Just had to do the same with some of mine. We had several days of a lot of rain. ended up having to take about 3 inches off to get beneath the affected area but I think I got far enough below it to stop the spreading. Good luck. Like stated early, make sure to use sterile practices. Only use those scissors on the affected bud spray with alcohol and then sterilize the scissors before using them again. Last thing you want to do is accidentally contaminate other buds with them.
I got a little OCD with it. I let alcohol soak on the blades, wiped off, used some coconut oil to remove the rest of the resin, scrubbed with Dawn, and then let some alcohol soak on the blades again.
Good luck. I hope you don't have to sacrifice too much, but taking some now is better than losing so much more.