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 ¾ 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.
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 ¼ inch green until 2 to 3 weeks after petal fall.
Conditions for infection
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 39°F 28 hours of continuous wetting is required for infection, while at 61 to 75°F, only 6 hours are required (See new updated tablein Tree Fruit Production Guide). After the fungus has penetrated, it continues to grow and enlarge beneath the cuticle. After 9 to 17 days (development occurs most rapidly at higher 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. Infection is rare above 78 degrees.
Sanitation can be effective for managing apple scab. Leaf tissue is necessary for the spores to survive and orchards are self-infecting since spores can travel about 100 feet.
To reduce the number of available overwintering spores present, apply a 5% solution of urea (46-0-0) in water to apple trees as leaves begin to fall in the autumn to aid in the breakdown of leaf tissue (42 lb urea in 100 gal. of water). Urea may also be sprayed on the leaves on the ground, after all of the leaves have fallen. Since nitrogen is being applied, one's fertilizer program needs to be adjusted accordingly. Shredding leaf litter using a flail mower or remove leaf litter by raking, sweeping, or vacuuming are additional options. Shredding leaf litter assists the decay of the plant material, as well as aids in the reorienting the leaves, thereby disrupting ascospore discharge.
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 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.
Growers are encouraged to rotate chemicals by FRAC groups when controlling for apple scab, as well as tank mixing with a broad spectrum fungicide such as captan or an EBDC.
The selection of scab-resistant varietiescan help reduce the need to control this disease. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.