Some of the most common diseases affecting apple trees include apple scab and powdery mildew. Apple scab appears as warty, brown lumps on the fruit and leaves, while powdery mildew is a velvety covering on the tree's branches and leaves.
Another common apple tree disease is black rot. This disease comes in three different types, including one that affects tomatoes and starts where the fruit blossoms, eventually spreading all across the fruit. Another iteration of black rot is frogeye leaf rot, which appears as gray or light brown spots on the leaves just as the tree's blossoms start to fade. A third type of black rot is black rot limb canker, which appears as depressions in the limbs of the tree; if left untreated, it can kill the tree.