In darkness, most Erysiphe pisi conidia responded rapidly to contact with a hydrophobic artificial substratum and released extracellular material (ECM) in the same way as on pea cuticle. On this substratum and barley leaf epidermis, conidia then produced a germ tube that emerged close to the substratum, contacted it, and differentiated an appressorium. By contrast, on a hydrophilic substratum, ECM release and germination were delayed and infrequent, and germ tubes often emerged and faced away from the substratum toward vertical light, thereby failing to make contact and form appressoria. This finding supported the hypothesis that ECM release is involved in both triggering germination and sensing substratum contact. Exposure to white light dramatically affected the germ tube emergence site so most emerged from a site in the conidial wall facing the light. Lateral light did not affect the frequency of germ tubes making substratum contact; but when lit from above, most germ tubes emerged up, facing away from the substratum. The germ tubes formed in light were longer than those formed in darkness, but no phototropism was found for the elongating tubes. Examination of Blumeria graminis indicated that its conidia and germ tubes are insensitive to white light.