When spots start appearing on your tree's leaves, this is usually sign of a fungal disease. Figuring out just what fungal disease is affecting your tree will help you determine how to best handle the situation. Follow this guide to figure out what fungus is infecting your tree.
Are the spots white and fuzzy?
If the spots on your tree's leaves are white (or pale green) and fuzzy, then downy leaf spot is probably to blame. Usually, the spots are round, though if two appear near each other they might look like blotches.
Downy leaf spot is caused by a fungus called Microstroma juglandis, and it most commonly affects hickory, pecan and walnut trees, though it can affect other species, too. The leaves that have developed spots will fall to the ground prematurely, and if you do not do anything to treat the disease, eventually some branches may die off.
Fungicides are not overly effective for treating this disease. The best way to deal with downy leaf spot is to collect all fallen leaves, nuts and branches, and burn them. The fungus over-winters in fallen leaves, so by removing these, you are interrupting its life cycle. Keeping your tree well watered and trimmed will also ensure it stays strong enough to fight off the infection.
Are the spots small and black?
Small black spots are typically caused by a disease called anthracnose. Caused by a number of related fungal species including Discula fraxinea and Stegophora ulmea, it can affect almost any species of tree.
Anthracnose does not usually cause serious harm to trees, but it is important to treat it since it can weaken the tree and leave it more prone to other more serious diseases. Having the tree sprayed with fungicides should help, as will cleaning up any fallen leaves. In some severe cases, anthracnose can cause cankers (sores) to develop on branches. If this is occurring, having these branches trimmed away is important since the cankers harbor the fungus.
Anthracnose and downy leaf spot are the most common fungal infections that cause spots on tree leaves. Both happen to be pretty harmless, though they can leave the tree susceptible to more serious infections. If you notice that entire branches seem to be dying or that large brackets are forming on the trunk, contact a tree services expert—your tree probably has a more serious ailment and will require more intensive treatment.