Q: Can you identify this disease and tell me how to control it? It started in several of my black-eyed Susan plants which made the stems turn black and completely die.
A: Looks like Southern blight (Rhizoctonia solani) . It’s a common soil fungus that causes stem rot when water accumulates around the stem of a plant.
If you have lots of clay in your soil you need to control water flow into the bed. A more permanent solution would be to lift any healthy plants and re-amend the soil to drain better internally.