It is hard to say what the cause is without having a sample of the plant to look at. It could be a fungal dieback. Another possibility would be that something such as a weed killer has been applied over the roots of the plant.
I suggest that you take a sample from the plant along with a photo of the plant in its growing location (if possible) to your County Extension Office to see if they can help diagnose the problem and recommend an appropriate course of action. Their contact info is:
Monday - Friday 8:00 am to 5:00 pm
2350 E. Michigan Street
Orlando, FL 32806
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