Evidence for Host Mediation of Preinfection Stages of Phytophthora infestans on the Leaf Surface of Solanum phureja

Four experiments were carried out to explore the possible relationships between certain preinfection developmental stages of Phytophthora infestans and certain host attributes of the diploid potato species Solanum phureja. Zoospores attached and germinated in a random manner on all leaf surface structures (undifferentiated epidermal cells, veins, trichomes and stomatal complexes). Appressoria and hyphal ramification in epidermal cells occurred more frequently on leaflet tips than on the base of leaflets. This occurred both in susceptible and resistant host varieties. Regardless of location on the leaflet, appressorium formation and hyphal ramification in epidermal cells was more frequent when zoospores had landed near stomatal complexes. Germination of zoospores occurred with similar frequency on resistant and susceptible cultivars, but appressorium formation and hyphal ramification in epidermal cells occurred more frequently on susceptible accessions. Zoospore germ tubes were longer on resistant accessions. These results indicate that appressorium formation may be mediated by host stimuli, and that host–pathogen signalling for preinfection processes is dependent on the host geno