Q: I have this fungus growing on this retaining wall. It has been there 10 years but this is the first time this has shown up. Do you know of anything I can use to kill it? I power washed it off but it came right back in about three weeks.
A: I don’t have great news. You may have to replace the wall.
The tag on every piece of treated wood you purchase states what kind of ground contact it is rated for. The American Wood Protection Association has developed standards and codes for where treated wood can be used. UC3 wood is used for above-ground decks, rails, etc. UC4A treatment is rated for ground contact in most situations, including landscape walls.
When you have a situation where there is a high likelihood of decay or extreme water exposure, UC4B treated wood is recommended.
My guess is your timbers were treated to the UC4A standard and the soil exposure conditions behind the wall have overwhelmed the treatment.
If you can find the AWPA tags and prove your timber failed unexpectedly, you might get some satisfaction from the treatment company.
Also see Fungus Growing in Treated Wood