Wetting and temperature requirements for infection of mature apples by Venturia inaequalis in South Africa*

Conidia of Venturia inaequalis were used to inoculate mature (picking ripe stage) Granny Smith fruit in greenhouse inoculation chambers and in the orchard. Wetting periods necessary for fruit infection were longer than those previously reported for leaves, while continuous wetting was followed by heavier infection than intermittent wetting. Temperatures during storage affected symptom development and the degree of infection was greatly influenced by the inoculum potential (number of conidia). At high inoculum potentials and with infection indices [fruit wetting period (h) × mean temperature (°C)] of 440–600, 601–1000 and >1000 subsequent disease development was ‘light’, ‘moderate’ and ‘heavy’, respectively if fruit was unprotected by fungicides. It was concluded that after conditions favouring infection, fruit should not be stored for long periods, but should be marketed for consumption immediately after harvest.