This plant disease, also known as frog-eye leaf spot, is caused by a fungus (Botryosphaeria species) that also attacks pears. The fungus spends the winter in rotted apples and in cankers. When temperatures reach 60°F and higher in the spring, spores are produced on infected tissues and splashed by water to the foliage and fruit. Branches and trunks may be infected with black rot, especially when the bark has been weakened by sunscald, cold, or heavy shading. Pruning cuts also encourage fungal infection. As the fungus decays the wood, cankers form, weakening the branches and reducing the overall vigor of the tree. Fruit infection is generally more severe in warm, moist areas, and canker formation is more prevalent in cooler climates.
Spray with a fungicide containing captan according to label directions as soon as infestation is noticed. The next spring, repeat the spray when the new growth appears. Remove all rotted apples from the tree and ground and destroy them. Prune out infected branches at least 12 inches below visible cankers. Densely branched trees should be thinned to provide adequate light and air circulation.
Plant Care for Apple