Holes in tree leaves can be an indication of several different tree problems, probably the most common of which is interference from insects and other tree pests. Insects very often feed on leaves and foliage, leaving behind feeding holes on different parts of the tree's anatomy. Look for excrement or honeydew on the leaves; also inspect the tree's twigs, branches and bark for exit holes made by boring insects. Tree-dwelling insects commonly breed on the underside of leaves, so look for nymphs or larvae underneath the leaves. Treat insects feeding on tree leaves with biological and natural measures first and with chemical insecticides as a last resort.
Holes in tree leaves could also indicate that your tree has been infected with a disease. Diseased leaves often turn yellow or brown and become brittle. Some leaves curl inward toward themselves and may fall prematurely. The diseases that infect trees are numerous, and each has its own unique symptoms. The best course of action is to research the most common tree diseases in your geographic area and the ones that most commonly infect the species of tree you grow and look for parallel symptoms alongside leaf holes to make an accurate diagnosis.
Winter Injury and Wind Damage
Another possible cause of holes in leaves in damage due to winter injury. Particularly if the species of tree is not winter hardy, it could suffer damage from a hard frost or high winds resulting in leaf holes. Inspect the leaves to see if they have turned brown or black around the edges and to see if they are beginning to curl inward. Wind damage typically gives leaves a torn or tattered look. You can do little to protect trees from winter and wind damage, but most trees recover from such damage as long as you keep them in good health.
Trying to diagnose a tree problem by a single symptom, such as holes in leaves, is difficult. As noted, there are myriad diseases that affect trees, and though each has its own unique symptoms, many share common symptoms as well. Learning the common tree problems that occur in your geographic area and the most common problems associated with the specific species of trees you grow is a good first step, followed by identification of parallel symptoms occurring alongside leaf holes.