Milkweed has turned black

Asked September 10, 2020, 9:16 AM EDT

Hi - I have a volunteer patch of common milkweed that has been growing, & overgrowing, amongst my irises for a number of years - we accept that, because we love to see the monarchs & their caterpillars. This year, the milkweed looks awful - it's looked awful most of the season - kind of discolored & "old-looking" - & overtime has become black (see photos). It is under the farthest stretch of the branches of a tulip poplar that we've been told has aphids (?) that secrete a liquid which falls on the ground underneath it. We see the results of this closer to the tree - all over my car, we've had to move some herbs, etc. There are other plants that grow under it just fine - part of a wildflower garden, a bridal wreath spirea shrub, some volunteer garlic chives. I don't know if this is related, because neither the tree nor the milkweed has moved & we have not had this problem before, & as I said, the milkweed is under only the very farthest reach of the tree.. maybe not even within it's reach. Any thoughts? What are the long-term effects on the milkweed? Anything we can do to prevent it in the future if it is not fatal? Also, while I'm at it - any thoughts about the tree-aphids(?)-honeydew? The tree is old & very tall - we cannnot examine the leaves or branches. Thanks!

Anne Arundel County Maryland

1 Response

The black growth is sooty mold, a fungus which feeds off of the sugars in honeydew (the sugar-water type of waste excreted by various sap-sucking insects) but which does not infect the plant itself. Although in extreme and chronic cases its blocking of sunlight from leaves could weaken the plant, this late in the season such leaf shading is not harmful to the plant's overall health. As the leaves shed this autumn, the sooty mold will disappear with it. It's a common organism in the environment and could re-appear in future growing seasons; as with many fungi, though, its preponderance is likely at least partly tied to weather conditions.

It's possible that insect pests on the Tulip Poplar are contributing to this problem, though Milkweed is known to harbor Oleander Aphids as well which can also create conditions for sooty mold. Aphid populations on Milkweed can reach explosive numbers in summer, though they don't cause much damage to the plants and are not worth resorting to insecticides to control. The fact that the nearby Marigolds and Dogwood look free of mold suggest that the problem is more localized to the Milkweed rather than honeydew raining-down from tree limbs above. An alternative explanation is that the leaves on these particular plants don't retain enough honeydew residue to support the growth of the mold.

Tulip Poplar more commonly has issues with scale than aphids, so if any pest population is booming in the tree, this one is more likely. Scale populations can reach outbreak levels due to various factors, one of which may be tied to area sprays for mosquito control. In either case, while the tree's health is not in much danger from even high scale numbers, you could have it inspected by an arborist or tree-care professional if its issues warrant treatment. Tulip Poplars are important members of the ecosystem, supporting a number of insects (and in turn, birds), so insecticide treatment should only be a last resort. Beneficial insect predators and parasites may catch-up to the pest population boom in time, which will then drastically reduce their numbers. (This happens with the aphids on the Milkweed too.) If you wish to search for an arborist, the second and third links on this page will lead you to search tools: