You have some form of fungal disease--It may be blight. I do know that the first line of defense is to remove all the infected leaves. You then need a fungicide and I can't advise you there either. When watering try not to get the leaves wet. Most leaf diseases spread the same way. Water splashes on the infected leaf, which splashes on to an uninfected leaf. The water carries the spores of the disease to the new leaf and it becomes infected. Eventually the whole plant dies. In addition some diseases live in the soil. In that case you have to solarize the soil by putting black plastic on it and let the sun kill it. Sorry to be the bearer of bad news, but maybe you can get rid of it if you are vigilant

Fungus in the soil. If you pull it up you'll notice the roots are very bad. This is bad this year because of the cooler temps we had in Spring and the wet weather. How has the weather been there this year? I had one plant like this in April. I sprayed all of my plants with Daconcil. That stuff works for me. It's too late when your plant is showing the signs of the problem. All you can dig it out and discard it.

Must agree with the above. We had so much rain this year and the Echinaceas are only starting to perform now that the rain has stopped. Thought we were going to lose quite a few before then. I must say the garden center Manager did a good job of 'up-selling' the fertilizers to you. I agree with Echinaceamaniac about the MG, and will add that the addition of compost to the soil should have been sufficient to start your plant off in the right direction. There was no need to buy fertilizer unless you knew that your soil was deficient, and from all indications, that was not the case since you say that all other plants in the same bed are doing fine. I would urge you to google the "ironate", do some reading, and then see if you wish to continue using it or dispose of it properly. I hope your plants come through and do well the rest of the season for you. :O)

Thanks for the heads up and the links. But I must say, the damage looks quite different than from aster yellows. The key is to know the difference.