Trees are an integral part of most landscaping, and many homeowners take them for granted because they seem as solid as, well, a tree. However, your trees are living things, and they can succumb to illnesses just like flowers, pets and even you. The best way to keep your trees "happy" and healthy is to monitor them closely for signs of trouble. Usually, the first signs of trouble will manifest themselves in your tree's leaves.
Leaf spots occur on nearly all leafy trees, and a variation can occur on evergreens as well. Leaf spot is caused by a fungal infection that attacks the "flesh" of the leaf and eats away at it. Initially this infection causes brown, black, purple, orange or yellow spots on the leaves. If the condition is not treated, these holes will wear through, leaving "shot-holes" in the leaves and eventually resulting in defoliation and even killing branches of the tree. Generally leaf spot problems can be controlled by sterile pruning and disposal of the infected debris rather than allowing it to lie on the ground near the uninfected plants. If the leaf spot has become too severe, however, you may need to treat it with a fungicide specific to the type of tree it has infected.
Blisters form on leaves as a result of an infection or a malfunction. They may look similar to human blisters or they may appear "corky" or look like warts. They can be the result of a fungus, in which case removal and disposal of affected foliage and contain the problem. If the blisters are a result of a condition called oedema, in which the roots of the tree take up more water than the tree can handle, creating blisters on stems, leaves and flowers, then you will need to control the amount of water the tree receives to the best of your ability until the condition passes.
Leaf scorch occurs when a tree's roots are unable to supply sufficient water to the leaves. This symptom can indicate root rot, or it can be a result of particularly hot or windy conditions. The use of road salt can also lead to leaf scorch. The outsides of the leaves turn brown as if they had been singed, and sometimes entire leaves will dry up. You can control leaf scorch by planting trees in groups in exposed areas and watering young trees deeply.