oh my gosh. I did manage to stop what I have now identified as Brown Rot on my Edward of York cross but after a quiet week my Ho Chi Minh rotted almost overnight! Almost every leaf was affected.
It was a 5-plant clump I have been growing for many years and was looking forward to many flowers next time....but last night I had to butcher it down to a single leaf and a tiny plantlet.
Physan everwhere, keeping my fingers crossed that it bounces back :-(
I broke down and disinfected my entire growing area today and physan-ed everything and everybody in there.
This is heartbreaking.

Now that you have the diagnosis: you can have the remedy. You are right to bring out the big guns on this.
And your quick action will save other plants.
I am sorry you had to go though this.

Brown rot aka Erwinia and Acidovorax(Pseudomonas) will be stunted with Physan but you really need a copper fungicide to kill it.
I have used both a general copper fungicide and Phyton 27 with reasonable results.

Copper fungicide like "Captan"?

Captan doesn't have any copper in it at all : Captan - Wikipedia, the free encyclopedia