I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
Very strange. The most common cause of such symptoms is soggy root conditions. I realize you indicated good drainage, so if we eliminate soggy roots then the cause must be some type of root rot. Sometimes these can be controlled by a fungicide drench if you catch it early enough and sometimes they cannot. If you want to be sure of a diagnosis and still have a living but sick plant, you can send it to the state plant pathology lab for a diagnosis. There is a fee for their service. Here is their web information:
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