Fusarium wilt of cotton is caused by the soil-borne fungus Fusarium oxysporum f. sp. vasinfectum (Fov). In Australia, 2 Fov strains are currently responsible for the disease, each belonging to a distinct vegetative compatibility group (VCG). Previous work showed that the 2 Australian Fov strains are genetically related to lineage A of native Fusarium oxysporum, suggesting their local origins.

This project aims to: 1) assess the risk of novel Fov pathotypes emerging from local Fusarium oxysporum population in cotton fields; 2) investigate the evolutionary potential for weakly pathogenic lineage A isolates of native Fusarium oxysporum to become more virulent; and 3) determine the competitive ability of different strains or genotypes of Fov.

Our work showed that Fusarium oxysporum present in the soil of cotton fields consists of 3 major lineages, A, B, and E. The presence of lineage A highlights the likelihood that new strains of Fov may emerge in the future. In addition, the difference in lineage composition between uncultivated soils and cotton fields is noticed, with the dominant form of Fusarium. oxysporum being lineage E in cotton fields but lineage B in uncultivated soils. The evolution of virulence in Fov was studied by carrying out the inoculation – re-isolation – re-inoculation infection cycles successively on a susceptible cotton cultivar. A mildly pathogenic isolate of lineage A became more virulent, causing more severe disease symptoms after 10 infection cycles, and importantly, correlated genetic changes were detected in its offspring isolates. This suggests that weakly pathogenic Fusarium oxysporum isolates of lineage A can become virulent Fov and continuous exposure to susceptible host plants plays an important role in this process.

Based on 350 isolates collected from 6 cotton growing regions in 2002 and 2004, 28 genotypes of Fov were identified, with 21 in VCG 11 and 7 in VCG 12. VCG 11 can be further divided into 2 subgroups and subgroup I-B may represent a new strain of Fov as all members of this subgroup are incompatible with either known Australian VCG. The occurrence of new Fov strains in cotton fields was also detected in other regions like Mungindi. VCG 11 is wides-spread, while VCG 12 is restricted to the Boggabilla area. No genetic structure was found among Fov populations at broad geographic scales, but interestingly some structure was found in the Boggabilla area, with the population in two fields being different from the others. In addition, variation in virulence was observed among different genotypes of Fov.

Field surveys showed that Fov populations may change over time because in one Boggabilla field the dominant Fov genotype was 11-A in 2002 but 11-B in 2004 and, furthermore, genotype 11-A was undetectable in 2006. This change is probably driven by the increasing level of resistance in newly released cotton cultivars planted in the field as our glasshouse trials showed that genotype 11-B is more aggressive than genotype 11-A on the tolerant cultivar, suggesting competitions between strains or genotypes of Fov.

Fusarium oxysporum occurs at comparable levels in the soil of cotton fields (Fov infested or free) in the Darling Downs, Boggabilla, and Moree, however, a clear declining tendency was observed in the frequency of Fov in Fusarium. oxysporum population from the Darling Downs soil to Moree soil. Future studies will focus on soil impacts on Fov since, as a soil-borne fungus, both aggressiveness and saprophytic ability are inevitably influenced by soil biotic and abiotic factors. Insights into how life history traits of Fov and soil factors interact to determine selection for virulence and persistence of the pathogen would aid developing effective disease management strategies.