Hi, I'm hoping to get some help IDing this pathogen. This is a 4 year old potted avocado I grew from seed, and has been doing great until about a year ago when this white/black spot started to slowly spread. I'm in Portland, Oregon and put the tree out in the summer and back inside during winter months.

I didn't think much of it at first but the bark is obviously cracking and its only getting worse, though the progression is very very slow. Possible ailments I was thinking it could be were also sunscald, some sort of freeze damage, or another abiotic factor. It did have some exposure to the sun over the summer but not all day or too much. It has been out during a couple light frosts, but it got worse over the summer when there was no frost as well. If I wash the area down with water, the white comes off but then comes back after a while (a few days time maybe?).

I'm having issues posting pictures, so I'll try to post a couple follow up ones as well. Let me know if you need any other info and what the solution to the problems are too. Thanks in advance for the help!