Attractants of Aphanomyces cochlioides Zoospores Contained in Sugar Beet Seedlings

Abstract

Zoospores of Aphanomyces cochlioides were attracted to hypocotyls of sugar beet seedlings and formed localized masses of encysted zoospores. From these sites of encystment, the zoospores invaded the plants. Sodium nitrate, potassium nitrate and sodium chloride were isolated from sugar beet seedlings as the attractant. The concentrations required for attraction ranged from 10^-3 to 10^-2 M. Tests using commercially available compounds indicated attraction could be attributed to nitrate (NO₃^-) and chloride (Cl^-) ions. The concentrations of these ions in hypocotyls were six and four times as high, respectively, as those in roots. The concentrations of these ions in exudates from hypocotyls and roots were almost the same. It is suggested that zoospores are attracted to hypocotyls by these ions and massed zoospores play a major role in the occurrence of damping off of sugar beet seedlings by A. cochlioides.