Population diversity of the Fusarium oxysporum f. sp. koae, the pathogen of Koa dieback in Hawaii

Koa (Acacia koa) is an important endemic Hawaiian tree species. Fusarium oxysporum f. sp. koae has been implicated in several recent and widespread dieback maladies of Koa in the Hawaiian islands. Although originally identified as a pathogen of Koa some 25 years ago in Hawaii, the recent outbreaks have caused considerable alarm. Samples of symptomatic Koa trees were collected from several locations on Hawaii. Isolates of Fusarium oxysporum were recovered from symptomatic tissue. Isolates were examined for pathogenicity on Koa seedlings in standardized inoculation tests. Isolates were also characterized based on colony morphology, vegetative compatibility, and mtDNA RFLPs, The majority of the isolates examined belonged to a single mtDNA RFLP haplotype and a single vegetative compatibility group (VCG). Also, isolates within the predominant VCG had a similar colony morphology. Isolates of F. o. koae are being examined further and also being compared to isolates from other leguminous hosts. These data may reveal if F. o. koae is an introduced or endemic pathogen to the Hawaiian islands.