. Plant Pathol. J. 4:122, 2005. (2) J. Morton. Pages 81-286 in: Fruits of Warm Climates. Creative Resource Systems, Inc., Winterville, NC, 1987. (3) K. O'Donnell et al. J. Clin. Microbiol. 42:5109, 2004. (4) K. H. Ooi and B. Salleh. Biotropia 12:31, 1999.