The temporal distribution and magnitude of ontogenic resistance in strawberry leaves to Podosphaera aphanis has recently been quantified. However, the degree to which the pathogen is inhibited at sequential stages of conidial germination, hyphal growth, haustoria formation, latent period, colony expansion, and sporulation on the adaxial and abaxial leaf surfaces of various strawberry cultivars remains unclear. Five developmental stages of strawberry leaves ranging from newly emerged and folded leaves to fully expanded and dark-green leaves were inoculated with conidia of P. aphanis. The percentage of germinated conidia significantly declined between leaf stages 3 and 5. Postgermination growth of the pathogen was sequentially reduced in all measured responses, and the latent period was increased. Haustoria were not observed in mature leaves. The failure of the pathogen to penetrate mature leaves was a consistent feature associated with the expression of ontogenic resistance in older, fully expanded leaves.