Fusarium oxysporum and F. sambucinum Associated with Root Rot of Amaranthus hybridus in South Africa.

Amaranthus hybridus has been identified as an important alternative vegetable crop with potential for increased commercial production in South Africa (1). In summer 1999, severe losses occurred in a large plot of 2-month-old A. hybridus plants on an experimental farm near Bloemfontein, South Africa. More than 90% of the plants were severely stunted, with chlorotic foliage that was wilted in most cases. Root rot was present in all symptomatic plants and was clearly visible as an amber to brown discoloration of tap and secondary roots; in severe cases, white mycelium was clearly visible on diseased root tissue. Isolations from symptomatic roots were made on potato dextrose agar (PDA) amended with streptomycin sulfate. Isolates (N = 121) were recovered from diseased roots (n = 89). The two most frequently isolated fungi were transferred to carnation leaf agar and identified as Fusarium oxysporum (n = 90, 74%) and F. sambucinum (n = 29, 24%). Pathogenicity tests with one isolate of each species were performed in the greenhouse on 1-month-old potted A. hybridus seedlings (10 plants per treatment). A single hyphal tip of each isolate was transferred to PDA and incubated at 25°C for 7 days in the dark. Five 4-mm-diameter mycelial plugs were taken and placed directly on the taproot of each plant, halfway along the length and ≈30 mm below the soil surface. Control plants were treated with uncolonized PDA plugs. Seedlings inoculated with either fungus exhibited wilting within 7 days; stunting, chlorosis (pale green to yellow), and root necrosis after 2 weeks; and mortality after 4 weeks. Inoculated plants were removed from pots after 3 weeks, roots were washed free of potting soil, and necrotic lesion length was measured. Necrosis and discoloration of root tissue were similar to those observed in field plants. The mean length of tissue necrosis induced by the fungi was 22.5 and 34.8 mm for F. oxysporum and F. sambucinum, respectively. F. sambucinum, thus, was more pathogenic than F. oxysporum despite being recovered significantly less often from field plants. Control plants inoculated with noninfested PDA plugs remained healthy. The presence of both pathogens was confirmed by reisolation from artificially inoculated taproots of all plants. No Fusarium spp. were recovered from the 10 control treatments. F. oxysporum has been reported on diseased red root pigweed (A. retroflexus) in the United States (2), but this is the first report of both F. oxysporum and F. sambucinum as causal agents of root rot in A. hybridus. These pathogens, therefore, must be considered a potential threat to commercial production of A. hybridus in South Africa and elsewhere. References: (1) W. J. Swart et al. S. Afr. J. Sci. 93:22, 1997. (2) R. M. Harveson and C. M. Rush. Plant Dis. 81:85, 1997.