Often we assume that trees are impervious to disease because they appear so big and solid. However, trees can fall prey to a number of infections if not closely monitored and cared for. Being aware of the signs and symptoms of diseases in trees will help you keep your trees "happy" and healthy for years to come.
Check the foliage for spots. If the leaves have yellow, brown, purple or black spots, your tree may have a fungal infection. These spots may have black centers, or the middles of the spots may be rotted completely away. Your tree could have leaf-spot disease, shot-hole disease or another similar fungal infection.
Examine the twigs on your tree. If the leaves have blotchy, dead, discolored areas and the twigs are also distorted by cankers or blisters, then your tree likely has an anthracnose infection. Often anthracnose will cause leaves to fall off the tree as well.
Look for a black, powdery coating on any part of the tree. Molds and mildew, powdery mildew in particular, cover the tree with a black, gray, pink or white residue. In cases of powdery mildew, the residue appears chalky. In cases of mold, the residue may be thicker and slimier.
Monitor your tree for wilting. There are a number of types of root rot that are also called "wilts." As the roots of the tree rot away and are unable to take up water, the tree will appear to wilt as if it were in a drought. Water your tree in the early morning using a drip hose. If it does not recover with adequate water, then you likely have a wilt on your hands.