Effect of leaf wetness duration, temperature, and conidial inoculum dose on apple scab infections

Apple seedlings (2 months old, 'Idared' x 'Golden Delicious') were inoculat ed with conidia of Venturia inaequalis in order to study the effects of ino culum 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 du ration. In one series of experiments, seedlings were treated with inoculum doses of 1.5, 5.4, 15.6, 32.2, 81.2, and 250 x 10(3) conidia/ml and leaves were kept wet during C3 infection periods at temperatures of 6, 11, 16, and 22 degrees C. At all four temperatures, disease incidence (scab lesions/pl ant) increased with increasing inoculum doses up to about 81.2 x 10(3) coni dia/ml. Disease incidence was lower at 22 degrees C than at the other tempe ratures. In a second series of experiments, seedlings inoculated with 10 x 10(3) 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 degrees C . Disease incidence increased with increasing duration of leaf wetness and generally leveled off between 150 and 200% of the C3 curve. At this inoculu m dose (10 x 10(3) conidia/ml), doubling the leaf wetness duration indicate d by the C3 curve resulted in high disease incidence, similar to levels obt ained with a higher inoculum (250 x 10(3) conidia/ml) and shorter wetness p eriod (1.0 C3).