Apple scab is caused by a fungus (Venturia inaequalis) that grows in the spring on apple tree detritus such as fallen leaves and fruit. As the weather turns rainy, and the humidity rises, the disease spreads. Apple scab attacks just about all parts of the apple tree, from foliage to twigs, and particularly the fruit. Leaves will eventually fall to the ground, the tree weakens and fruit and leaves develop olive green spots, then cracks. In the end, the fungus will overwinter in the litter on the soil around the tree, to begin the cycle again the following spring.
Determine which products are registered in your area for treatment of apple scab. A reputable nursery or your county cooperative extension office will be able to help you with this. Alternatives, according to plant pathologists at Cornell University, include products such as Immunox MPF, Greencure or Spectracide.
Don protective clothing, such as long sleeves, gloves and a breathing mask. Fungicides generally contain toxic chemicals.
Spray the tree, according to package directions, for the first time at the pink stage. This is at the end of winter dormancy when all of the blossom buds in the flower cluster are pink. Spray the tree again at bloom, at petal fall and then, finally, two weeks after petal fall.
Prevent apple scab by raking up and disposing all leaves and other debris in autumn. If your sprinklers get water on the tree's foliage, water in the morning so the leaves have a chance to dry before the coolness of the evening sets in.
Apple scab-resistant apple trees include Goldrush, Jonafree, Enterprise and Pristine.