Characterisation of a population of Fusarium oxysporum f sp vasinfectum causing wilt of cotton in Australia

Following the discovery of fusarium wilt in Australian cotton crops in 1993, isolates of Fusarium oxysporum f. sp. vasinfectum were collected from 6 cotton farms on the Darling Downs of Queensland. Using a range of procedures the Australian isolates could not be differentiated from each other, but they did differ from foreign isolates of the pathogen in a number of characteristics. Pathogenically, the isolates behaved similarly to race 6 of the pathogen when inoculated onto differential lines. Using aesculin hydrolysis tests, however, it was difficult to match local isolates with any of the known races. Additionally, none of the foreign isolates examined produced detectable volatile compounds when grown on a starch substrate, while all Australian isolates produced a distinctive odour during these tests. The local strain was not vegetatively compatible with any of the foreign isolates and belonged in a single, unique vegetative compatibility group. It is speculated that the Australian strain arose locally, perhaps from a minor population becoming prominent in response to aide-scale planting of highly susceptible cotton cultivars. These findings have significant implications for control of the disease and spread of the pathogen in Australia.