I haven't identified the disease, but all of the possibilities appear to be fungal, and I know that one basic preventive measure is to clear the fallen leaves away from the tree. I want more clear advice on what that means: how thoroughly must the leaves be cleared, and how far from the trunk?

The trees grow next to the base of a retaining wall about a foot high. In two cases there's a chain link fence along the top of the wall with a hedge behind it. In one case there's a storage shed flush with the wall, and on another side is a chain link fence that separates my property from a neighbor's.

I've raked up most of the leaves in the accessible areas for three or four feet from the base of each tree, except in the last case, where the property line is only a couple of feet from the base. Cleaning under the hedge would be very difficult. Going into the neighbor's yard is out of the question.

How good a job have I done? If I need to do more, any suggestions on the measures that would be most effective?

I have the book from UC Davis "IPM for stone fruits". The apricot diseases that might damage your fruit are brown rot, Powdery mildew, shot hole, and jacket rot. Look into those on the UC Davis pest management website. Maybe you can identify the issue.

Cleaning up the leaves can't hurt but is probably not the source of any infection.