Why Are There Black Spots on Tree Leaves?

Have you noticed black spots on tree leaves? Sometimes this may indicate that your tree is sick. But, other times, it could just be a cosmetic issue. If you want to know what is going on with your trees, then we invite you to keep reading this blog!

When you notice a brown or black spot on a leaf, it may be the cause of a fungal disease called tar spot. You will mostly find this type of leaf spot on maple leaves. So, in this blog, we’ll focus mainly on what happens to maple trees when this disease invades them.

If you wish to keep your tree looking beautiful, then we recommend you pay attention to this. Learn what may be wrong and how you can fix it. Also, don’t hesitate to ask your trusted arborist for help.

Black Spots on Tree Leaves: What Are They?

Some leaf spot diseases can weaken trees, while others don’t really affect the health of the tree. For that reason, you must learn how each disease can affect your trees.

When there are black spots on tree leaves, it could result from a fungal disease called tar spot. It mostly affects maple trees, specifically Norway, silver, and red maples. This fungus is likely to happen if you have been getting lots of rain.

The spots on maple leaves are not as noticeable when they are starting to appear. But, we recommend you take a look at your tree if you believe it could have this disease.

What Is Tar Spot Disease?

As we said before, “tar spot” is a fungal disease. The fungus is called Rhytisma acerinum. It gets its name because the spots resemble black tar.

You will most likely see this spot disease in early June and August. During June, the spots on maple leaves start developing. They will have a light green appearance and are very tiny. So, you would probably not notice these green spots if you were just glancing.

When August comes around, the tiny spots will get wider and darker on the maple leaf. You will know that it is this disease as the spots will literally look like clumps of tar landed on your tree leaves. The spots may also be visible on your trees’ seeds.

Besides Norway, silver, and red maples, tar spot also affects the following trees:

• Oregon maple
• Sugar maple
• Box elder
• Sycamore

Be careful not to think that your maple trees can only get tar spots. Bacteria or other pathogens cause a few other fungal leaf spots. In fact, almost all trees are prone to more than one leaf spot disease. So, we recommend that you ask your arborist as soon as you notice something odd on your tree leaves.

What Happens to the Health of the Tree?

Fortunately, the health of the tree is not affected by this leaf spot. It is only considered a cosmetic issue.

However, it can cause your leaves to start falling earlier than they should. This is because leaf spots interrupt photosynthesis. So, it weakens your trees’ leaves. But, this happens on rare occasions. Most of the time, you won’t see any damage to your trees, besides the cosmetic issue we mentioned before.

Most of the time, tar spot only affects a small percentage of a tree’s leaves.

You should contact a professional if you notice that most or all of your tree leaves start falling two to four years in a row. If this happens, then the problem could be beyond tar spot. When leaf loss happens a lot, it results in reduced tree growth. It can also make your trees more prone to pests and other diseases.

What Are the Symptoms?

As we mentioned, the symptoms of this spot disease start during the summer. You will rarely notice the spots as they are light green or yellow. They start appearing on the upper leaf surface.

Then, later in the summer, the black spots start appearing, and they become bigger. If you examine them closely, you’ll see that the spots have a ridged appearance on top and cupped on the bottom.

The infected leaves will fall eventually. This symptom becomes more noticeable when there are too many leaves falling when they shouldn’t.

Life Cycle of This Fungal Disease

It’s important that you understand the life cycle of tar spot to learn how it spreads to your trees. This organism survives on infected fallen leaves during the winter. When springtime comes, the fungal spores are blown by the wind into new leaves.

How Can You Treat This?

We mentioned that the black spots on the infected leaves have fungal spores. When a leaf drops, it will still have these spores. Even though they are on fallen leaves, the wind can blow them and carry them to new leaves.

So, one way to treat this and prevent it from spreading is to rake and destroy leaves on the ground. As a result, the spores will not land on new leaves. You should try to destroy every leaf in the area. If you leave even one leaf behind, the spores on that leaf can eventually land on a new leaf and then spread to the rest of the tree.

There is another way you can treat tar spot, but it is a bit more challenging. You can have an arborist apply a fungicide when the next spring season comes around. For this to work, the fungicide you apply needs to cover every single leaf on your tree. This can be quite difficult on mature trees. So, you should use this method on smaller trees.

We recommend this treatment only if your trees get tar spot too often and the issue has become prevalent. If not, then raking and destroying the leaves should suffice.

Black spots on tree leaves may not be something you should worry about if the issue does not persist. But, if you see that it is happening continually, then you should get help. Also, make sure that you’re taking note of how many leaves are falling and how often.

If you need more assistance regarding these black spots, make sure to give us a call.