There are several reasons for apple leaves to curl. One of them is simply fall weather, and a natural part of the growth cycle of all deciduous trees. If leaves are turning color, curling or dropping in spring or summer, it can be a sign of a serious problem. The University of Kentucky at Kearneysville lists more than 25 diseases of apple trees that can affect its leaves, blossoms, bark and roots. Only a few of these bacteria, fungus and insects cause apple leaf curl as a symptom.
Apple Leaf Curling Midge
Unroll a tightly curled apple leaf with your fingers. If you see tiny, pinkish-orange larvae, you've got apple leaf curling midge (Dasineura mali). According to Washington State University, tiny female flies deposit eggs on the leaf surface. When they hatch, larvae eat the leaf margins, causing them to curl and eventually drop. They don't hurt fruit production, but can eventually hurt the vitality of the tree if it's deeply infested. Control apple leaf curling midge by pruning off and destroying affected foliage.
Brown, dying and curling leaves can be a sign of a dangerous bacterial disease called fire blight. Starting in the spring blossoms, it moves back into the shoots, leaves and twigs. Not only does the bacteria prevent fruit from forming, it often kills the tree. North Carolina State University recommends controlling fire blight with 1 teaspoon of streptomycin per gallon of water, sprayed during spring bloom.
Rosy Apple Aphid
Watch for apple leaves that turn bright crimson in spring. This is an early sign of rosy apple aphid, the worst of the several aphids that attack apple trees, according to the University of Kentucky. These aphids distort and curl leaves by injecting their digestive juices into leaf tissue. Small numbers of aphids can seriously damage an apple tree, and once the leaves curl and form a protective home for colonies, they are difficult to reach with sprays. Use a 1 to 2 percent solution of insecticidal soap spray or summer horticultural oil to control aphids.
Look for black specks and a grayish film on curling leaves, particularly on the leaf margins. This fungus is called sooty mold, and it usually follows aphid infestations, because it feeds on their sticky excretions, or "honeydew." Sooty mold doesn't harm the leaf other than blocking some light from its surface, and can easily be controlled if aphid populations are addressed with insecticidal soaps first.