Apple Disease - Apple Scab

Symptoms

Apple scab can be observed on leaves, petioles, sepals, fruit and pedicels and, less frequently, on young shoots and bud scales. 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. 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 "pinpoint scab" or "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. During late winter and early spring, ascospores mature (primary phase). During the spring, when overwintered leaves on the orchard floor become wet from rain or heavy dew, ascospores are forcibly discharged and disseminated by wind and will initiate infections on new growth. 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. Mature ascospores peak from late pink through petal fall, which is the most critical time for protection during the primary phase of the disease. especially if ideal environmental conditions favoring disease are present.

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 table in 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°F.

Disease Management

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 percent 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 pounds of urea in 100 gallons of water). Urea may also be sprayed on the leaves on the ground, after all the leaves have fallen. If urea is not used, any ammonium-based nitrogen will be sufficient.  Since nitrogen is being applied, adjust your fertilizer program 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, preferably before wetting events. 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 cover period. The most important time during this period will be from late pink through petal fall. This will be the greatest threat for infection if disease conditions occur since the maximum amount of mature ascospores will be available. If disease conditions are favorable from late pink through petal fall, growers are encouraged to use complete sprays for greatest protection.  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 occurs, 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 ("spraying by the numbers"), as well as tank-mixing with a broad-spectrum fungicide, such as a rainfast mancozeb to minimize fungicide resistance. During very wet conditions early in the season, using a rainfast mancozeb or adding a spreader-sticker adjuvant to the tank mix when a non-rainfast mancozeb is used will allow mancozeb to persist a little longer despite heavy rainfall. This will allow a buffer time for reapplication of fungicides. The apple scab fungus is prone to fungicide resistance and fungicide resistance management is necessary.

The selection of scab-resistant varieties can help reduce the need to control this disease. Specific chemical recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.