Maple Tar Spot

Maple Tar Spot

Maple leaf tar spot on dried & pressed leaves
Maple leaf tar spot on dried & pressed leaves | Source

Maple tar spot is easily noticed on the leaves of infected maple trees. Tar spot symptoms appear as black spots on the leaves with yellowish rings around a black center. The disease causes the tree to become very unsightly and appear unhealthy.

Maple tar spot is a fungal infection that is relatively easy to control using cultural methods instead of chemical control, although chemical control can be used as a last resort. Ambition and a little time is all that is needed to control maple tar spot.

Maple Tar Spot on Living Tree

Maple tar spot on living tree
Maple tar spot on living tree | Source

What is Maple Tar Spot?

Tar spot is a common, visually distinctive disease caused by fungi of the genus Rhytisma. The disease is merely a cosmetic problem most of the time, which appears as black and brown spots on leaves. The fungi usually survives and reproduces in fallen leaves and twigs.

Life Cycle of Rhytisma Tar Spot Fungi
The spores of the fungi have a sticky coating that allows them to easily attach to foliage. The spores penetrate the leaves once attached. Yellow spots begin to appear on the leaves during the spring. The fungi continues to reproduce and the spots begin to blacken. The leaves drop once autumn arrives, but the fungi survives in leaf litter until the following spring. The cycle repeats once spring arrives. The infection of tar spot is localized to the spotted areas on the leaves and the disease is mostly a cosmetic issue, rather than a detrimental disease.

Maples Susceptible to Tar Spot
The fungi most commonly affects silver and Norway maples. Other maples can be contract the fungal disease such as big leaf, mountain, red, Rocky mountain, sugar, and sycamore maple.

Notice the blackened and crispy centers and the slight yellow ring about the spot
Notice the blackened and crispy centers and the slight yellow ring about the spot | Source

Maple Tar Spot Symptoms

The first symptoms of maple tar spot are small, yellowish spots that form on leaves in late spring. These spots may remain small or grow up to an inch in diameter. The center of a tar spot becomes raised and eventually turns black towards the end of summer. The tar spots feature patterns that resemble fingerprints when viewed up-close. Infected trees are usually pretty healthy except for the ugly tar spots on the leaves.

Fallen Maple Leaves

Cleaning up leaf litter reduces risk of maple tar spot
Cleaning up leaf litter reduces risk of maple tar spot | Source
Clean up leaf litter to reduce maple tar spot infection
Clean up leaf litter to reduce maple tar spot infection | Source

Maple Tar Spot Cultural Control

The best method of maple tar spot control is prevention. Preventing the fungal tar spot infection is relatively easy. The fungi spreads and reproduces via fallen leaf litter, especially during autumn and spring.

Leaf Litter & Sanitation
Always remove and destroy dropped leaves and trimmings by burying, composting, or burning. This will prevent the future release of fungal spores into the air. Removal of tree litter should occur before the leaves and trimmings dry out and become brittle. Brittle leaves make raking and collecting difficult, and spores are easily dispersed when leaves crumble.

Avoid Watering Foliage
The foliage should be entirely avoided when watering, but sometimes sprinklers cause water to land on the foliage. Never let sprinklers water the foliage of a tree before nightfall. Leaves that are wet during cool nighttime temperatures are disease prone. Splashing water from droplets can easily spread the fungi onto stems and leaves.

Maple Tar Spot Chemical Control

Proper sanitation may not be enough and a fungicide control can be considered during severe cases of tar spot disease. Copper fungicides can be sprayed in early spring when leaves are budding and twice more throughout the season in attempt to reduce the symptoms of tar spot disease.

Fungicidal control is not normally used because tar spot disease is mostly cosmetic. Thee susceptible maple tree species are usually low in economic value and reduction of tar spot is usually of little concern to homeowners and growers. The most important practice is to simply keep yard debris and litter to a minimum.

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