I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Something is wrong in the roots. Consider dry soil or possibly soggy soil due to lots of water and poor drainage, or a root rot disease. Contact you local County Extension Office for information on getting a plant sample analyzed to help determine the cause of the problem.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!