Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Author "Bothma, Sheryl"

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    Mutation breeding of Cavendish bananas for resistance to Fusarium oxysporum f. sp. cubense tropical race 4
    (Stellenbosch : Stellenbosch University, 2021-03) Bothma, Sheryl; Viljoen, Altus; Rose, Lindy J. ; Mostert, Diane; Jankowicz-Cieslak, Joanna; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Bananas (Musa spp.) are among the world’s top ranking agricultural commodities and are cultivated in more than 120 tropical and subtropical countries. The crop provides a stable source of nutrition and household income to millions of people around the world. Among the hundreds of bananas cultivated, the Cavendish banana is the most popular, accounting for 47% of global production. Commercial Cavendish plantations are established from tissue cultured material and are cultivated in large monoculture systems. These cultivation practices leave the Cavendish banana susceptible to many biotic and abiotic stresses. Banana Fusarium wilt, caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc), is present in most banana-growing regions in the world. Foc is classified into three races based on its pathogenicity to different banana varieties. Foc races 1 and 2 cause disease to Gros Michel, several dessert varieties, and some cooking bananas like Bluggoe. Foc race 4 is divided into subtropical (STR) and tropical (TR4) race 4 strains, which affect Cavendish bananas in both the subtropics and tropics, respectively. Foc TR4 is considered a global threat to banana production due to its devastating effect on Cavendish and other banana varieties susceptible to Foc races 1 and 2. The resilience of its chlamydospores, which can remain viable for decades, makes Foc a challenging pathogen to manage. Prevention of the banana Fusarium wilt through phytosanitary regulation and the use of clean planting material is important. However, once the Foc has established in a field, the only reliable management option is the replacement of susceptible with resistant banana varieties. Currently, no resistant banana variety meets the requirements of growers and consumers to replace the Cavendish banana on the global export market. Resistance to Foc TR4 in banana can be obtained by conventional or unconventional breeding. Bananas developed by conventional breeding are often not popular with consumers due to the unfavourable change in its taste. Unconventional breeding has produced a genetically modified Cavendish banana resistant to Foc TR4, which is not accepted by the general public. Mutation breeding is, however, not considered genetic modification, and has successfully produced crops with improved yield, performance and disease resistance. In this study, mutation breeding was used to develop Cavendish banana mutants resistant to Foc TR4. Meristem cutting produced in vitro were first treated with a range of ethyl methane sulfonate (EMS) concentrations and gamma irradiation loads to determine their chemical toxicity and radio-sensitivity, respectively. Survival rate, growth characteristics and Fusarium wilt resistance were assessed after mutation induction, and the optimised dosages for bulk mutagenesis established as 1.5% for EMS and 20 Gray for gamma-irradiation. Twelve weeks after inoculation, one gamma-irradiated mutant and five EMS-derived mutants were identified as putatively resistant to Foc TR4. These mutants will in future be tested for field resistance to Foc TR4, agronomic performances and post-harvest fruit quality. If resistant with good agronomic traits, such a plant can be used to replace susceptible Cavendish bananas in fields infested with Foc TR4.