Apple Scab
Plant disease · Venturia inaequalis
Description
Apple scab is a fungal disease of apple trees caused by Venturia inaequalis. It also affects crabapples. The fungus survives winter mainly in fallen, infected leaves. In spring, rain releases spores that infect new growth, especially during extended periods of leaf wetness. Infected leaves develop olive-green to dark, velvety spots that may merge; leaves can yellow and fall early. Fruit develops dark, rough, corky patches. As apples grow, affected skin may crack or the fruit may become misshapen. The blemishes are mainly cosmetic, but severe infections can reduce fruit quality and weaken a tree through repeated early leaf loss. Warm, wet spring weather favors the disease, and susceptible varieties can develop new infections throughout a rainy growing season. Spots already present will not disappear when conditions improve.
Symptoms
- Spots on leaves
- Yellowing leaves
- Leaves dropping
- Cracked or split fruit
How to Identify
Look for olive-green, slightly velvety spots on young leaves in spring, often followed by yellowing and early leaf drop. On fruit, look for dark, corky, scab-like patches; growing fruit may crack around them. Finding characteristic spots on both leaves and fruit is a strong clue, particularly after a wet spring. Unlike insect feeding, the patches are not holes or bite marks. Unlike sooty mold, they do not simply wipe off the surface. Cedar-apple rust typically produces brighter yellow-orange leaf spots, often with structures on the leaf undersides. A local extension service can help confirm uncertain cases.
Prevention
Choose scab-resistant apple or crabapple varieties where possible. Each autumn, rake up and dispose of fallen leaves, or mow them finely so they break down faster; follow local guidance if composting diseased leaves. Prune during dormancy to improve light and airflow through the canopy, and avoid crowding trees. If irrigating, water the soil rather than wetting the foliage. Check trees regularly from budbreak onward, especially during rainy springs. In a garden with a history of severe scab, a preventive fungicide program may be needed for susceptible varieties. Applications must begin before or around the first spring infection periods, following the product label and local extension advice. Good sanitation and airflow reduce risk but may not prevent every infection.
Treatment
Start with nonchemical care: collect fallen infected leaves, improve airflow through appropriate pruning, and avoid wetting foliage when watering. Removing badly damaged fruit can improve the harvest’s appearance, but it will not cure infected leaves or stop spores already present. Existing leaf and fruit spots cannot be repaired. If scab has repeatedly been severe on a susceptible tree, consider a fungicide labeled for apple scab and home fruit trees. Fungicides work mainly by protecting healthy growth, so begin at the label-directed spring growth stage, before expected infection, and repeat only as the label and weather conditions warrant. Products containing sulfur or copper may be options where labeled, but both can damage plants under certain conditions; copper can also russet fruit. Follow all label directions, including temperature precautions, harvest intervals, and restrictions on combining sprays. Resistant varieties offer the most dependable long-term reduction in spraying.
Plants Affected
Not sure this is your problem? Try the problem diagnoser.