sootymold3.jpgYou might think that in the winter, Monument Valley Park wouldn’t have anything interesting going on with its plant life. Not so! If you spend any time in the park in the winter (or anytime after the leaves have fallen from the trees), you may have noticed that many trees look like they have been burnt. Their bark looks scorched. Most of the trees I’ve noticed like this are along the middle path of the northern loop (though my photos are from the trail that heads south towards the Uintah St underpass). Map.

What we’re seeing on the trees is called sooty mold. The mold is made up of several different fungi. The fungi are there because during the summer, aphids enjoyed a feast in the tree leaves above, dropping their signature “honeydew” that any gardener knows as the sticky waste matter aphids excrete. The fungi like to feast on the honeydew. (Ewwwwww.) The sooty mold often ends up on the trunk and branches of the same tree that the aphids have been feeding on. But sometimes, it occurs on other understory plants that happen to be under the tree, or even on hardscape like curbs or sidewalks.
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Other insects produce honeydew as well, such as scale or whiteflies. But aphids are responsible for most of the honeydew and the sooty mold that follows in our area.

The sooty mold doesn’t cause any direct harm to the trees. If it covers leaves of smaller plants, it may interfere with photosynthesis in the summer. But for larger trees, it does not cause much of a problem, other than making them look like they’ve been burned.

When I took the Colorado Master Gardener training course, I learned that while sooty molds can be found on lindens, boxelders, and maples, in our area, it primarily occurs on American elms. This became a key part of tree ID for me. And now that I’ve shared the secret with you, you can amaze your friends and family by casually mentioning “Hmmm…that looks like an American elm to me” – in the middle of winter when the tree has no leaves!