Apple Tree Scab is caused by the fungus Venturia inaequalis. Symptoms of Apple Scab appear in the spring as small dark, olive green spots on the leaves. A severe infection may cause the entire leaf to turn brown and (drop)from the tree. The fruit becomes marred by slightly raised black spots.
Planting scab-resistant varieties is the easiest way to avoid Apple Scab. They include Pristine, Williams' Pride, Redfree, Nova Easygro, Liberty, Freedom, Enterprise,
Controlling Apple Scab
As with other Apple tree pest insects and diseases, you should take any preventive action until you know you have a problem be it insect or disease. If last year you did not have any apple scab on your trees and fruit then there is no need to do anything except keep the 3 to 4 inches of organic mulch around each tree 365 days a year.
If last year your trees suffered some apple scab, then here is what you do:
Late February - Apple Tree Scab can be controlled organically with a dormant oil spray applied in later winter when the air temp is above 40F. Dormant oils are highly refined oils that will smother the Apple Scab fungus.
Week Before Bloom Starts – Spray tree the new biological fungicide Actinovate. Wait three weeks for bloom to be finished and begin a bi-weekly spray program with Actinovate till harvest time.
Groundfalls and leaf litter should always be removed from beneath the trees to deprive the disease of a place to spend the winter.