u/Furgling

We pulled off the linoleum in the bathroom and found these mold spots by the corners where the tub meets the walls. The OSB still seems solid by poking with a screwdriver.

https://postimg.cc/gallery/KG7T06K

I'm not sure if mold could have grown under the tub. I'm hoping not to have to remove the floor or the tub. I don't know how serious this is.

Any advice?

We removed 40 year old linoleum and each square was pretty much outlined in a dark line just from dirt getting in. However, under the fridge and stove there was more dark spots. I don't know if it's the heat through the linoleum making the dark patches or if it's mold.

I know there are mold kits at Lowe's but I read those are inaccurate as they can just catch mold spores in the air.