This plant disease is caused by a fungus (Venturia inaequalis). It is one of the most serious diseases of apples in areas where spring weather is mild (60° to 70°F) and wet. The fungus spends the winter in infected leaf debris on the ground. In the spring, spores are produced and blown by the wind. If there is adequate moisture on the foliage and fruit, the fungus infects them, and spots develop. The infected tissues produce more spores that infect other leaf and fruit surfaces, where further spotting and decay occur. As the temperature increases, the fungus becomes less active. However, late-summer rains initiate additional spore production, and apples that are infected when they are almost ready to be harvested will develop scab lesions after being picked.
Unless severely infected, the apples are edible. To prevent recurrence of the disease next year, remove and destroy leaf debris and infected fruit in the fall. Next spring, spray with a fungicide containing captan or triforine.
Plant Care for Apple