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.

First Report of Macrophomina phaseolina Causing Pre-harvest Cassava Root Rot in Benin and Nigeria.

In diagnostic surveys conducted in parts of Benin and Nigeria to determine the incidence of pre-harvest cassava root and stem rot during the dry season, Macrophomina phaseolina (Tassi) Goidanich constituted 14.2 and 18.7% of the total fungi (n = 201) associated with cassava root and stem rot from Benin and Nigeria (1). Pathogenicity of M. phaseolina on cassava was tested with cv. Agric. Inocula for pathogenicity tests were prepared by incubating 5-mm-diameter mycelial plugs for each of five isolates (Mp 1 to Mp 5, all collected from Benin) with 500 ml of autoclaved, sterilized, dehusked rice seed for 14 days at 30°C. Five 30-cm-long stem portions per isolate were cut from healthy cassava, surface disinfested in hot water (52°C, 5 min), and planted into 1-liter pots containing autoclaved, sterilized sand mixed with 10 ml of air-dried inoculum. Five plants per isolate similarly treated but not inoculated served as controls. Plants were watered once a week, and maintained in a greenhouse under natural light at 28 to 30°C. Lower leaves of inoculated plants gradually wilted, usually preceded by chlorosis, and brown to black lesions formed on the lower stem portions of some roots. Control plants remained asymptomatic. Plant height and percentage of leaf wilt (determined by counting the number of leaves wilted per plant and dividing by the total number of leaves per plant) were measured on a weekly basis for 8 weeks for each of the control and inoculated plants. At the end of 8 weeks, lesion length on the lower stem was measured. There were significant differences (P < 0.05) in length of the lesions and percentage of leaf wilt induced by the different isolates of M. phaseolina. Isolate Mp 1 induced the longest lesion (7.2 cm), followed by Mp 4 (4.1 cm), Mp 3 and Mp 5 (3.8 cm each), and Mp 2 (1.2 cm). Mp 4 induced the highest percentage of wilted leaves (53%), followed by Mp 1, Mp 3, and Mp 5 (30%), and Mp 2 (10%). All five M. phaseolina isolates (except Mp 3) reduced plant height, compared with control treatments. M. phaseolina was isolated from all infected plants, and the identification was independently confirmed by the International Mycological Institute, Surrey, UK. This is the first report of M. phaseolina causing pre-harvest cassava root rot in Benin and Nigeria. Reference: (1) W. Msikita et. al. Plant Dis. 81:1332, 1997.

Sexual reproduction in the soybean sudden death syndrome pathogen Fusarium tucumaniae.

We investigated the sexual reproductive mode of the two most important etiological agents of soybean sudden death syndrome, Fusarium tucumaniae and Fusarium virguliforme. F. tucumaniae sexual crosses often were highly fertile, making it possible to assign mating type and assess female fertility in 24 South American isolates. These crosses produced red perithecia and oblong-elliptical ascospores, as is typical for sexual members of the F. solani species complex. Genotyping of progeny from three F. tucumaniae crosses confirmed that sexual recombination had occurred. In contrast, pairings among 17 U.S. F. virguliforme isolates never produced perithecia. Inter-species crosses between F. tucumaniae and F. virguliforme, in which infertile perithecia were induced only in one of the two F. tucumaniae mating types, suggest that all U.S. F. virguliforme isolates are of a single mating type. We conclude that the F. tucumaniae life cycle in S. America includes a sexual reproductive mode, and thus this species has greater potential for rapid evolution than the F. virguliforme population in the U.S., which may be exclusively asexual.

Fusarium species of the Gibberella fujikuroi complex and fumonisin contamination of pearl millet and corn in Georgia, USA.

This study was designed to identify and compare the Fusarium species of the Gibberella fujikuroi complex on pearl millet (Pennisetum glaucum (L.) R. Br) and corn (Zea mays L.) crops grown in southern Georgia, and to determine their influence on potential fumonisin production. Pearl millet and corn samples were collected in Georgia in 1996, 1997 and 1998. Three percent of the pearl millet seeds had fungi similar to the Fusarium species of the G. fujikuroi species complex. One hundred and nineteen representative isolates visually similar to the G. fujikuroi species complex from pearl millet were paired with mating population A (Fusarium verticillioides (Sacc.) Nirenberg), mating population D (F. proliferatum (Matsushima) Nirenberg) and mating population F (F. thapsinum (Klittich, Leslie, Nelson and Marasas) tester strains. Successful crosses were obtained with 50.4%, 10.1% and 0.0% of these isolates with the A, D and F tester strains, while 39.5 of the isolates did not form perithecia with any tester strains. Two of the typical infertile isolates were characterized by DNA sequence comparisons and were identified as Fusarium pseudonygamai (Nirenberg and O'Donnell), which is the first known isolation of this species in the United States. Based on the pattern of cross-compatibility, conidiogenesis, colony characteristics and media pigmentation, a majority of the infertile isolates belong to this species. Fumonisins FB(1) and FB(2) were not detected in any of the 81 pearl millet samples analyzed. The species of the G. fujikuroi species complex were dominant in corn and were isolated from 84%, 74% and 65% of the seed in 1996, 1997 and 1998, respectively. Representative species of the G. fujikuroi species complex were isolated from 1996 to 1998 Georgia corn survey (162, 104 and 111 isolates, respectively) and tested for mating compatibility. The incidence of isolates belonging to mating population A (F. verticillioides) ranged from 70.2% to 89.5%. Corn survey samples were assayed for fumonisins, and 63% to 91% of the 1996, 1997 and 1998 samples were contaminated. The total amount of fumonisins in the corn samples ranged from 0.6 to 33.3 microg/g.