Assessing Variations in Host Resistance to f sp. Race 4 in Species, With a Focus on the Subtropical Race 4.

f. sp. () has severely curtailed banana production in the tropical regions of the world. The tropical race 4 (TR4) of 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 race 4 is urgently needed. To characterize sources of resistance, we have assessed the resistance response of 34 cultivars with plants grown under controlled settings. Amongst diploid banana cultivars carrying the AA genome, resistance is found in sub-species including 'Pahang' and 'Calcutta4.' In the polyploid group, the hybrids such as 'FHIA-18' and 'FHIA-25' are highly resistant against both -TR4 and subtropical race 4 (-STR4). Interestingly, 'FHIA-2' and 'CAM020' appear to be resistant to -TR4 but susceptible to -STR4, suggesting potential differences in the resistance mechanisms against the different race 4 strains. Using a GFP tagged -STR4 strain challenged onto both resistant and susceptible lines, a high inoculum dosage rapidly induced vascular wilt in the susceptible 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 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.