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Fusarium oxysporum f. sp. cubense (Foc) has severely curtailed banana production in the tropical regions of the world. The tropical race 4 (TR4) of Foc was detected in Australia in the 1990s and it is virulent to all Cavendish type banana cultivars, which represents the majority of banana production in Australia. Genetic resistance to Foc race 4 is urgently needed. To characterize sources of resistance, we have assessed the Foc resistance response of 34 Musa cultivars with plants grown under controlled settings. Amongst diploid banana cultivars carrying the AA genome, resistance is found in Musa acuminata sub-species including malaccensis ‘Pahang’ and burmannica ‘Calcutta4.’ In the polyploid group, the hybrids such as ‘FHIA-18’ and ‘FHIA-25’ are highly resistant against both Foc-TR4 and subtropical race 4 (Foc-STR4). Interestingly, ‘FHIA-2’ and ‘CAM020’ appear to be resistant to Foc-TR4 but susceptible to Foc-STR4, suggesting potential differences in the resistance mechanisms against the different race 4 strains. Using a GFP tagged Foc-STR4 strain challenged onto both resistant and susceptible M. a. malaccensis lines, a high inoculum dosage rapidly induced vascular wilt in the susceptible M. a. malaccensis lines at 2.5 weeks. This was associated with an accumulation of micro-conidia in the rhizome and the movement of the fungus through the xylem vessels. In contrast, the fungal movement was restrained in the rhizome of the resistant M. a. malaccensis lines and no sporulation was observed. Overall, this research suggests that the resistance response is dependent to an extent on inoculum dosage and that the plant host’s response, in the rhizome, plays an important role in inhibiting the fungus from spreading to the rest of the plant. Identifying race 4 resistant accessions can help to understand mechanisms of resistance and provide banana breeders with the genetic resources to integrate resistance genes into commercial varieties.

Fusarium wilt, caused by the fungus Fusarium oxysporum f.sp. cubense (Foc), is one of the most important and destructive diseases in banana crops worldwide. There have been numerous studies into the infection process of this soil-borne pathogen; however, the extent of research into the movement of the pathogen through the rhizome and into the rest of the plant is limited. Furthermore, little is known about the movement of the pathogen once it reaches the aerial components of the plant. A strain of Foc sub-tropical race 4, genetically transformed with green fluorescent protein (GFP) gene, was used to monitor the movement of the pathogen through two susceptible cultivars, Cavendish ‘Williams’ (Musa AAA) and Lady Finger (Musa AAB). Visualization of the pathogen in planta demonstrated its presence in the roots, the rhizome and in the outer leaf sheaths of the pseudostem prior to the appearance of external symptoms. Within the non-senescing leaf sheaths, the migration of Foc was confined to the xylem vessels; this included those where the vascular tissue was visibly discolored, as well as those, which were apparently healthy. As senescence of leaf sheaths occurred, chlamydospores developed within the gas spaces, while formation of sporodochia, and hyphal growth were apparent on the outer surface of senescing leaf sheaths. These results generate a greater understanding of the epidemiology of Foc, providing much needed knowledge to assist in the future management of Fusarium wilt incursions, as well as enhancing protocols for ongoing on-farm hygiene and biosecurity.