Apple trees are highly desirable by home gardeners for their spring beauty and delicious fruit. That fruit, however, can be disfigured or destroyed by any of several different types of apple diseases. Diseases that affect apple trees can affect not only the wood, leaves or flowers of the tree, but the developing apples as well. Luckily, many of these diseases can be treated before the apples are completely ruined, and some can even be prevented altogether.
Cedar apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. This fungus often travels from junipers to nearby apple trees. The fungus overwinters on cedar trees in galls, then travels on water (wind-blown rain) to nearby apple trees. Unusually wet springs increase the incidences of the disease.
The primary symptom of cedar-apple rust is the appearance of distinctive, bright orange specks or spots on the skin of young apples. The spots are slightly raised. As the apples ripen, the lesions merge and become brown. They may also crack open.
Cedar-apple rust can be prevented with regular applications of fungicide and by planting cedar and apple trees far from each other.
Fire blight is a highly destructive disease of apple trees, according to Wayne Wilcox, a horticulturalist with Cornell University. The disease is caused by the bacteria Erwinia amylovora. Apples infected with the bacteria become black and shrivel up. The apples also quit ripening, and cling to the tree rather than dropping to the ground. In some cases, the apples will ooze a thick, bacterial liquid.
This disease can be prevented with control measures that include choosing hardy rootstock and spraying young trees with a bactericide. Infected trees should be removed and destroyed to prevent the spread of the disease to nearby trees.
Apple scab is a fungal disease. The fungus, Venturia inaequalis, spreads on water; primarily wind-blown rain. Rainy weather, especially in the spring, can cause this disease to spread rapidly from tree to tree.
Olive-green lesions appear on developing apples, becoming larger and darker green, with a velvety texture, as the apples mature. The fruit becomes deformed and cracked, and drops from the tree before it is fully ripe.
Apple scab can be prevented by spraying the tree with a fungicide when the tree pinks out, when it blooms, when the petals fall, and again two weeks after the petals have fallen. Clear fallen leaves away from the tree in the fall, as the fungus can overwinter on the leaves and infect the tree again in the spring.