GARDENING: Mold buildup on trees could be from insect manureFloyd is a horticulturist with Texas AgriLife Extension Service. He can be reached at 498-4071 in Ector County or 686-4700 in Midland County or by email at

As trees begin to push new leaves in the spring, interesting questions roll into the office. Budding leaves cause people to notice things they hadn’t before.
This week, one caller sent in a photo showing lots of black powdery stuff on the surface of their pecan tree. It turns out the dark chalky coating was a buildup of mold caused by insects feeding on the foliage.
Pecan trees are notorious for attracting aphids. These tiny little plant juice suckers slurp up fluid from leaves only slightly faster than they expel it. Sometimes aphid populations become so large that anyone standing under an infested tree may think it’s beginning to rain.
Aphids make lots of this stuff, affectionately called honeydew. You’ve already figured out that it’s actually bug poop. Honeydew tastes sweet, at least to ants (I’ve never tasted it, but you go right ahead). Ants protect aphids to harvest this sugary material which sticks to cars, benches, driveways, and anything under the tree. That’s right, if you park under a pecan tree and are curious about that mystery goo on your windshield every morning, it is insect manure. There’s no pleasant way to describe it.
Bark sometimes collects enough honeydew to grow a sooty colored mold, which causes branch surfaces to turn dark. The mold isn’t usually a problem, but sometimes it’s thick enough to keep leaves from photosynthesizing tree food. Systemic insecticides control aphids.
It’s not always the dark colored bark that causes alarm. Sometimes tree owners notice a contrasting splotch of silver-grey bark in the middle of a mold-coated branch. This lighter bark is the pecan tree’s normal bark color, revealed as older bark naturally sloughs off.
The next time you notice a dirty black varnish on your pecan tree, check the foliage for aphids.
To learn more about horticulture in West Texas, call the AgriLife office at 498-4071.