Maple trees (Acer spp.) provide cooling summer shade across U.S. Department of Agriculture plant hardiness zones 3 through 9. Red-or purple-leafed maples provide three seasons of vibrant garden color, while green-leafed varieties come into their full glory with Technicolor-blazing fall foliage. The trees’ brilliant displays fade from luminous to lackluster, however, whenever fungal leaf diseases leave their marks.
Anthracnose (Kabatiella apocrypta) fungal disease surfaces as wet leaf lesions that age to reddish-brown or black. as they age. The disease also discolors and splits twig and branch bark. Because anthracnose spreads in cool, wet windy spring weather, it's a minor threat to maples growing in Mediterranean climates. Elsewhere, adequately watered and fertilized trees have the resources to withstand serious infection. Other protective measures include pruning their canopies in winter to increase airflow and sunlight exposure, irrigating them from beneath and removing infected leaf and twig debris as soon as it drops.
Phylliosticta Leaf Spot
Active during wet spring and summer weather, Phyllosticta leaf spot fungus (Phyllosticta minima) blemishes maple leaves with red- or purple-margined yellow spots. Its rain-splashed spores damage the lower foliage and tender new growth. Severely infected branches may defoliate. Preventive measures include regularly watering from beneath and removing potentially infectious fallen leaves and affected branches.
Maples with yellow or greenish-yellow speckling on their new spring leaves have tar spot (Rhytisma acerinum, R. punctatum) infections. By late summer, the speckles merge into tarlike patches composed of spores and infected leaf tissue. While tar spot seldom does lasting damage, infected maple leaves may drop before putting on their autumn show. As with anthracnose and Phyllosticta, clearing away diseased foliage and irrigating the trees from beneath slow the spread of this fungicide-resistant fungus.
Taphrina Leaf Curl
Taphrina leaf curl -- also known as maple leaf blister (Taphrina carveri) -- strikes unfurling maple leaves in early spring. Infected leaves develop raised grayish or black spots. As the spots coalesce, the leaves may bulge, distort and curl. Taphrina is most dangerous at temperatures lower than 61 degrees Fahrenheit with continuous rain lasting least 12 1/2 hours. Mature maple leaves resist infection. Manage Taphrina by maintaining vigorous, well-watered and properly fertilized trees.
Colonies of black, calico, cottony maple or painted maple aphids congregate on maple leaves to consume sap. During feeding, the tiny, pear-shaped pests drench the foliage and branches in transparent, gooey waste. This honeydew lures airborne sooty-mold spores to the trees, where they germinate into powdery, dark-gray or black fungal mats. Thick sooty-mold layers interfere with photosynthesis, potentially weakening the trees. Pruning badly infested twigs, hosing off small aphid colonies or spraying large ones with horticultural or neem oil applied according to label specifications eliminates the pests. Without their honeydew to feed on, the sooty mold gradually withers away.
- Fine Gardening: Genus Acer
- University of California Integrated Pest Management Program: Maple
- University of California Integrated Pest Management Program: Anthracnose
- Utah State University Cooperative Extension: Anthracnose of Shade Trees
- University of Nevada Cooperative Extension: General Care of Maples: Managing Phyllosticta Leaf Spot Disease
- Michigan State University Diagnostic Services: Maple Tar Spot
- University of California Integrated Pest Management Program: Leaf Spot Diseases
- Iowa State University Extension: Maple Leaf Blister: Black Leaves on Maple Explained
- University of California Integrated Pest Management Program: Taphrina Leaf Curl
- University of California Integrated Pest Management Program: Quick Tips -- Aphids
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