Kenaf (Malvaceae; Hibiscus cannabinus L.) is being commercially cultivated in Winterton, South Africa for its high-quality cellulose fibers with approximately 2,000 ha currently under cultivation. In 2004, 25% of 1-month-old kenaf plants grown from seed were observed in the field with severe wilting followed by lodging and mortality within 1 week. Isolations from diseased stem and root tissue on malt extract agar (MEA) consistently yielded Fusarium verticillioides (Sacc.) Nirenberg (2). Pathogenicity tests were conducted by inoculating kenaf seedlings with inoculum prepared from barley grains that had been colonized by the pathogen in vitro for 2 weeks prior to being finely ground in a laboratory mill. Fifty seeds from each of eight kenaf cultivars were incubated at 25°C on sterile filter paper to ensure germination and the absence of pathogens. Germinated seeds were sown in pots (400 cm3) containing steam sterilized loam soil (200 g) by placing 20 germinated seeds from each cultivar, with four replicates (5 seeds per pot), on the soil in each pot and covering them with 100 g of the same soil. Inoculum powder was sprinkled on the surface of the soil in each pot and covered by 100 g of soil. Pots were maintained in a glasshouse at an ambient temperature of 25°C. Sterile ground barley seeds served as the control treatment. Pots were watered daily with 20 ml of water and observed periodically for seedling emergence. The percentage of diseased seedlings was recorded after 3 weeks and the experiment was repeated. Wilting had occurred in 85% of seedlings when they were approximately 4 cm high and all diseased seedlings had died within 1 week thereafter. Subsequent examination revealed dark brown lesions girdling the stem and decayed roots in all instances. No symptoms developed on control plants. From means of combined data, the greatest seedling mortality was observed for cv. Gregg (65%) and the least for cv. Cuba108 (5%). Mean mortalities for the remaining six cultivars ranged from 30 to 55%. The pathogen was reisolated on MEA from all diseased seedlings. To our knowledge, this is the first report of F. verticillioides occurring on kenaf in South Africa. The only other report of Fusarium sp. causing serious damping-off of kenaf is from Iran (1). The potential impact of the pathogen on kenaf production in South Africa must be considered in the implementation of disease control measures.
References: (1) J. M. Dempsey. Kenaf. In: Fiber Crops. The University Press of Florida, Gainesville, 1975. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006.