Hello!
This is my first time posting so nice to meet you all!
I had this plumeria for the past year and this morning I have noticed this at the base. I don't know what's wrong with it and it looks healthy otherwise!!! I hope it's something it can recover from!!
It's stem rot. It can recover, but you're gonna have to start over. Cut the plant off, let callous over, and replant. Your soil looks like it holds too much water. Need to add about half perlite in with it to speed up draining and drying.
Look deep into nature, and then you will understand everything better.
