Apple Scab Tree and Shrub DiseaseApple Scab Disease

Apple Scab Disease is caused by the fungus, Venturia inaequalis and it affects almost all fruit trees, including crabapples, purple plums, sand cherries and mountain ash trees. Disease development is favored by wet, cool weather that generally occurs in spring and early summer. Both leaves and fruit can be affected. Infected leaves may drop resulting in unsightly trees, with poor fruit production. This early defoliation may weaken trees and make them more susceptible to winter injury or other pests. The disease causes scab-like lesions on the leaves, causing them to brown, curl and fall prematurely.  The infected leaves, when left on the ground, re-infect the new growth in a continuous cycle throughout the season. The disease is most severe during a cool, wet and windy spring.  In the spring, the fungus in old diseased leaves produces millions of spores. These spores are released into the air during rain periods in April, May and June. They are then carried by the wind to young leaves, flower parts and fruits. Once in contact with susceptible tissue, the spore germinates in a film of water and the fungus penetrates into the plant. Depending upon weather conditions, symptoms (lesions) will show up in 9 to 17 days.
A timed, early spring and summer spray schedule is critical to managing the disease and will restrict the spread of apple scab lesions, helping the tree to maintain its aesthetic beauty. Regular pruning is also needed to control the disease, by allowing proper air flow through the tree. Any fallen leaves should be removed immediately.