Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum (Fov), is a destructive disease of cotton (Gossypium hirsutum L) in almost all cotton producing countries of the world. First reported in 1993, this disease is now widespread in Australia and is causing substantial losses. Previous studies identified two distinct genotypes in Australia (VCGs 01111 and 01112) that were morphologically distinct from the eight races of Fov found outside Australia, but prior to this study the origin(s) of the Fov in Australian cotton fields were unclear.

There are 17 native Gossypium species or wild cottons in Australia, some of which have ranges that overlap cotton-growing regions. Wild crop relatives are a traditional source of novel resistance genes for many plant diseases, and preliminary studies of Australian Gossypium species suggested they may contain some useful levels of Fusarium wilt resistance. At the same time, however, it was possible that the native species could be harbouring potential cotton pathogens. The main objectives of this project was to explore the risk and the potential of the Australian Gossypium species, specifically 1) to screen accessions of native Gossypium species for Fov resistance; 2) to determine if Fusarium wilt pathogens occur in native Gossypium populations; and 3) to investigate the genetic relationships between Fov causing the disease in Australian cotton fields and indigenous F. oxysporum associated with native Gossypium populations.

Screening the Australian Gossypium species identified a range of accessions that will be useful in the continuing efforts to new cotton cultivars with improved levels of Fusarium wilt resistance. Although there was considerable variation in Fusarium wilt resistance among the Australian Gossypium species, G. sturtianum emerged as a possible source of novel resistance genes. At the same time, a number of susceptible G. sturtianum accession were identified that can be used to generate segregating populations for genetic analyses. Future genetic analysis will assist cotton breeders by providing a clearer picture of how Fusarium resistance is controlled genetically.

Simultaneously, it has become clear that while the native Gossypium species are not harbouring cotton field pathogens, there are some of the native soil fungi of potential concern and continuing vigilance would be appropriate.

Fusarium wilt is an endovascular disease, and while endovascular fungi are commonly found in the stems of the wild Gossypium species, with only one exception, none of the 600+ isolates tested were related to cotton field pathogens. More importantly, pathogenicity trials of these endovascular fungi established that none of these isolates have the ability to invade cotton through the roots and cause wilt symptoms, and therefore are highly likely to give rise to new cotton field pathogens. It is unlikely that the native Australian Gossypium species are harbouring potential new pathogens that will impinge upon the Australian cotton industry in the future.

Surveys of soils from the native cotton populations and native vegetation in the Norwin—Boggabilla region, where Fusarium wilt was first detected, identified a range of diverse Fusarium oxysporum genotypes. Broadly speaking, these genotypes fall into one of five distinct lineages, designated A to E. Pathogenicity trials established that 14% of these Fusarium oxysporum genotypes could induce mild Fusarium wilt symptoms on cotton. While none of these isolates currently are virulent enough to cause plant death, isolates in lineage A have emerged as the closest known relatives of the cotton field pathogens, and it is now certain that the origin of the cotton field pathogens can be traced to native Fusarium oxysporum genotypes. Whether other Australian genotypes have the same potential to develop into new cotton field pathogens will be the focus of ongoing research.