Appressorium formation in vitro by Phytophthora palmivora started after about 1 h of incubation of zoospores in an aqueous solution and was influenced by topographical signals, substrate hydrophobicity and available nutrients. Free floating, non-adhering germlings did not form appressoria. On smooth substrates, no appressoria were formed under high nutrient conditions (20% pea broth) independent of the hydrophobicity of the contact surface, while under low nutrient conditions (5% pea broth) appressoria formed on smooth substrates with hydrophobic, but not on smooth substrates with hydrophilic, surfaces. If the same substrates were scratched, appressoria formed over these scratches independent of levels of exogenous nutrients and substrate hydrophobicity. These findings are summarized in a model of signalling for appressorium induction.