Answer: Celosia is a good, tough summer annual and one of the few flowers that laughs at the Texas heat. Your plants died because of a problem below ground, not a disease of the foliage. I don't think the mums are the culprit, but there are several possible causes:
Root rots can quickly destroy a root system. When you dig up the plant, the roots will appear to be grey/brown and water-soaked, rather than a healthy, creamy-white color. To remedy root rots, avoid keeping the soil too wet. In severe cases a fungicide drench may be necessary. Most often, cultural (watering) measures are all that is needed.
Grubs and certain other soil insects can feed on roots. Once the roots sustain heavy damage, the top of the plant will quickly decline in the summer heat. Sift through the soil beneath affected plants and look for grubs. I've noticed them to be especially bad beneath outside porch or patio lights where their "mommas", June bugs, tend to gather in the evenings.
Another consideration would be moles. You didn't mention their tunnels being evident, but you may want to take another look. As summer heat and lack of rain causes the soil to dry outside our landscape borders, moles really build up in our lawn and landscape where the soil stays moist and grub, earthworms and other food sources are plentiful.
If your investigation turns up more details, submit another question and I'll be happy to try to help narrow the diagnosis and recommend a control.
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