Characterization of diseases of kenaf (Hibiscus cannabinus L.) in South Africa

English: Kenaf is propagated by seed and thus its cultivation depends on good quality seed and uniform emergence of seedlings. The aim of chapter two of this study was to investigate factors that are related to the establishment of kenaf from seed. Consequently, a total of nine fungal genera were identified from surface disinfested kenaf seeds of ten cultivars. In all cases Alternaria spp. was the most common followed by Chaetomium spp. The cultivar, Whitten had the highest incidence with 60% of seeds contaminated. Of the fungal species isolated from nine genera, Fusarium subglutinans was considered to be the most potentially pathogenic. In the glasshouse trials, kenaf cultivars artificially inoculated with F. subglutinans produced disease symptoms which resulted in damping-off of seedlings. This suggests that the pathogen can be expected to influence kenaf production in South Africa. Kenaf seed treatment with ComCat® and thiram did not improve seed germination.A stimulatory effect of ComCat® on seedling emergence, fresh and dry root weight as well as fresh foliage of kenaf seedlings was not observed. ComCat® was nevertheless involved in the increase of dry weight of the above ground parts. The latter suggests that it has a potential role in improving dry matter production of kenaf. In the field trials, kenaf plants inoculated with Botrytis cinerea displayed brown necrotic lesions and girdling of the stem which resulted ill loding. There was variation in susceptibility between cultivars. Everglades 41 and SF 459 had the largest and smallest lesion lengths respectively. Surface wetness and temperature are important factors in epidemics of il. cinerea. It was found that optimum temperature for mycel ial growth of B. cinerea isolated from kenaf plants occurred between 15 and 20°C. Studies on the effect of irrigation on the incidence of grey mould, found no significant difference in grey mould incidence between three moisture regimes. Trials were conducted in vitro to determine possible fungicides thatwould be employed for the control of B. cinerea. Benemyl displayed the highest inhibition. Variation in sensitivity to benornyl was observed between B. cinerea isolates. The objectives of the fourth chapter were to characterize Pyhtium group G on kenaf in terms of its pathogenicity to kenaf cultivars, optimum temperature requirements and its sensitivity to selected fungicides. In pathogenicity trials, the artificially inoculated fungus colonized the cambial tissue of all ten kenaf cultivars. Reisolation of Pythium group G from artificially inoculated tissue confirmed its pathogenicity to kenaf plants. Seedl ing damping-off studies were conducted by artificially inoculating kenaf seedlings grown in pots in the glasshouse. Mortality of kenaf seedlings occurred rapidly but no significant difference in susceptibility was observed between cultivars. Growth studies conducted in vitro found that the optimum temperature for mycelial growth for Pythium group G ranged between 20-30°C. Screening of six fungicides was conducted in vitro to determine their inhibitory effect on radial colony growth of Pythium group G isolate. Dichlorophen and mancozeb/metalaxyl were found to be the most effective fungicides. Results of this study established that B. cinerea and Pythium group G are virulent to kenaf and could hamper its establishment as a new crop in South Africa. Satisfactory control of the pathogens could be achieved by the integration of cultural practices and chemical control. Effecti ve programs to monitor the distri bution of the pathogens and control strategies should be implemented to prevent serious losses to kenaf.