Popular landscape trees along the coasts and in the central United States, red Japanese maple trees can develop several fungal diseases, though most will not severely harm the tree. Since fungal diseases live on through winter, the best disease safeguard is to remove fallen leaves when the tree sheds its leaves in autumn. Learn to distinguish symptoms of major diseases in order to diagnose your tree's problem.
Japanese red maples are susceptible to leaf blight caused by the fungus Kabatiella. This often occurs in conjunction with anthracnose, a separate fungal disease. Symptoms of leaf blight include circular reddish brown lesions and blotches on the leaves. Japanese maples can lose their foliage prematurely as a result of leaf blight.
Anthracnose can be caused by several different fungi, including Discella, Discula, Monostichella and Kabatiella. Japanese maples that get Kabatiella-caused anthracnose display symptoms of leaf blight in addition to trunk symptoms, but the other fungi do not lead to leaf blight. All types of fungi cause round fruiting cysts to form on plant leaves. Warm weather combines with rains cause anthracnose outbreaks. Gardeners can limit the spread of anthracnose by raking fallen leaves and disposing of them properly, spraying trees with a fungicide designed to control anthracnose, and irrigating and fertilizing trees to keep them healthy.
Japanese maples that receive too much sun and dry winds can become scorched, developing yellow or brown patches. Known as scorch disease or leaf scorch, this is often a sign that trees aren't receiving enough water or that the soil is contaminated by salt, fluorides or another material. Scorching can also occur when the Japanese red maple has other fungal or bacterial infections. If the scorch doesn't clear up with increased watering, it is likely due to contaminated soil or symptomatic of another disease.
Tar spot is a less common disease of the red Japanese maple and is caused by the fungus Rhytisma acerinum. Affected leaves develop yellow or green splotches; inside the splotches, a black tarry spot grows. Leaves develop brown undersides, with the yellow or green coloring only on the top. Sometimes, leaves fall off affected trees but generally not in large quantities. While this disease is easy to identify and may cause alarm, it does little damage to red Japanese maples and can be treated with a fungicide designed to treat anthracnose.