Authors
W. J.
Swart
and
M. T.
Tesfaendrias
,
Department of Plant Sciences, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
; and
W.
Botha
,
ARC Plant Protection Research Institute, Private Bag X134, Roodeplaat 0001, South Africa
Kenaf (Hibiscus cannabinus L. Malvaceae) presents a source of high-quality cellulose fibers and is being investigated in South Africa for commercial production. In March 2001, 12 5-month-old kenaf plants grown from seed in experimental plots near Bloemfontein, South Africa, displayed large, black, sunken lesions (10 to 20 cm long) at the base of the stem, and severe root rot. A study was undertaken to characterize the pathogen, and to determine the relative susceptibilities of five kenaf genotypes being considered for commercial cultivation. Isolations from diseased tissue on malt extract agar consistently yielded a fungus identified as Pythium group G (1). Four-month-old kenaf plants were artificially inoculated in the field by inserting wooden toothpick tips colonized by the pathogen approximately 25 cm above soil level into the stems of 10 plants of each of five genotypes. Inoculation points were wrapped using Parafilm. The fungus was highly virulent to all five kenaf genotypes in two experiments, with mean cambial lesion lengths of 117, 119, 120, 122, and 139 mm at 7 days after inoculation for Tainung-2, Cuba 108, SF-459, El Salvador, and Everglades 41, respectively. Lesions ranged from 44 to 164 mm, with an overall mean of 124 mm for all five genotypes. No lesions developed in control plants. Although Everglades had the longest lesions, there were no significant differences (P < 0.05) among genotypes. Koch's postulates were completed by reisolating the fungus from all inoculated plants. To our knowledge, there are no published reports of Pythium group G causing stem or root rot of kenaf.
References: (1) M. W. Dick. Keys to Pythium. University of Reading Press, Reading, UK, 1990.
