My Maple Tree's Leaves Are Turning Black
- Tar spot is a fungal disease that affects the leaves of the maple, causing raised black spots on the upper surface of the foliage. The first sign of infection often appears in June as pale, yellow spots -- approximately 1/8 inch in diameter -- begin to form. Gradually, the spots enlarge as the season progresses, with dark spots beginning to develop inside a yellow border. The black spot continues to thicken, eventually appearing similar to a spot of tar.
- A different type of tar spot appears on Norway maples, with as many as 50 tiny spots dotting each individual leaf. They grow to a large, dark mass of nearly 2 inches in diameter. The fungus on all maples over-winters on the surface of infected leaves that fall to the ground. In the spring, spores escape from the old leaves, are carried by the wind and infect a new batch of foliage.
- Raking and destroying infected leaves in the fall is the most effective way to manage tar spot disease. Application of fungicide is often ineffective as the chemical must cover the surface of all leaves, a difficult proposition with large maple trees. Additionally, infected trees in adjacent yards or landscapes can easily transmit the fungi. While leaves may wither and drop, the overall health of the tree is rarely affected; damage is primarily aesthetic.
- Phyllosticta Leaf Spot, caused by the pathogen Phyllosticta minima, is a frequent problem on silver, red, Japanese and Amur maples. Initializing as tan spots with purple and red borders, the leaf spot eventually causes black, fruiting bodies to form inside the established lesions. The damage often stands out brightly on the tree, but injury is primarily to the beauty of the maple and application of fungicide is a rare necessary.