Why does my milkweed look diseased or yellow?
Disease isn't always the culprit when milkweed doesn't appear healthy. If there is a possibility that herbicide was applied in the general vicinity, this could potentially be a cause for unusual or sick-looking milkweed. If spray drift is unlikely the cause, there are many different diseases that affect milkweed, including everything from fungi to viruses. One common bacterial disease is called milkweed yellows phytoplasma. The University of Minnesota Monarch Lab has a post about how to identify and deal with this particular milkweed disease.
It can be difficult to identify what is affecting your specific plants, as there is no comprehensive diagnostic manual for milkweed diseases. The Xerces Society Milkweed FAQ provides some general advice about dealing with diseased milkweeds:
"When milkweeds in home gardens or landscape features display symptoms of infection, removing and disposing of the diseased tissue will help prevent the infection from spreading to healthy plants."