Hi,

I'm no expert, but am very familiar with cannas. I notice you're in Oklahoma. How is the weather in your area? If the temps are getting up into the upper 90s like in my area, the west side receives a lot of the heat. If your house is brick, or even wood painted white which reflects the light, the heat index may be reaching very high temps. This will force cannas to wilt and die back - from experience. Is there any discoloration on the leafs, similar to fertilizer burn? If so, this may very well be the problem. You say the rhizomes are healthy. Look at the dirt around them. If you notice white running mycelium (rootlike shoots) and strong smell of decay, you might have mold/virus problem. If you compost or you top dress with compost, some woody stems in the compost carry a mold/virus combo that attacks the roots (same one that attacks hostas). I had an epidemic three years ago. I hope this helps.

Thanks Wildgrdnr! Perhaps the heat is the problem. We did have record high temps at the end of May. I will just keep an eye on them and see if the problem continues or escalates.
On a positive note, I don't smell anything and the dirt around them looks fine. I had not even thought it might be a virus or mold.