Anthracnose in maple trees is quite common. The cool, wet weather of spring often brings on the disease. Infected leaves develop small, irregularly shaped brown spots in the beginning of the growing season. As the disease and the season progresses, the spots grow and may eventually cover the entire leaf. Severely affected leaves eventually become distorted, curl up and drop. Anthracnose is not a severe maple disease, but it should be treated promptly by stopping the spread of the fungus. Recurring infections may reduce the tree's growth and leave it susceptible to other diseases in its weakened state.
Prune affected leaves by hand as soon as they begin to develop spots. These spots harbor the fungus responsible for anthracnose. Removing them early can stop the spread of the disease.
Rake any leaves from around the base of the tree. The fungus overwinters in these leaves. They should be burned or thrown away, but not composted. Sterilize the rake by wiping it down with alcohol after you use it.
Prune the maple tree to thin its crown. This will improve light penetration and air circulation which will make the leaves dry sooner. Anthracnose needs high levels of moisture to infect new tree buds. Small branches can be pruned with lopping shears, but larger ones should be sawed off. Wait until late fall, when anthracnose is dormant, to prune.
Fertilize your maple tree in fall. Anthracnose weakens maple trees, and they benefit from a dose of fertilizer after an attack. But you should wait until fall when the fungus is dormant. Stimulating new growth in the spring will simply provide more new growth for the fungus to feed on. Use a fertilizer prescribed for use on maple or deciduous trees. Follow the manufacturer's instructions for application rates and amounts.