At this time of the year, the energy is being placed into tip growth and flowering. The bottom leaves have done their job and are expended. This process will speed up into the fall as more bottom leaves are dropped until all the leaves drop and the plant goes dormant. Just pick off the yellow leaves as they fully yellow or as the stem droops so they do not attract whiteflies and trash them. This is a sign that the seasons are changing for the plumies. I find that each plant does this at a different time and rate. Some will start now but it will take many months to drop all leaves. Others stay green and then go yellow very quickly. I do not see a clear pattern on any set of plants. The actual cause of yellowing is the cutting off of the photosynthesis which makes the leaf green so the yellow shows through just like in the turning of the leaves in New England. Bill