This thesis is concerned with the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc), which causes Fusarium wilt of banana. Although this disease is best known for its impact on the export banana trades, many important banana cultivars are also affected. The purpose of this study is to analyse isolates of Foc from Australia and Asia to enhance our understanding of the phenetic and genetic diversity of this fungus.
Isolates of Foc from wilted banana plants in Australia and Asia were characterised using vegetative compatibility analysis and production of volatile compounds on starch substrate. DNA analysis using the RAPD-PCR method, and pathogenicity were also used to characterise isolates of Foc from an outbreak of Fusarium wilt in Cavendish plants at Carnarvon, Western Australia.
Six vegetative compatibility groups (VCGs) were identified among 140 isolates of Foc from Australia. Eighty four isolates from Queensland and New South Wales belonged to the race 1 VCGs 0124 and 0125 and 44 isolates recovered from southern Queensland and northern New South Wales belonged to the race 4 VCGs 0120, 0129 and 01211. Race 4 was not detected in the major Cavendish production area of North Queensland. Twelve isolates which belonged to VCG 0128 were recovered from Kamerunga in North Queensland. Australian isolates of Foc in VCG 0128 have previously been described as race 2 since they have been recovered from ABB cooking bananas (Bluggoe and Blue Java). However, in this study, three of the isolates that belonged to VCG 0128 were recovered from race 1 susceptible plants (Sugar and Tuu Gia). Further analysis is required to determine whether these isolates represent a new race of Foc.
Gas chromatography was used to detect volatile compounds produced by isolates cultured on starch substrate (steamed rice). Isolates could be divided into either the "odoratum" or "inodoratum" group based on the presence or absence of volatile compounds. Absolute correlation was found between volatile production and race for isolates in race 1 (VCGs 0124 and 0125; "inodoratum") and race 4 (VCGs 0120, 0129 and 01211; "odoratum"). The "odoratum" isolates could be further divided as number of chromatogram peaks equated with VCG.
Twenty isolates of Foc from wilted Cavendish plants at Carnarvon in Western Australia did not belong to any previously described VCG, but did show vegetative compatibility amongst themselves and with 14 isolates from wilted windbreak plants (an unknown edible diploid) which had been lyophilised and stored in the Western Australia Department of Agriculture in the late 1960s and early 1970s. This unique VCG was assigned the VCG code number of 01220.
On the basis of volatile production and RAPD-PCR analysis, isolates in VCG 01220 were similar to Australian race 1 isolates. However, field observations and a glasshouse pathogenicity test using three-month-old banana plants, demonstrated that isolates in VCG 01220 differed from Australian race 1 isolates in their ability to attack Cavendish plants. In the field, isolates in VCG 01220 were able to systemically infect Cavendish plants, particularly where plants were growing in poorly drained soils which had been over-irrigated. The race classification of this population of Foe is uncertain.
VCG analysis and volatile production were also used to characterise 109 isolates of Foc from Asia, which represents the first comprehensive study of isolates from this region. Seven of the 16 previously described VCGs of Foc and four new VCGs were identified among the Asian isolates. Sixteen of these isolates were of unknown VCG and were not compatible amongst themselves. Volatile analysis was used to divide isolates of Foe from Asia into either the "inodoratum" or "odoratum" group. The gas chromatogram profiles for many of the Asian "odoratum" VCGs were very similar to those of Australian race 4 VCGs. Evidence to support the hypothesis that Foe originated in Asia and coevolved with Musa aeuminata and Musa halhisiana is presented.
Once the population structure of Foc at field screening sites in southern Queensland had been determined, the tetraploid banana FHIA 01, or "Goldfinger", from a breeding program in Honduras, was evaluated for resistance to races 1 and 4 Foc. FHIA 01 showed resistance to both races 1 and 4 Foe after 6-7 months and appears to be the first bred banana with commercial potential. Screening potential new varieties for disease resistance over a single season may not be adequate, however, and longer trials in multiple locations is recommended.
Results from a preliminary study conducted in subtropical southern Queensland to investigate the relationship between cold tolerance and resistance to race 4 Foe are also presented. In this study, the race 4 resistant Cavendish cultivar Dwarf Parfitt sustained less damage to photosynthetic mechanisms after periods of winter chilling than Cavendish cv. Williams. Dwarf Parfitt was also able to maintain its leaf area and a higher chlorophyll concentration than cv. Williams throughout the winter period. It was concluded from these results that a correlation did exist between cold tolerance and resistance to race 4, and that further investigations were warranted to examine the significance of this in relation to the susceptibility of Cavendish cultivars to race 4 Foc in subtropical climates.
This study contributes to our understanding of the biodiversity of Foc in Australia and Asia and provides evidence in support of the proposal that this pathogen coevolved with its diverse banana hosts in Asia. Results presented in this thesis also contribute to a better understanding of the complex host-pathogen interactions of this disease.