butterfly weed dying
Several of the branches of one of our butterfly weed plants have died and there are many small, round things (mushrooms? insect eggs?) in the soil where those branches used to be rooted. What are they and did they kill the plant? Will they spread?
Montgomery County Maryland
This looks like Southern blight. It is caused by the fungus Sclerotium rolfsii. This fungus can attack probably all herbaceous perennials and is favored by hot weather.
It generally causes the top of the plant to collapse with quite visible fine white mycelia and mustard seed like fruiting bodies on the stem and at the base of the plants. Oxalic acid is released and girdles the tissue but does not kill the roots. The plants and fungus comes back. It can filter down into the mulch.
We recommend that you remove the mulch and plants, including the top several inches of soil to help prevent more infection. Minimize the use of a deep mulch and keep away from the stems of the plants. Practice good sanitation and Do not compost material killed by southern blight.
Here is more about Southern blight prevention: https://extension.umd.edu/hgic/topics/southern-blight