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

[摘要] English: Kenaf is propagated by seed and thus its cultivation depends on good quality seedand uniform emergence of seedlings. The aim of chapter two of this study was toinvestigate 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 tencultivars. 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 thefungal species isolated from nine genera, Fusarium subglutinans was considered to be themost potentially pathogenic. In the glasshouse trials, kenaf cultivars artificially inoculatedwith F. subglutinans produced disease symptoms which resulted in damping-off ofseedlings. This suggests that the pathogen can be expected to influence kenaf productionin 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 wellas fresh foliage of kenaf seedlings was not observed. ComCat® was nevertheless involvedin the increase of dry weight of the above ground parts. The latter suggests that it has apotential role in improving dry matter production of kenaf.In the field trials, kenaf plants inoculated with Botrytis cinerea displayed brownnecrotic lesions and girdling of the stem which resulted ill loding. There was variation insusceptibility between cultivars. Everglades 41 and SF 459 had the largest and smallest lesionlengths 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 isolatedfrom kenaf plants occurred between 15 and 20°C. Studies on the effect of irrigation on theincidence of grey mould, found no significant difference in grey mould incidence betweenthree 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 interms of its pathogenicity to kenaf cultivars, optimum temperature requirements and itssensitivity to selected fungicides. In pathogenicity trials, the artificially inoculated funguscolonized the cambial tissue of all ten kenaf cultivars. Reisolation of Pythium group G fromartificially inoculated tissue confirmed its pathogenicity to kenaf plants. Seedl ing damping-offstudies were conducted by artificially inoculating kenaf seedlings grown in pots in theglasshouse. Mortality of kenaf seedlings occurred rapidly but no significant difference insusceptibility was observed between cultivars. Growth studies conducted in vitro found thatthe 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 radialcolony growth of Pythium group G isolate. Dichlorophen and mancozeb/metalaxyl were foundto be the most effective fungicides. Results of this study established that B. cinerea and Pythium group G are virulent tokenaf and could hamper its establishment as a new crop in South Africa. Satisfactory controlof the pathogens could be achieved by the integration of cultural practices and chemicalcontrol. Effecti ve programs to monitor the distri bution of the pathogens and control strategiesshould be implemented to prevent serious losses to kenaf.

[发布机构] University of the Free State