Have you checked for insects? Look for a beetle that is yellow striped. Look at the roots for beetles, and then look at the underside of the leaves for eggs. The eggs are tan in color, and appear to be glued to the leaves in small clusters. If you find bugs, it's time to treat for them. I use Sevin dust. Put it in an old sock and shake it around the root area. You must remove the eggs from the leaves too. They will stick to masking tape. Good luck.