It’s always a surprise to me to see a plum tree in an old field. They’re not very common around here, but you occasionally see one. The fruits are tasty if you get them when they’re ripe before everyone else (wildlife) gets them.
There are several species of plums around, but this species has the spines plus leaves with sharp teeth. I’m no plum expert, but the books say that this combination points to American Plum.