Apple scab is Pennsylvania's most important apple disease, attacking wild and cultivated apple and crabapple. Early season disease management is directed primarily at controlling apple scab.

Symptoms

The first infections often occur on the leaves surrounding flower buds. Dull, olive-green areas visible on the undersides of leaves are the first evidence of the disease. By tight cluster, when the first leaves are mature, lesions occur on the upper surface of the leaf. As the lesions (infected areas) become older, they assume a definite outline as olive-green or brown circular spots. Leaves are susceptible to infection for about 5 days after they unfold. Severe early leaf infection can result in dwarfed, twisted leaves, which may drop later in the season. Early infection may occur on the calyx (blossom end of the fruit) or on the flower or fruit pedicel. Severe pedicel infection results in fruit drop. Fruit may become infected at any time in its development. Typical fruit lesions are distinct, almost circular, rough-surfaced, olive-green spots. Heavily infected fruits are usually misshapen and may crack and drop prematurely.

When leaf infection is active just before harvest, the fruit may become infected.These spots do not show at harvest time but develop slowly while the apples are in storage. This phase of apple scab disease is termed storage or "pinpoint" scab.

Disease Cycle

Apple scab is caused by the fungus Venturia inaequalis, which overwinters in infected leaves that have fallen to the ground. Fruiting bodies are produced within the dead leaf tissue. As spring approaches these begin to mature and produce spores (ascospores) that are discharged into air currents and carried to developing apple buds. The fruiting bodies in the fallen leaves must be wet for the spores to discharge. The ascospores are not all discharged with the first spring rains, for they mature over a 4- to 6-week period.

When the spores land on wet apple buds, leaves, or fruit, and if they remain wet for a few hours, they germinate and grow into the apple tissue. The time required for germination and penetration depends on temperature and the presence of a wet surface. After the fungus has penetrated, it continues to grow and enlarge beneath the cuticle. After 8 to 18 days (development occurs most rapidly at high temperatures) a visible scab lesion is produced. Scab spores form on the surface of the lesion and are easily dislodged when the lesions are wet. The spores are splashed by rain and blown by wind to new leaf and fruit surfaces within the tree. They germinate on wet surfaces, infect the tissue, and produce a new lesion. In this manner, several secondary infection cycles may occur in the course of a growing season.

Disease Management

Scab infections may be prevented by applying fungicides at regular intervals throughout the growing season. The object is to provide a protective coating that will inactivate any spores landing on the fruit and foliage. It is critical to control scab early in the season from bud emergence through the second spray after blossom petals fall (second cover period). If scab infection can be prevented during the time all the ascospores are discharged from the fruiting bodies in the fallen leaves, the disease cycle is broken and no further source of infection remains for the rest of the season. However, if the cycle is not controlled, and leaf and fruit infection does occur, then conidia are produced on these lesions and scab will remain a constant threat all season whenever wet weather occurs.

The proper selection of cultivars can help reduce the need to control this disease. Scab-resistant apple cultivars are available from most nurseries. These apple cultivars are not susceptible to apple scab, therefore no fungicide application is required to control apple scab. Orchard sanitation is also important in the prevention and spread of this disease. Keeping the orchard floor free of leaf litter aids in disease control.