Abstract
Kenaf provides a source of high quality cellulose fibers and its cultivation is being investigated in South Africa with a view to commercial production. In March 2001, kenaf plants in experimental plots near Bloemfontein, South Africa, developed large, black sunken lesions (10–20 cm long) at the base of the stem, and severe root rot. The causal agent was subsequently identified as Pythium group G and various studies were undertaken to characterize the pathogen and the disease it causes on kenaf. The relative susceptibilities of the kenaf cultivars available for commercial cultivation were determined by means of artificial inoculations in the glasshouse. Mean lesion lengths ranging from 43.4 mm to 58.3 mm with an overall mean of 52.7 mm, were obtained seven days after inoculation of potted plants grown in the glasshouse. Significant differences (P < 0.05) were found between the mean lesion lengths of the 10 inoculated cultivars. Everglades 41 had the longest mean lesion length (58.3mm) while Everglades 71 had the shortest (43.1 mm). The ability of Pythium group G to cause damping- off on kenaf was tested by artificially inoculating seedlings in the glasshouse. Mean damping-off of seedlings ranged from 35–60% and no significant (P < 0.05) difference in susceptibility was observed between cultivars. The optimum temperature range for growth of Pythium group G was 20–30°C. Six fungicides were screened in order to determine their inhibitory effect on the in vitro radial growth of an isolate of Pythium group G. Dichlorophen and mancozeb/metalaxyl were the most inhibitory. This study has important implications for the cultivation of kenaf in South Africa.
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