Have you cut them open to see what is going on inside? If bugs get to them and lay their eggs inside them the larvae will eat tunnels through the fruit and the tree will drop it early.

Tree not old enough to sustain fruit? Too small? Drought? Heat? Could be a number of things.

Pollination issues also would be right up there.

The the dropped green fruits are fine inside; not worm-riddled at all for either type of tree.

Only after the plums get damaged do they usually ripen before dropping. The plum trees are not too old. Some are 5 years old and some are 15 years old and range between those ages. They've dropped their fruit like this every year.

The apple tree is at least 20 years old. But, for the past 17 years it has dropped it's fruit in this manner. Every year it is completely loaded with apples. The deer end up getting every one.

I have many apple trees and many types of apple trees. Pollination shouldn't be a problem for those. The plum trees shouldn't have a problem either.

As far as heat and drought go, it rarely ever reaches above 100F in the summer. It's been a cool rainy summer here this year and the trees are still dropping their fruit.

There is possibility of some nutrient deficiency in the soil that your fertilization may not be compensating for. I am not an expert to say which lements/compounds can help but such deficiency is a possibility.

ottawan, that may be a problem. The soil around here isn't very good. I've been googling and have finally found a link that has quite a fair amount of data on fruit drop:

http://www.umanitoba.ca/afs/hort_inquiries/fruit_and_fruit_plants/premature_fruit_drop.html

This pretty much tells me what I need to know. The rest of detective work on my part now. Thanks to those that have answered. :)

You might have the Plum curculio attacking your fruit. Go on the internet to read up on it. Fruit will always drop early with this, so you need to use a fruit tree spray at petal drop and up to 6 weeks after. Have you thought of applying Kaolin clay, its not a chemical and it works for me. Then bag your apples.

You may also be having pollination problems. Unpollinated flowers will bear fruit, but they abort.

Unless it's simply over-pollination, I am leaning towards this being a soil/nutrient problem. Only because of how recurrent it is.

I would consider your watering regimen. If you let them dry out or do not water regularly you could get fruit drop because your trees are stressed. Just a thought. I water regularly with soaker hoses and don't have any fruit drop.