I'm aware that there is a problem in PA with plum pox virus. It's possible that's what's causing your fruit drop. I think it' s only been found in orchards in Adams County so far. So maybe it's not likely, but if that's what it is, it's important for PA Cooperative Extension to know.

The Backyard Fruitgrowers is a great source of information. You can read about plum pox there and email for advice.

Here is a link that might be useful: BYFG

Good advice above. It could be PPV Plum Virus, however, it could also be something cultural with unfavorable environmental conditions...weather and pollination, or nutritional as something lacking in the soil. You should have your soil tested. There is also an insect, plum curculio, which could affect early fruit drop. If you live near farms which spray herbicides, then that could also be a factor with drift.
Call the Bucks Co. Extension office and speak to the agent. S/he will advise you how to proceed.
http://bucks.extension.psu.edu/
http://sharka.cas.psu.edu/

Most common cause of fruit drop like this is insects. Commonly plum curculio.

George

Here is a link that might be useful: Plum Curculio

I made an assumption that if there had been no problem for several years, and then it suddenly appeared, that it would not be commonly encountered insects. But maybe that's not a good assumption - have people found that when they first plant a fruit tree, things go well for a few years, then the insects, diseases "discover" you? Inquiring minds want to know.

When we built our house 35 years ago, we planted a plum orchard, 12 trees, 2 each of 6 varieties. We had bushels of fruit for 4 years, then started getting early dropping from the curculios. They brought the virus, and at about 10 years all the trees had died. We tried a few more, but they became virused before they even grew to full fruiting size.

I miss the plums too.

George

Thanks for all your advice. I will give the Extension offices a call!

We inherited an old plum tree when we bought our house. It's hideously disfigured from black knot disease. I've tried to prune out the diseased parts, but the tree basically looks awful each season. Even so, it usually produces loads of yummy fruit. I think plum trees are pretty vigorous. Hopefully your extension office will be able to diagnose your tree's problem. It'll probably come back nicely after treatment.

i finally gave up and cut mine down. i hated to do it, but it was a mess. the bark was all dried up and falling. fruit would grow mostly at the top and birds and squirrels (i think) would get them and then they would all drop at once. did indeed bloom very pretty, but the site of the rest of it didn't warrant saving. i held out as long as i could.

My parents have a plum tree in their backyard. It would bloom and attempt to fruit, but never got much fruit due to drop and what looked like disease. My dad finally cut it down to about a foot above the crotch. We placed a big flat rock on top of the branch stubs so it fit really nice, almost like a table. The tree ended up growing around the rock holding it securely, and the new growth looks nice compared to the gnarled, black knotted part that was there before. I'd say if you're at the point where you're going to cut it down, try cutting all of the top growth off.