Answer: Cultural stress can cause curling leaves; be sure to water deeply once each week during the growing season. If you do not see aphids actively feeding on the leaves, the problem is probably not due to insect feeding but to a fungal disease. The most common on apple trees is caused by the fungi Sphaerotheca species or Podosphaera species and is most prevalent during dry, hot periods. New growth is often stunted, curled and distorted. Fruit may turn russet-colored and develop poorly.
If the disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apple trees and crabapple trees: thiophanate-methyl, basic copper sulfate, lime sulfur, wettable sulfur (minimum 80 percent), propiconazole (do not eat fruit) or triadimefon. Some of these chemicals can injure the tree if applied at the wrong time. Apply all chemicals according to directions on the label.
Best wishes with your apple tree!
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