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!
I suspect it is a fungal disease caused by the Rhizoctonia fungus. I would try to avoid overwatering and also see what can be done to remedy the tendency of this area to stay wet. With warming weather this particular disease will go away on its own.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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