Yeah definitely fungus I would try copper sulfate they sell it and any hardware store
Sounds like a blight. Buy some organic fungicide. Many good brands. Read the labels
Call them they are very knowledgeable
You can also go to their website and select the contact tab. You will see ask the expert and from here you fill out the form with your question. You will usually get your question answered that day if not, definitely by the next business day.
I suggest calling and asking to speak with someone in the perennial department about the issue.
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