Apple scab occurs wherever apples are grown and may be a very serious disease on susceptible varities. The disease can also infect crabapple and mountain ash. Scab diseases similar to apple scab occur on pear, firethorn, and hawthorne. The scab-like leaf spots and fruit spots, from which the name was developed, may cause defoliation and reduction in fruit quantity and quality.
The disease may affect leaves, petioles, pedicels, fruit and twigs. The symptomatic spots are most noticeable on leaves and fruit. Infections first appear as olive-green spots with indefinite borders. With age, these spots become more prominent and darken to greenish-black with a velvety appearance (see picture on left). Severe spotting will cause leaves to senesce and fall off. Spots on young fruit result in deformation and cracking (see picture on right). If infection is severe, the fruit may drop off before ripening. Defoliation may result in a reduction of flower bud formation so that bloom or fruit yield the next year will be reduced.
This disease, caused by the fungus Venturia inaequalis (anamorph Spilocaea pomi), may be quite severe when rainy, cool weather occurs in the spring. Fungal spores are produced in early spring on dead, fallen apple leaves about the time buds begin to develop. These spores are splashed by rain and blown by wind to land on developing plant tissue and initiate infections. After spots appear on the newly formed leaves, more spores are produced that spread infection to other parts of the tree. Again, rainy weather greatly encourages spore spread and infection during the secondary phase of spore production. The fungus overwinters on fallen leaves.