Fusarium wilt is considered by many growers, ginners, consultants and other industry personnel as the most important constraint to sustainable cotton production to have developed in recent years. The discovery of Fov in many new cotton districts sent shock waves throughout the industry. Districts which have had the disease confirmed from plant samples include: Bourke; the upper Namoi; Warren and Narromine in New South Wales and; St George; Moura and Theodore in Queensland. This project had a number of objectives to address knowledge gaps and obtain data to improve the management of this disease.

In excess of 660 specimens, suspected of being fusarium wilt, have been examined during the course of the project. Approximately 50% of these tested positive for Fov with most being strain VCG 01111 which was the original strain identified from the Darling Downs. No additional pathogenic strains have been identified to date. A reference collection of preserved strains of Fov is being maintained at the DPI&F laboratories at Indooroopilly. A database, which includes all records isolations of Fov made at the Indooroopilly laboratories, has been completed and is searchable under several fields such as, VCG, cotton variety, state, district or year. Fusaric acid production by various isolates of Fov does not appear to correlate with the virulence of Fov isolates in any way and acid production is not likely to be a useful tool to identify pathogen diversity.

Seed studies showed that Fov was not detected in any of the seed, hand picked from healthy plants in any season. Furthermore, in 2002 and again in 2004, Fo was not detected in seed hand picked from plants showing wilt symptoms in several localities in Queensland and northern New South Wales. The level of Fusarium oxysporum was generally low (8-11%) in seed from infected plants and from ginned cotton, harvested from known infested fields. Results from studies on persistence of Fov in seed showed that the fungus did not persist for more than six months when infected seed was stored in paper bags in the laboratory. Hence, it is unlikely that acid delinted, fungicide treated seed will transmit Fov and Dr David Nehl (NSW DPI) and Dr Joe Kochman (QDPI&F) have used these results to revise the cotton seed production protocols, should Fov be identified in seed production areas. This draft protocol has been circulated to Industry bodies for comment.

Many of the current strategies to manage the disease have been developed as a result of the project work carried out at Mr Graham Clapham’s property ‘Cowan’. The “Cowan” trial site is recognised by the cotton industry as a high disease incidence site, providing unbiased information on disease management practices. Some specific outcomes include: (i) the development of the ‘F rank’ system to allow growers to compare resistance of varieties to Fov, (ii) the identification of germplasm with resistance to the pathogen which has lead to breeding and release by CSIRO of Sicot F1, a variety with significantly improved resistance to Fov and with acceptable yield and quality characteristics and, (iii) the identification of some agricultural practices, such as planting date and crop rotations, that may reduce the incidence of the disease. The management strategies that have been developed have not increased pesticide usage.

A PCR based diagnostic assay has been developed to detect the two Australian strains of Fov. The TaqMan® assay format offers the following advantages over the gel-based diagnostic test for Fov: (i) increased specificity, (ii) increased sensitivity, (iii) faster sample processing (2-3 hours with no post PCR processing such as running gels), (iv) reduced risk of contamination, (v) quantitation, and (vi) it is amenable to high sample throughput. To date, 1706 samples have been tested, of which 455 were positive for Fov, 1246 were negative and 5 were cross-specificities (0.29%). Most importantly, there were no false negative results recorded – in all cases where fusarium wilt was known to occur, Fov was correctly detected and identified. All samples have been correctly diagnosed and results correlated with those from traditional identification based on morphology, vegetative compatibility and pathogenicity tests.