Addition of certain polyphenols to a liquid or solid basal medium induced wide differences in the growth rate of seven isolates of Phytophthora fragariae Hickman, representing three pathogenic races. The presence of gallic, proto-catechuic and chlorogenic acids, D-catechin and DL-tyrosine at a concentration of 0.001 M generally increased growth, whereas growth was restricted or inhibited by the same concentration of cinnamic acid, pyrocatechol and p-cresol. There was no similarity in the pattern of response to a given set of polyphenols either between isolates of the same pathogenic race, or between isolates from different pathogenic races; all isolates behaved differently, irrespective of source and pathogenicity. There were wide discrepancies between the patterns determined from measurements of radial growth on solid media and of dry weights of mycelia from liquid media.
Only very low levels of extracellular polyphenol oxidase activity could be detected in cell-free culture filtrates.