Fusaric Acid Production and Pathogenicity of Fusarium oxysporum f. Sp. Vasinfectum

Fusarium wilt of cotton caused by Fusarium oxysporum f. sp. vasinfectum (Fov) occurs in most cotton growing countries. The recent discovery of new pathotypes not previously found in the U.S. is of particular concern to the cotton industry. In addition, a virulent Fov biotype has been identified in Australia that can cause > 60% plant mortality. In 2002 /2003, several shiploads of cottonseed were imported into the U.S. from Australia as feed for dairy cows. We isolated 17 Fov isolates from about 17,000 of these seeds. The most virulent isolate belonged to race 3 based on molecular phylogenetic analysis and was vegetatively compatible with the Australian biotype. While the Australian biotype has not been found in U.S. fields, a newly discovered Fov race 4 is of increasing concern due to losses of Pima cotton in California. In contrast to race 1 isolates, which are often associated with root-knot nematodes and thrives in light textured sandy acid soils, race 3, race 4 and the Australian biotypes attack cotton seedlings without root-knot nematode in heavy alkaline clay soils.  They also produce prodigious quantities of fusaric acid (FA) when grown on Czapek media. FA is a potent phytotoxin especially to cotton and evidence implicated it in the pathogenicity of races 3, 4, and Australian biotype Fov isolates. We confirmed that FA is derived in part via a polyketide synthase (PKS) through isotope labeling studies. We identified and cloned a gene cluster containing the corresponding PKS gene and an amino acid kinase gene.  Targeted gene disruption of either of these genes in an Australian biotype Fov isolate resulted in complete blockage of FA production.  Bioassays were carried out with these FA knockout mutants to investigate the role of FA in the pathogenicity of the Australian biotype isolate.  Results from these studies will be presented.