Property, Gardens & Trees

Tar Spot Disease

Why are my Maple leaves getting spots?

Don’t worry, it is a very common thing to get yellow or black spots on your maple tree leaves.  This is caused by a fungal disease (Rhystisma acerinum), also known as tar spot. Tar spot is rarely harmful enough to affect the health of trees, but makes trees look unpleasant. Heavy infections can also cause early leaf drop.  The best way to help control the fungus, is to rake and destroy leaves in the fall.

What is Tar Spot Disease?

Tar spot is a fungal leaf disease that does occur on several plants, but it is most common on maple trees. It does not cause permanent damage to the tree.

Tar spot is an unsightly condition on the leaves for maple trees. It starts with small yellow spots on growing leaves, and by late summer these yellow spots expand into large black blotches that look like tar has been dropped on the leaves. This is because a fungal pathogen, Rhytisma acerinum has developed on the leaves of the maple.When the fungus initially infects a leaf, it causes a small 1/3 cm (1/8 inch.) wide yellow spot to form. As the summer weather progresses, that spot spreads, eventually growing up to 2 cm (3/4 inch.) wide. The spreading yellow spot will also change color as it continues to grow, slowly turning from a yellow-green to a deep, tarry black.  That is why it is referred to as a tar spot.

A variety of factors contribute to the spread and recurrence of this fungal infection. Some factors, such as weather, are not able to be controlled. However, there are some ways in which you can help to prevent infection. Proper garden and yard sanitation will help reduce the spread of tar spot. Overwintering in fallen leaves, one of the best practices is the removal of all your maple’s fallen leaves and bagging or composting them to eliminate the closest source of tar spot spores. If you leave the fallen leaves on the ground until spring, the spores on them will likely reinfect the new foliage and start the cycle again.

Trees that have trouble with the fungal disease year after year may also be struggling with excessive moisture. You’ll do them a great favor if you increase the grade around them to eliminate standing water and prevent moisture build-up.

Young trees may require treatment, especially if other trees have had a lot of their leaf surfaces covered by fungal disease in the recent past. If you’re planting a younger maple in an area prone to the fungal disease, though, applying a fungicide, Safer Defender Garden Fungicide 1L Ready-to-Use or Safer 3-in-1 Garden Spray 1L Ready-to-Use, at bud break and twice again in 7- to 14-day intervals is recommended. Once your tree is well-established and too tall to easily spray, it should be able to fend for itself.