What is the plant?
Thank you very much, Sandy.
You are welcome!
Sandy, just for your information, I kept the mystery plant to see what would happen next and found out (which I suspected) that the plant itself is an Echinacea but a Rudbeckia. I do believe your suggestion, however: that the plant has aster yellows. I will be getting rid of that plant in the near future as per your suggestion. Thank you again. Here is an updated photo of the mystery plant (at the bottom of the photo with the three petals).
Thank you for the update! Getting rid of it is the only way to protect you other Echinacea plants. Have a good weekend, Sandy