Abstract
Zoospores of Aphanomyces raphani were attracted to the hypocotyls of cabbage seedlings and the hypocotyl exudates. The attractant was identified and its effects on zoospores of three Aphanomyces spp. were investigated. Indole-3-aldehyde in the cabbage hypocotyl exudates was identified as an A. raphani zoospore attractant. This compound attracted at the concentration of 10-9mol., and zoospores massed within a few seconds at the open ends of capillary tubes containing 10-7-10-8mol. solution. This sensitive reaction might account for the accumulation of zoospores on the hypocotyls of cabbage seedlings. As indole-3-aldehyde did not attract zoospores of A. cochlioides and A. euteiches, the attractive effect was found to be specific to A. raphani. Compounds structurally related to indole-3-aldehyde had almost no effect on A. raphani zoospores at concentrations lower than 10-6mol.
