Answer: Several fungal leaf spot diseases can infect maple, including anthracnose, Phyllosticta leaf spot, and tar spot. Extended periods of cool, moist weather as new leaves are emerging, favor these diseases.
Symptoms of anthracnose (Discula sp.) include light brown, purple, or black spots that vary in size and shape. On sugar maple, spots are light brown with darker margins, whereas spots on Norway maple are purplish in color. Spots can develop anywhere on the leaf surface and are not restricted by veins.
Phyllosticta (Phyllosticta minima) leaf spot is characterized by tan or brown spots with darker margins. Sometimes the diseased centers dry and fall out leaving holes in the leaf. Later, fruiting bodies of the fungus (black pepper-like dots) develop within the leaf spots. These fruiting structures are diagnostic of Phyllosticta.
Tar spot (Rhytisma acerinum), another leaf spot disease of maple, has very distinctive symptoms and is easily identified by the raised black spots on the upper leaf surfaces. These spots resemble a puddle of tar. Tar spot most frequently infects red maple.
These fungi overwinter on leaves and twigs that were infected the preceding year. Extended periods of cool, moist weather in May favor a high incidence of fungal leaf spot diseases. Infection is most severe in the lower third of the tree, where the relative humidity remains higher, and on newly emerging leaves.
Chemical treatment is usually not necessary. The spotting may appear unsightly, but rarely causes more than minor damage to the tree. Cultural methods such as mulching, proper watering, and fertilizing help minimize the occurrence and impact of fungal leaf spot diseases. Raking leaves in the fall and pruning dead or dying branches remove fungal material and help reduce new infections in the spring. In addition, thinning out excessive twig and branch growth promotes air circulation by reducing the amount of moisture present in the lower canopy. This, in turn, reduces the severity of disease. An inch of water supplied weekly during dry periods and early spring fertilization boosts vigor, so trees are better able to withstand the stress of leaf loss.
If trees are severely stressed from defoliation or have been severely defoliated three out of five years, fungicides may be applied. Available fungicides are preventive, not curative, and therefore must be applied before leaf spotting occurs. Fungicide application should begin at bud break to protect the young, succulent growth. Repeat applications, according to fungicide label, during cool, wet weather. Read the label carefully and apply only as directed. Application to large trees requires special equipment to ensure adequate coverage. Fungicides containing copper are currently labeled for fungal leaf spots of Maple.
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