Even at a young stage the fruits of apples and plums look completely different. Just miniature versions of their mature selves. So if it looks like a plum, it's a plum. But if you really still don't know for sure cut one in half. Plums have a single stone. Apples have several seeds.

I looked carefully at the damaged fruit, removing the drop of hardened sap, and see not a pinprick but a small semicircular slice -- so this is plum curculio, yes? And I will need to pick off all the fruit and destroy it.

I checked the native plum and I am seeing the same there.

I went around and picked up all the dropped fruits from all the trees. Tomorrow I will remove all the fruit from the plums, and check the apples and pears carefully also.

From what I read, I shouldn't be insulating my potted trees with leaves in the fall either. But, I will need to use something.

These plums are still too small even now, to bag to prevent this next year; I guess I will have to spray, and maybe try to wrap the tiny fruits loosely with plastic wrap.

I would appreciate any further suggestions. Is there any purpose to spraying now?

Floral UK, I am sure that it is not an apple. Just not absolutely sure if it is plum, or perhaps nectarine.

Never wrap the fruit in plastic - it will just heat up and rot. If you need to bag the fruit, use a product intended for this purpose (a stretchy knit fabric that looks like footies) or a paper bag.

And yes, it does sound like plum curculio, but photos to confirm would help. Lots of information online on how and when to treat.