Rapid progression of disease in susceptible and resistant banana cultivars inoculated with Fusarium oxysporum f. sp. cubense Race 1 and Subtropical Race 4 in tissue culture

In Australia, banana tissue cultures are produced within a Quality Banana Approved Nursery Scheme (QBAN) to eliminate spread or introduction of pests and diseases such as Fusarium oxysporum f. sp. cubense (FOC) which is the current main disease threat to the Australian banana industry. Tropical Race 4 (TR4) of this persistent soil borne fungus was recently found in the major production area of North Queensland and attacks the commercial Cavendish cultivar ‘Williams’. Growers want evidence that FOC will not be carried in banana tissue culture and will not be suppressed due to culture conditions or an immune cultivar. To determine how FOC would grow in banana tissue cultures, two-week-old banana tissue cultures growing in Murashige and Skoog media were inoculated with FOC under commercial conditions, to determine survival of plants and rate of disease development. Tissue cultures of cultivars ‘Lady Finger’, susceptible to Race 1 (R1) and Subtropical Race 4 (STR4), ‘Williams’, immune to R1 but susceptible to STR4, and ‘Goldfinger’ immune to R1 and highly tolerant to STR4 were inoculated with R1 and Subtropical Race 4 FOC. Within three days the Fusarium had grown to cover more than 50% of the culture surface and within 10 days 100% of FOC inoculated cultures were fully colonised and within 2-3 weeks all plants were dead. All non-inoculated plants remained healthy. Both FOC Races grew rapidly in tissue culture with fluffy white mycelia covering the surface of media and plants. Fusarium growth was not restricted in banana tissue culture, where growth was rapid and very obvious. In all cultures inoculated with any FOC Race, the banana plantlets were rapidly invaded and killed, regardless if that cultivar was resistant to FOC under field conditions. Fusarium can be easily detected in banana tissue culture and rapidly kills plantlets before completion of one subculture cycle