Not a plum, that's 100% absotively posilutely a persimmon, you can tell by the leaves and the flower part at the top of the fruit. The bark is also easy to identify. I have 3 huge ones at the edge of a field, VERY similar to that setup picture NCRemi posted. My stand is almost in the exact place, but a little further into the woods. I checked em this past weekend when I went camping, and they are looking excellent. I also found a big muscadine vine comin out of a tree right beside the persimmon trees that I had never seen before. Not to mention the countless white oaks all around my stand up on a hill above all this luscious fruit. I didn't get to hunt much last year early on, so I missed the parties all the deer had when they fell. I hunted em twice with no luck, didn't hit it perfect I guess. But this year I'm gonna be sure at least two nannies die with a mouthful of fruit under this stand!
Here's a page from a great site.
http://www.oplin.org/tree/fact%20pages/persimmon/persimmon.html