Department of Plant Pathology, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
ARC Institute for Industrial Crops, Private Bag X2075, Rustenburg 0300, South Africa
Kenaf (Hibiscus cannabinus L.) (Malvaceae) is a source of high-quality cellulose fibers and is being investigated in South Africa with a view to commercial production. In April 2001, 20 to 30% of 5-month-old kenaf plants grown from seed in experimental plots near Rustenburg, Northwest Province, South Africa, were affected by gray mold caused by Botrytis cinerea Pers.:Fr. Infected plants displayed brown necrotic areas that girdled the stem, resulting in wilting and lodging in at least 50% of observed cases. Symptoms included extensive growth of mycelia and gray conidia on stem lesions. Microscopic examination revealed hyaline, one-celled conidia and conidiophores conforming to the description of B. cinerea. Plating of diseased stem tissue on malt extract agar (MEA) consistently yielded B. cinerea. Koch's postulates were satisfied by applying toothpick tips (5 mm) colonized by B. cinerea on MEA to the stems of 10 120-day-old greenhouse-grown plants of each of five kenaf cultivars. A colonized toothpick tip was placed on the stem of each of five plants per cultivar at a point ≈15 cm above soil level. Another five plants of each cultivar were wounded once using a sharp dissecting needle, and a colonized toothpick tip was placed on top of each wound. Corresponding control treatments consisted of five additional plants per cultivar, each wounded and mock-inoculated with sterile toothpick tips. Inoculation points were wrapped in Parafilm. The experiment was conducted twice. Developing lesions were measured after 7 days. Mean lesion lengths for the two treatments, nonwounded and wounded, on the five cultivars were, respectively: 32.4 and 35.2 mm for Everglades 41; 14.9 and 53.8 mm for Cuba 108; 39.5 and 55.8 mm for El Salvador; 19.0 and 44.3 mm for SF459; and 12.4 and 43.9 mm for Tainung 2. The Newman-Keuls multiple comparison test revealed no significant difference (P < 0.05) in means among cultivars for the wounded treatment. For the nonwounded treatment, Everglades 41 and El Salvador were significantly more susceptible (P < 0.05) than the three remaining cultivars. No lesions developed on control treatments. The fungus was reisolated on MEA from all artificially inoculated plants. The pathogen is reported to cause serious losses in yield and fiber quality of kenaf in Spain (1). This is the first report of B. cinerea on kenaf in South Africa, and its potential impact on kenaf production in this country should be taken seriously.
Reference: (1) A. De Cal and P. Melgarejo. Plant Dis. 76:539, 1992.