Very strange. This is not a normal symptom at all. Either the soil is suddenly staying too soggy wet or something chemical may have been applied to the soil around them. It is also possible that a root rot has affected the plants. If this is the case a fungicidal drench for root rots may be helpful but there is no way to know for sure unless your County Extension Office can assist with a proper diagnosis. In the mean time if you can provide adequate care and keep the soil moist but not soggy they plant will have the greatest chance of recovering. Pruning out dead areas is fine.
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