Technical Abstract: Initiation of asexual sporulation in powdery mildews is preceded by a period of superficial vegetative growth of mildew colonies. We found evidence of signaling in Erysiphe necator that was promulgated at the colony center as early as five days after inoculation and stimulated sporulation throughout the colony. Removal of the colony center after putative signal promulgation had no impact upon timing of sporulation by young (48-¬hr-¬old) hyphae at the colony margin. However, removal of the colony center before signaling nearly doubled the latent period. A relationship between inoculum density and latent period was also observed, with latent period decreasing as the number of conidia deposited per mm2 was increased. The effect was most pronounced at the lowest inoculum densities, with little decrease of the latent period as the density of inoculation increased above 10 spores/mm2. Light was furthermore shown to be necessary to initiate conidiation of sporulation-competent colonies. When plants were inoculated and maintained in a day/night cycle for 36 hr, but subjected to darkness after 36 hours, colonies kept in darkness failed to sporulate for several days after plants kept in light had sporulated. Once returned to light, the dark-suppression was immediately reversed, and sporulation commenced within 12 hr. These results suggest that after a period of vegetative growth, light is required for promulgation of a quorum sensing signal that triggers synchronous conidiation throughout an E. necator colony.