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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.
Symptoms
Two types of leaf spots appear as a result of blotch. The less frequent occurs on the leaf between the 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 may drop off.
New shoot infections at first look similar to petiole infections, except they are longer and are seen more easily. They occur at the juncture of the petiole with the shoot (node) or between the nodes. Once the lesion is established, it may 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 may 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.
Disease cycle
The causal fungus remains alive throughout the year in the twig and limb cankers. The first infections in spring occur about petal fall on leaves, young fruit, and new shoot growth from 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.
Disease management
Routine fungicide applications will normally control this disease in Pennsylvania. Summer fungicide applications should not be extended beyond 14-day intervals.