ABSTRACT

Several methods were employed to detect and identify the pathogen in seed. Seed was plated on agar media in Petrie plates and fungi that grew from the seed were identified by microscopic examination, using taxonomic characters such as spore types and spore formation. Fungi identified as Fusarium oxysporum (Fo) in the cotton gin samples were subjected to further testing with DNA Amplification Fingerprinting (DAF) diagnostic tests developed specifically for Australian isolates of Fusarium oxysporum f.sp. vasinfectum (Fov).

The level of Fo was generally between 8 and 11% in seed from infected plants and from ginned cotton, harvested from known infested fields. Some of the seed samples yielded high levels of other Fusarium species, mainly F. equiseti. Fo was not detected in any of the seed, hand picked from healthy plants, even those growing in known Fov infested fields.

Fo did not persist in seed stored at room temperature for more than six months although other species of Fusarium persisted, but at lower levels than originally detected. The reason for this relatively short persistence of Fo is unknown and is somewhat surprising, as the fungus is known to survive in soil for many years. These results indicate that relatively short-term storage may prevent the dispersal of Fov in seed lines.

The apparent control of Fo in seed by metalaxyl, reported previously, was corroborated by results obtained when two different formulations of the product were compared as seed dressings. Both prevented isolation of Fo from seed. The efficacy of metalaxyl is surprising but, as it is routinely used as a component of the planting seed dressing used on cotton in Australia, it is likely to be a factor in reducing the risk of dispersing the pathogen in seed.

Commercial scale fumigation of fuzzy cotton seed stacks with methyl bromide eliminated Fo from the seed.