i have a 5yr old stanley plum tree. every year it gets a multitude of blosoms and seems very healthy. The fruit gets the size of grapes then they get pinkish spots on them and fall off. only4 or 5 develop into mature fruit. Please help me!

You didn't mention your location, but I suspect Plum curculios are causing this. They are an insect that attack plums. Don't let the ruined plums remain on the ground because the adults will emerge from them and the cycle will continue.

Spraying in the early spring is a good control against this.

I googled and found this information about Plum Curculios insects:

http://en.wikipedia.org/wiki/P...

And, this pages offers information and control info: http://nysipm.cornell.edu/fact...

There are causes other than/or in addition to plum curculio that could be involved.
http://www.gardenguides.com/10...

My landlord had a lovely Stanley plum tree in Enfield , Connecticut but the beautiful ripe fruit always had some kind of worms - fully ripe fruit with worms, dang.