How Do You Treat a Magnolia Tree Infested With Ants and Sooty Mold?

The Real Problem Is Scale Insects

Sooty mold on a leaf.
Nigel Cattlin/Corbis Documentary/Getty Images

"How do I treat ants and sooty mold on a magnolia tree?" asked one reader recently. It is a common query. His question continued as follows:

"We have a four-year-old magnolia tree (4 inches in diameter and 13 feet in height) that has really never looked great but has always had nice white flowers in spring, at least. The leaves never got that real dark green look. Now, this summer, the leaves have started looking a lighter green and the bark has mold on it.

I have washed the mold off the bark and noticed at the base of the tree the bark is coming off about 4 inches up. I pulled the loose bark off, and under it there was a ton of ants. The layer of bark under that is still solid, but a few bore holes (the holes only go in a little bit, I think). We are in Florida."

The answer to the question follows:

Problems With Sooty Mold and Ants Actually Stem From Scale Insects

The ants and mold (it is probably the mold called "sooty mold") may be the effect, rather than the cause of your magnolia tree's problems. The real cause may be magnolia scale bugs. The scale insects secrete a liquid named "honeydew" that invites sooty mold. And because this honeydew -- true to its name -- is a very sweet substance, the ants like to feed on it. You can find information on organic ant control in a separate resource.

What Is Sooty Mold?

"Sooty mold" sounds disgusting, and it has an appearance to match.

It appears as a sickly, black film covering the bark and/or leaves of a plant. Known in scientific lingo as Capnodium, sooty mold is a kind of fungus.

As the University of Florida Extension points out, the sooty mold fungus, in and of itself, does not cause a direct problem for your plant, but rather an indirect one: namely, it inhibits the photosynthesis in a plant.

Photosynthesis is the very important process whereby the leaves of a plant convert nutrients into carbohydrates, by harnessing the energy derived from the rays of the sun. With a layer of sooty mold fungus covering the leaves of a plant, the sunlight cannot get through properly. The result is that the necessary photosynthesis can't take place, and the health of the plant suffers accordingly.

Scale insects are commonly the culprit behind this chain of events on magnolia trees. To fight these insects effectively, learn how to practice magnolia scale control. Other sucking insects also secrete honeydew, including:

  1.  Aphids
  2. Mealybugs
  3. Whiteflies

Take careful note that it is the sucking insects, rather than the sooty mold fungus or the ants, that you should be fighting. The fungus and the ants will go away once you have eliminated the sucking insects.