CHARACTERIZATION 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 collecte d from 6 cotton farms on the Darling Downs of Queensland. Using a rang e of procedures the Australian isolates could not be differentiated fr om each other, but they did differ from foreign isolates of the pathog en in a number of characteristics. Pathogenically, the isolates behave d similarly to race 6 of the pathogen when inoculated onto differentia l 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 compound s when grown on a starch substrate, while all Australian isolates prod uced 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 t hat the Australian strain arose locally, perhaps from a minor populati on becoming prominent in response to aide-scale planting of highly sus ceptible cotton cultivars. These findings have significant implication s for control of the disease and spread of the pathogen in Australia.