Trees serve functional and decorative purposes in the landscape. Although homeowners strive to maintain optimum tree health, sometimes they display black spots on their leaves. Also called tar spot, the black spot disease doesn't harm the trees but damages its appearance, making it unattractive, but you can treat infected trees and prevent the spread of the disease to nearby trees.
Tar spot is a fungal disease that manifests itself as black spots on the leaves of infected trees. Early symptoms of the disease include yellow spots on the foliage in spring that turn black as the disease develops. The spots, composed of fruiting bodies, start out small but increase in size as the disease progresses, especially in damp weather, to form raised, tarlike growths on leaf surfaces. Diseased leaves fall to the ground.
Caused by the fungi in the genus Rhytisma, tar spot survives winter on fallen leaves. The two most-common fungi responsible for tar spot are Rhytisma punctatum and R. acerinum. The fungus grows on or in buds, leaves and fruit. R. acerinum causes larger spots, while R. punctatum causes smaller spots and, sometimes, speckled tar spots.
The black fruiting structures on the leaves split and release spores into the air when the leaves mature in spring. The spores drift with the wind and are carried to the foliage of nearby trees. They also spread through splashing rain or irrigation water, and insects. During extended periods of wet, cool weather, the spores germinate and enter leaf surfaces. They grow within the leaf for a month before exhibiting symptoms of the disease. When heavily infested leaves fall from the tree, they usually land face up, which makes it easier for the black fruiting structures to release the spores, spreading them to nearby trees.
Tar spot doesn't damage the health or affect the survival of infected trees, because most of the leaf area remains green during summer, allowing the leaves to still perform photosynthesis. You can manage tar spot, however, by raking up spotted leaves in fall and destroying them immediately to keep the spores from appearing in spring. Prune infected parts of the tree with sharp, sterilized pruning scissors and discard immediately. Although fungicides are available, they're not usually recommended. Where treatment is absolutely necessary, apply an organic spray composed of horticultural oil, baking soda and water. You can also apply protective fungicides to high-value landscape or fruit trees from the time the buds break until leaves mature. Also, mulch the base of your trees heavily, fertilize regularly, and apply water with a drip irrigation system or soaker hose during periods of drought.
- University of Illinois Extension; Fungal Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest; July 1998
- University of Massachusetts Extension; Giant tar spot; Daniel H. Gillman; 2005
- National Gardening Association; Pruning Fruit Trees; Lee Reich
- Purdue University Extension; Tar Spot on Maple; Peggy Sellers
- Iowa State University Extension; Minor leaf spot diseases of maple: tar spot and leaf blister; Rosanne Healy; August 2007