The 2012 ASHS Annual Conference

Fusarium Wilt is a lethal, vascular wilt disease of palms. It is caused by the fungus Fusarium oxysporum f. sp. palmarum (F. o. palmarum) in Washingtonia robusta and Syagrus romanzoffiana. Currently, there is no treatment for this disease. This study was conducted to determine if F. o. palmarum has in vitro sensitivity to phosphite and to compare the results with greenhouse trials to determine if phosphite has direct and/or indirect effects on the pathogen in W. robusta. We tested two isolates of this fungus: one isolated from W. robusta (PLM-249A) and one isolated from S. romanzoffiana (PLM-140B). Seven concentrations (0, 1, 5, 10, 20, 40, or 60 µg·mL^-1) of potassium phosphite (phosphite) were used to evaluate their effects on in vitro fungal growth, using 1/5-strength potato dextrose agar (PDA) as the base medium. Growth of both fungal isolates consistently decreased as the phosphite concentration was increased. Growth suppression of the two isolates was similar at 1, 5, 10 and 20 µg·mL^-1, but PLM-140B had greater growth suppression than PLM-249B at 40 and 60 µg·mL^-1. Forty greenhouse-grown Washingtonia robusta seedlings were treated with soil drenches or foliar sprays of phosphite, each at concentrations of 2.64 ml·L^-1 and 5.28 ml·L^-1. Five weeks later they were inoculated with F. o. palmarum to determine the efficacy of phosphite in controlling Fusarium wilt in vivo.