Cause of mold on wood floor

[Erin K.]

I have a one story house with a crawl space below it. Tenant moved out and in the bedroom corner, a little in the closet corner and a corner in the kitchen, there is mold growing on the wood. He had a dog. Has anyone seen urine cause mold like this? I'm not sure how to post a pic. I don't know if moisture in the crawl space could cause it, but it seems odd that it is just in the corners. Thoughts?