well, i wouldn't consider myself an expert in this field, but from my current issues and what i'm seeing on my plants, you should check the underside of the leaves for spider mites. when they puncture the leaf to drink the sap, the leaf dies around the hole. the mites usually live on the underside of the leaves and can be all white, all red, or yellow with black spots. you should check for those and let us know.

hope this helps!