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Apple Scab
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. 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 pedicel (fruit stem). 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 up to 3/4 inch in diameter. 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 scab.
Disease cycle
The apple scab fungus (Venturia inaequalis) 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. This period usually coincides with the time that elapses from 1/4 inch green until 2 to 3 weeks after petal fall.
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. At 40°F almost 48 hours of continuous wetting is required for infection, while at 65 to 75°F, only 10 hours are required (Table 2-1). 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. On its surface appear more spores (conidia), which are easily dislodged when the lesions are wet. The spores are splashed around 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.
Scab-control fungicides may be protectants or dual-action materials that combine both protectant and eradicant properties. Protectant fungicides, to be effective, must be present on the leaf or fruit surface before the spores land. Materials of this type are captan, ferbam, thiram, and wettable sulfur. If the germ tube has already penetrated the leaf or fruit, these materials will not stop infection. In contrast, an eradicant fungicide can be applied to infected surfaces after the infection has occurred and will stop the infection process. This property of certain fungicides is quite useful, as the grower may apply the fungicide a short time after the beginning of a rain period and still prevent infection. Procure, Rubigan, and Nova will control scab up to 96 hours after infection has occurred, but they have few protectant properties.