Apple seedlings (2 months old, 'Idared' × 'Golden Delicious') were inoculated with conidia of Venturia inaequalis in order to study the effects of inoculum dose and leaf wetness duration on development of apple scab symptoms. For each experiment, the C3 curve (indicating heavy infection levels) was used as the basis for relating infection to temperature and leaf wetness duration. In one series of experiments, seedlings were treated with inoculum doses of 1.5, 5.4, 15.6, 32.2, 81.2, and 250 × 103 conidia/ml and leaves were kept wet during C3 infection periods at temperatures of 6, 11, 16, and 22°C. At all four temperatures, disease incidence (scab lesions/plant) increased with increasing inoculum doses up to about 81.2 × 103 conidia/ml. Disease incidence was lower at 22°C than at the other temperatures. In a second series of experiments, seedlings inoculated with 10 × 103 conidia/ml were kept moist for infection periods ranging from 0.6 to 2.0 times the C3 leaf wetness duration curve at 6, 11, 16, and 22°C. Disease incidence increased with increasing duration of leaf wetness and generally leveled off between 150 and 200% of the C3 curve. At this inoculum dose (10 × 103 conidia/ml), doubling the leaf wetness duration indicated by the C3 curve resulted in high disease incidence, similar to levels obtained with a higher inoculum (250 × 103 conidia/ml) and shorter wetness period (1.0 C3).