Host-pathogen relationships of Fusarium wilt of bananas

Abstract

This thesis is concerned with a new race of the banana wilt pathogen Fusarium oxysporum f. sp. cubense (FOC) which has the ability to infect Cavendish cultivars (Musa sp. ) previously resistant to all known races of the fungus. The purpose of this study was to gain more knowledge of the population biology of the fungus and to develop a reliable growth cabinet or glasshouse assay to screen for resistance in progeny generated by conventional or unconventional means. To study the genetic relatedness of Australian isolates of FOC, vegetative compatibility (heterokaryosis) was used to subdivide the populations of the pathogen. One hundred and forty-eight isolates were placed into six vegetative compatibility groups (VCGs) or genetically isolated populations. VCG correlated well with virulence on specific cultivars in the field. All putative race 1 isolates were members of either VCG 0124 or 0125 and, with one exception, isolates in these groups were only recovered from cultivars susceptible to race 1. Race 2 isolates were placed in VCG 0128 while all race 4 isolates were found to be members of either VCG 0120 or 0129. One isolate from the diploid SH3142, a product of the Honduran breeding program, was found to be incompatible with all other isolates. The race designation of this isolate is unknown. The abovementioned VCG analyses plus field surveys suggest that wilt in Cavendish cultivars in Queensland is due to the previous widespread distribution of Cavendish competent strains (VCGs 0120, 0129) in plantings of Lady finger which is the major Australian race 1 susceptible cultivar. Lady finger is also susceptible to race 4. Thus it does not appear that race 4 has evolved as a simple mutation from race 1. Race 4 isolates were not detected in north Queensland where more than 80% of Cavendish production is located. While reports exist claiming the reliability of root dip methods for resistance screening and race determination using one to two month old banana plants derived from tissue culture, this technique has not been successful using Australian isolates of FOC. In endeavouring to develop a reliable glasshouse assay to screen for resistance and race designation, various aspects of the host-pathogen relationship were investigated using plants derived from tissue culture. Studies concerned with the determination of the inoculum density and inoculum potential of FOC in naturally infested field soil were conducted and they showed that FOC makes up a large proportion (approximately 32%) of the total soil Fusaria in infested fields. The influence of several factors on disease development including temperature, light, inoculum concentration and plant age were also investigated.