Okay,for now lets say it’s not fusarium or verticillium. The symptoms for over fertilization are burning of the leaves, darker green than normal leaves, overabundance of leaves, no flowers. This doesn’t sound like your problem. So lets examine the roots. Dig one plant up like you were going to replant it, saving as much of the root mass as possible. Shake the soil from the roots and examine them Closely. If they look black or dark brown and are small or missing cut into one and examine the interior. If it’s dark( it should be white) you have a root rot. Probably a fungus called Phytophthera. This is a fungal disease of very soggy ground very common in nurseries. Because it’s another soil fungus it’s also hard to eradicate. The best control is getting resistant varieties and starting over, after you have fixed the wet ground problem. If the ground is not wet and also not dry we can try to fix the too much fertilizer theory. This you do by watering the bed excessively. You would be trying to flush the nitrogen out of the soil. This isn’t so good for the environment, since the nitrogen has to go somewhere.
But the other possibility is that your Mums just fried in the dry weather. This year has been extremely dry and most of us are not watering enough, even though we think we are.
If it’s none of these things I’m stumped. If they are coming back that’s great. Watch them closely, don’t fertilize, water enough, check the soil and hope for the best.