Photo courtesy of Morton Arboretum

Apple scab is a disease of Apple, Crabapple, Pear, Mountain Ash, Pyracantha, and other members of the rose family. Development  is favored by wet, cool weather that generally occurs in spring and early summer.            

Damage

Both leaves and fruits are affected. Symptoms first appear in the spring as lesions(spots) on the lower leaf surface, the side first exposed to fungal spores as buds open. This is followed by early defoliation. This early defoliation weakens trees and makes them more susceptible to other problems.            

Cause and Development

Apple scab is caused by a fungus. This fungus survives the winter in the previous year’s diseased leaves that have fallen under the tree. In the spring, the fungus in old diseased leaves produce million of spores. These spores are released into the air during rain periods in April, May and June. They are 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. The disease continues to develop and spread throughout the summer. Because a film of water on leaves and fruit is required for infection to occur, apple scab is most severe during years with frequent spring rains.            

Control

The primary form of control for apple scab is done with fungicide sprays. Timing and proper application technique are essential. To ensure coverage of newly emerging tissues and to replace the chemical as it is lost to weathering, fungicides must be re-applied on a regular schedule. This consists of at least three(3) treatments, spraying every 7 to 20 days as weather permits during springtime. There are some methods involving direct trunk injection in the fall for control the next year but their effectiveness is yet to be determined.            

Other considerations

Applying foliar fungicide applications as a preventative treatment for trees which have a history of apple scab is very important for their health and beauty. There are other things you can do that can help them:            

  • Proper health care such as mycorrhizae, beneficial bacteria and fertilizer treatments when necessary.
  • Supplemental watering during dry periods keeping in mind not to wet the foliage as this can contribute to spreading the fungus.
  • Pruning to allow more air and light into the interior of the tree.
  • Picking up leaves that have apple scab to help keep the fungus from spreading.
  • Proper mulching.