Blotch of Apple
Blotch is caused by the fungus Phyllosticta solitaria, which can infect the fruit, leaves, and twigs of apple and crabapple trees. Only occasionally seen in Pennsylvania fruit orchards, this disease does not pose a very large problem for apple producers here.
Two types of leaf spots appear as a result of blotch. The less frequent appears between the leaf veins as a small, light-gray spot with a dark dot in its center. The more common leaf spots occur on the veins, midribs, and petioles (leaf stems) as long, narrow, slightly sunken, light-colored lesions. These contain several dark dots--the fruiting structures (pycnidia) of the fungus. When petiole infections are numerous, leaves might drop off.
New shoot infections look similar to petiole infections at first, except that they are longer and more visible. They occur at the juncture of the petiole with the shoot (node) or between the nodes. Once the lesion is established, it might continue to enlarge for 3 or 4 years, becoming noticeably larger than the diameter of the normal limb. In this manner, the organism causing apple blotch establishes itself in the tree.
Fruit infections vary in size from small, dark spots to large blotches that can cover much of the fruit surface. Edges of the larger lesions are irregularly lobed with many radiating projections. Large lesions often cause the fruit to crack.
The fungus overwinters in twig and limb cankers. The first infections in spring occur at about petal fall on leaves, young fruit, and new shoot growth and are caused by spores oozing from the cankers. Secondary infections from spores produced in the pycnidia can occur until late summer. Frequent rains and temperatures above 75°F favor the disease.
Routine fungicide applications normally will control this disease in Pennsylvania. Summer fungicide applications should not be extended beyond 14-day intervals. Control also is achieved by planting disease-free nursery stock and resistant varieties such as Delicious.