Black Leaf Spots on a Maple Tree
A fungal infection called tar spot causes black spots on maple leaves. The disease usually appears during wet years. It causes little damage and is rarely a cause for serious concern.
The Rhytisma acerinum and Rhytisma punctatum fungi cause tar spots on maple leaves. The pathogens spend the winter on infected leaves and mature during the spring, releasing tiny spores that travel on the wind. The spores create new infections if they land on the leaves of a susceptible host.
Pale yellow or greenish-yellow spots appear on infected foliage during spring and early summer. The yellow areas grow and their color becomes more pronounced as the disease progresses. Raised, tar-like black splotches appear within the yellow spots. The Rhytisma punctuatum fungus creates numerous small spots, according to the University of Nebraska-Lincoln, while the Rhytisma acerinum fungus creates several large spots.
Tar spot, which peaks shortly before the leaves fall, does not harm the tree. It is primarily a cosmetic problem that does not require chemical treatment, according to the Michigan State University Extension. Removing fallen leaves from the ground reduces the number of spores that can infect new growth the following spring.
Maple Tree Leaf Fungus
Anthracnose (Kabatiella apocrypta) fungal disease surfaces as wet leaf lesions that age to reddish-brown or black. Because anthracnose spreads in cool, wet windy spring weather, it's a minor threat to maples growing in Mediterranean climates. 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. Its rain-splashed spores damage the lower foliage and tender new growth. Severely infected branches may defoliate. Maples with yellow or greenish-yellow speckling on their new spring leaves have tar spot (Rhytisma acerinum, R. punctatum) infections. Infected leaves develop raised grayish or black spots. 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.
- Purdue University: Tar Spot on Maple
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- 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
- 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