First Report of Sclerotium rolfsii on Kenaf in South Africa.

Kenaf, Hibiscus cannabinus L. (Malvaceae), is being planted commercially in South Africa for the high quality cellulose fibers that it produces. In a January 2001 survey of 3-month-old kenaf plants grown from seed in experimental plots near Rustenburg, Northwest Province, 30% of plants were observed with severe wilting. Stems at ground level of all infected plants had sunken tan lesions, white mycelial strands, and small, dark brown, 1 to 2 mm diameter sclerotia. Isolations from diseased stem tissue on malt extract agar (MEA) consistently yielded a fungus conforming to the description of Sclerotium rolfsii Sacc. (teleomorph Athelia rolfsii (Curzi) Tu & Kimbrough). Pathogenicity tests were conducted by applying toothpick tips (5 mm) colonized by S. rolfsii on MEA to the stems of 120-day-old potted plants of 10 kenaf cultivars in the greenhouse. 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. Control treatments consisted of five plants per cultivar each wounded and inoculated with sterile toothpick tips. All inoculation points were wrapped using Parafilm, and the experiment was conducted twice. Lesions were measured after 10 days. Mean lesion lengths for the 10 cultivars were as follows: Dowling (34.9 mm), Cuba 108 (38.6 mm), Gregg (41.1 mm), Everglades 41 (44.2 mm), SF459 (44.9 mm), Tainung 2 (45.8 mm), El Salvador (45.9 mm), Whitton (46.1 mm), Everglades 71 (46.4 mm), and Endora (54.0 mm). The Newman-Keuls multiple comparison test revealed that cvs. Dowling and Endora were significantly more resistant and more susceptible (P < 0.05), respectively, than the other cultivars. Lesions did not develop on control plants. 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 (1). To our knowledge, this is the first report of S. rolfsii on kenaf in South Africa. Commercial plantings of kenaf in South Africa are expected to exceed 500 ha during the next 2 years, so its potential impact on kenaf production in this country will be significant if efficient disease control measures are not practiced. References: (1) J. M. Dempsey. Kenaf. Pages 203-304 in: Fiber Crops. The University Press of Florida, Gainesville, 1975.

First Report of Basal Stem Rot Caused by Pythium Group G on Kenaf in 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.

First Report of Botrytis cinerea on Kenaf in 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.