SUN ETD - Theses and Dissertations

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Browsing SUN ETD - Theses and Dissertations by Author "Bailey, Wendy"

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    Investigating global spread of Fusarium oxysporum f. sp. cubense race 4 isolates
    (Stellenbosch University, 2024-12) Bailey, Wendy; Mostert, D.; Viljoen, A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    Fusarium wilt, caused by the soilborne fungus Fusarium oxysporum f. sp. cubense (Foc) is one of the most devastating constraints to banana production worldwide. The pathogen can persist in the soil for decades due to the production of resilient survival structures called chlamydospores. The first Fusarium wilt epidemic occurred in Latin America when the favoured Gros Michel banana succumbed to Foc race 1 infection. The disease was only managed when Gros Michel was replaced with the resistant Cavendish varieties. Foc Tropical Race 4 (TR4) is the cause of the second Fusarium wilt epidemic, currently severely affecting Cavendish production worldwide. Foc TR4 can also affect other popular dessert varieties, as well as cooking bananas that is important for food security. In the last two decades Foc TR4 has spread to 24 banana growing countries due to the global expansion of Cavendish production. Currently, no resistant variety is ready to replace the popular Cavendish banana. Therefore, prevention, strict biosecurity and regular surveillance is of utmost importance to contain further spread of this devastating pathogen. In this study, surveys were conducted in Laos, Vietnam, and Tanzania to determine the occurrence and spread of Foc TR4. Isolates were characterised with molecular detection tools and vegetative compatibility testing. In Laos, Foc TR4 was only present in large-scale Cavendish plantations and therefore, most probably imported with contaminated planting material from China. In Vietnam, the TR4 infected plants were found in small-holder Cavendish farms and backyard gardens with no direct link to Chinese growers. Pathogen spread was therefore attributed to the local and regional movement of infected plant material. Other VCGs identified in both countries on Pisang Awak included VCGs 0123, VCG 0124/5, VCG 01218 and VCG 01221. In Tanzania, Foc TR4 was not present and only Foc VCGs in races 1 and 2 were identified. Foc VCG 01214, which has previously only been reported in Malawi, was for the first time identified in Mtwara, Zanzibar and Ruvuma Provinces in Tanzania. Foc is divided into three different pathogenic races and more than 24 different vegetative compatibility groups (VCGs). The relationship between VCG and race is complex, with a single race linked with several VCGs and vice versa. Foc VCG 0120 populations is particularly confounding, as it is classified as Foc race 1 when it causes disease to Gros Michel in the tropics but is classified as subtropical race 4 (STR4) when it causes disease to Cavendish under subtropical conditions. In this study, phenotypic, genotypic and pathogenic variability between nine Foc VCG 0120 isolates from the tropics and subtropics were investigated. Foc VCG 0120 isolates were genetically homologous based on the core genes, but significant differences in growth rate, spore sizes and virulence were present among isolates. Foc VCG 0120 isolates caused more severe disease at 21°C than at 27°C, agreeing with the hypothesis that the ability to cause disease to Cavendish under subtropical conditions is linked to predisposition to cold stress.