I suspect a water issue, too much or too little.
Does the planter have drainage holes or are the plants sitting in muddy wet soil and getting root rot?
If there is good drainage, then are you watering enough or are the plants suffering from water deficit/drought stress as a cause for the drying up of the foliage?
If the watering is as good as can be, what is the condition of the soil? Is it a fertile potting soil with good organic matter content that retains soil moisture, or sandy or course and porous.