Epidemiological and biological studies of Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense tropical race 4

Fusarium wilt of banana is caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. cubense (Foc). The ‘tropical’ race 4 (Foc TR4) strain of this disease is particularly devastating, as it is capable of infecting and consequently killing most commercial banana varieties including ‘Cavendish’, the dominant cultivar of trade worldwide. There are significant gaps in the knowledge of Foc TR4 which seriously hinders efforts to control the disease and its spread. Current management techniques predominantly involve limiting the spread of the pathogen within and between farms. In this research I have investigated aspects of the current management of Fusarium wilt including: the efficacy of sterilants used to minimise spread; the role of sorghum (currently used as a cover crop) as a potential host and the use of cassava as a biofumigant. Also investigated has been the role of insects as potential vectors of Foc; an aspect not previously considered in plantation management of Fusarium wilt. Also with a view to refining the diagnostic for this particularly aggressive strain of Foc, the presence of pathogenicity genes known in other formae speciales of F. oxysporum were investigated.This study demonstrated a surface sterilant which is effective in inhibiting the germination of Foc TR4 spores. Various conditions were assessed, including concentration, exposure time as well as effectiveness once the chemical had been exposed to heat and sunlight for various times; conditions which would realistically occur within a plantation in a tropical region. The sterilant Sporekill® was determined to be more effective than Farmcleanse® in inhibiting the germination of Foc TR4 spores and may be a more effective product to use in footbaths and for disinfecting machinery.Intercropping with cassava has previously been associated with a reduction in Fusarium wilt; in this study the direct effect of cassava root exudates on Foc TR4 was investigated. In vitro analysis of cassava root exudate showed an inhibitory effect on the growth of Foc TR4 and may have a similar effect in the field. Current farm management practices use sorghum as a cover crop in banana plantations following heavily infections of Foc TR4. In this study sorghum was shown to act as an alternative host for the pathogen. Thus, limiting the presence of sorghum, and other alternative hosts, may assist with the decline of the Foc TR4 inoculum level in infested soils. These findings will lead to an increased understanding of this disease enabling development of optimal control and containment strategies.This project provided an understanding of the pattern and method of spread of the pathogen on banana plantations. Field analysis determined that Foc TR4 spreads randomly throughout a banana plantation, rather than exclusively by plant-to-plant contact. Potential contributing factors to the spread may include insect vectors, such as the banana weevil borer, Cosmopolites sordidus. Analysis of these insects collected from a Foc TR4 infected plantation revealed that it is possible for C. sordidus to act as a vector for Foc TR4. Eradicating these insects may assist with the control and containment of thispathogen.The research presented here also revealed the presence of putative pathogenicity genes in Foc TR4 similar to that previously reported in F. oxysporum f. sp. lycopersici (Fol), the fungus that causes Fusarium wilt on tomato. A range of genes identified as ‘secreted in xylem’ (SIX), were analysed for their presence in Foc. Homologues of SIX1, SIX7 and SIX8 were found in pathogenic isolates of Foc, with SIX7 and SIX8 only found in race 4 isolates of Foc. The presence of these putative genes in Foc suggests they may play a role in pathogenicity and cultivar specificity and by that very role would be ideal targets for a molecular diagnostic. In the absence of genetic resistance, the most effective means of control of Foc TR4 is quarantine; an accurate diagnostic tool is therefore essential in implementing effective quarantine to minimise spread into new regions.