Grown for their beautiful foliage and the shade they provide, maple trees (Acer spp.) struggle with several diseases. Growing in U.S. Department of Agriculture plant hardiness zones 2 through 10, depending on the species, the thin-barked, shallow-rooted trees are often attacked through wounds caused by yard maintenance. Proper care and wound prevention help keep maples healthy. It's important to identify diseases correctly when you do find problems, but you should also understand that it may already be too late to treat.
Several leaf diseases affect maples, but few cause significant damage. Fungal pathogens that attack stressed or wounded trees are usually to blame. Symptoms peak during wet, cool weather and minor leaf drop results. Anthracnose is the most serious. Symptoms include scorched-looking, red-brown leaf tissue accompanied by telltale brown fungi. To be effective, fungicide must be applied before the disease appears and must thoroughly cover all leaves, branches and twigs. On large trees, promoting good health and sanitation is more practical. Mix chlorothalonil-based fungicide concentrate at a rate of 2 1/4 teaspoons per 1 gallon of water, or according to the label rate, and spray all tree parts thoroughly. Repeat every seven to 14 days, as needed. Wear protective clothing and goggles when you spray.
Maple trees in urban landscapes are sometimes hit with verticillium wilt. Infected soil or pruners transmit the soil-borne fungus. The fungi block the transport of water and nutrients through the tree's vascular system. Buds fail to open, leaves look scorched and entire branches turn brown and die. Dark streaks show beneath the bark, and dark stains flow with the grain along infected branches. Diseased branches must be removed. Prune small branches with a sharp pruning saw sterilized with household disinfectant before and after every cut. Leave large branches to a professional arborist. Water the tree thoroughly, but limit fertilizer while it recovers. Affected trees remain stable when the branches are removed.
Unsightly calluses and sunken areas on maple tree trunks and bark are caused by canker diseases. Common to maples, the fungal pathogens enter the trees through wounds. The disease begins in bark layers, then extends into inner wood and often leaves oozing, wet areas on the bark. The cankers provide entry points for insects or other diseases and weaken the tree. In some cases, the canker-stricken areas can be cut out, but this is best done by an arborist. The resulting wounds can be more detrimental than the cankers. Severely affected trees should be removed. Minimize wounds and infections by always sterilizing pruning tools, taking care with mowers and weed trimmers, and promoting maple tree health.
Rots and Decay
Fungal rot diseases attack maple trees stressed by injury or environmental conditions and often enter through exposed wounds. The diseases reveal themselves when fungal fruiting bodies appear as mushrooms at the tree's base. There's no treatment at this point. Rot and decay diseases often contribute to what is known as dieback-decline in maple trees. In this condition, many pathogens combine to destroy maples slowly or suddenly. Dark, stringy fungal networks can be found under the tree bark and mushrooms proliferate around the roots. The diseases make affected maples unstable and they could fall on nearby buildings or people. Remove maples that are decayed and rotten as soon as possible.