Black Tar Spot: What It Is & Should You Be Concerned?

Recently, there has been a noticeable increase in concern from homeowners about black spots suddenly appearing on tree leaves. At first glance, it can definitely look alarming, and many people worry their tree is dealing with serious disease or beginning to decline.

In most cases, what you're seeing is called Black Tar Spot - a very common fungal disease found on maple trees throughout Wisconsin and the Midwest.

The good news? While it may look ugly, Black Tar Spot is primarily a cosmetic issue and is typically not harmful to healthy trees.

What Is Black Tar Spot?

Black Tar Spot is a fungal disease caused by fungi within the Rhytisma family. It most commonly affects maple trees, especially Norway Maples and Silver Maples, although other varieties can also develop it.

The disease begins forming in spring during cool, wet weather. Early in the season, you may notice pale green or yellowish spots on the leaves. As the summer progresses, those spots darken and develop into large black blotches that resemble drops of tar.

By late summer and fall, the fungus becomes much more noticeable, which is usually when homeowners begin spotting it and calling with concerns.

Why Is Black Tar Spot So Common?

Black Tar Spot tends to thrive in environments with prolonged moisture and humidity. Wet spring weather creates ideal conditions for the fungal spores to spread and infect newly developing leaves.

The fungus survives the winter in fallen infected leaves on the ground. When temperatures warm up and moisture increases in spring, spores are released and carried by wind or rain onto new leaves.

That means some years are worse than others depending on weather conditions. After particularly wet springs, Black Tar Spot tends to become far more widespread and noticeable.

Is Black Tar Spot Harmful to Trees?

Fortunately, in most cases, Black Tar Spot is not dangerous to established, healthy trees.

Despite its appearance, the fungus usually does not create serious health problems or kill trees. The biggest issue is cosmetic appearance. The black spots can make trees look stressed or unhealthy even though the tree itself is often completely fine.

Occasionally, trees with heavy infections may experience:

• Minor premature leaf drop

• Mild stress during severe outbreaks

• Reduced aesthetic appearance

However, healthy trees are generally able to tolerate Black Tar Spot without long-term damage.

A mature maple tree dealing with Black Tar Spot alone is usually not at risk of dying from the disease.

Should You Treat Black Tar Spot?

In most situations, treatment is not necessary.

At Your Personal Gardener & Arborist, we believe in honest recommendations. Since Black Tar Spot is mainly cosmetic and does not typically threaten the health of the tree, chemical treatments are usually not worth the cost for homeowners.

Fungicide applications would need to occur very early in the season and often require multiple properly timed treatments to be effective. Even then, results can vary depending on environmental conditions.

Because of this, we generally recommend focusing on good tree care practices rather than chemical control.

What Can You Do to Reduce Black Tar Spot?

Although you may not be able to completely eliminate Black Tar Spot, there are several steps homeowners can take to help reduce the severity year after year.

1) Remove Fallen Leaves

This is the single most important step.

Since the fungus overwinters in infected leaves, removing and disposing of fallen leaves in autumn can help reduce the number of fungal spores present the following spring.

Raking leaves thoroughly from around the tree can make a noticeable difference over time.

2) Promote Overall Tree Health

Healthy trees are better equipped to tolerate environmental stress and fungal issues.

Proper watering during dry periods, mulching correctly, and avoiding root damage can all help maintain strong tree health.

3) Proper Pruning & Airflow

Selective pruning can improve airflow throughout the canopy, helping leaves dry faster after rain or humidity. Moisture sitting on leaves for extended periods creates ideal fungal conditions.

When Should You Actually Be Concerned?

Although Black Tar Spot itself is usually harmless, it's still important to monitor the overall health of your tree.

You should consider having your tree professionally evaluated if you notice:

• Significant dead branches

• Sparse canopy growth

• Cracks or decay in the trunk

• Mushrooms growing near the base

• Multiple years of declining health

• Severe stress beyond leaf spotting

Sometimes homeowners assume Black Tar Spot is causing tree decline when another underlying issue may actually be present.

Final Thoughts

Black Tar Spot is one of the most common fungal issues we see on maple trees throughout Wisconsin. While black spots can certainly look concerning, the disease is usually nothing more than a cosmetic issue for healthy trees.

In most cases, treatment simply is not necessary because Black Tar Spot is not going to kill your tree.

If you ever have concerns about the health of your trees or want help determining whether something more serious may be going on, the team at Your Personal Gardener & Arborist is always happy to help.