Generally nice looking trees. Others can give you more and better advice, but here goes on a little.

Prune out all dead wood/branches on all trees. They could use some additional pruning to open them up and perhaps lower their height to a more reachable level, but read up on pruning and renovating older trees for that.

You may want to remove some of the lowest branches on the plum to get them off the ground so that you can get a lawn mower under it or reach under to add mulch/remove weeds. The plum looks like it may be a european plum. The second picture shows some pectin oozing from the plum, probably some kind of insect damage. My plums exude pectin also, but it doesn't affect the edibility of the plum. That might not be the case in your area. The third picture is of a fruit mummy from last year. Mummies should be removed in winter to reduce disease and insect pest carryover.

The 4th picture with the hole in the trunk is a peach tree, not an apple. It has lost a major branch in the past, but most of the tree looks good other than the dead/stunted branches to the left. There isn't much you can do to stop any further decay inside the main trunk from the open hole, however the tree isn't that tall or heavy so it may live years more before the trunk damage ever causes it to fall over. Since it isn't that big a tree, it probably won't hurt anything when it does, so I don't see a reason to remove the tree prematurely unless the dead and stunted branches on the left are a sign of disease that will soon kill the rest of the tree.

The grey scales are lichen and quite normal on trees where there is moist air/rain and shade. They won't hurt the tree and add a lot of interest to the trunk and branches.

rastoma-
On the idea of pruning, there are a lot of really good videos up on YouTube these days... including ones for rejuvenating neglected trees. I would spend a little time watching them. Then come back here with any questions you might have. I know I've found them quite helpful.
-Glenn

Excellent, thank you both.

I'll read up on pruning and then come back.

Since there was no fruit except on the plum and pear I guess I got the other mixed up :)

Until I learn more about pruning, is there anything else I need to do in the meantime? I've seen all kind of 'fruit tree food' and fertilizer at various garden stores. I've been afraid a lot of that is more gimmick then good. But is there something that's cost effective to feed the tree or keep pests off? Or is too late in the season for that?

What about mulch around the base of the trees? Other than looking good, does that have any benefits?

May I emphasize that this is not the time for major pruning. Save that for when they are dormant. Now is the time for general clean-up. Clean out that rotted hole in the peach tree and seal it with pruning seal. clean out any diseased stuff (the dried up plum) and remove it completely from the orchard area so it doesn't promote return of the infection.

Prune as needed while they are dormant. Keep in mind to prune for general shape first. Get rid of crossing branches. Remember that stone fruits generally need somewhat aggressive pruning as they tend to bear on one to four year old wood. That means that you want to remove about 25% of the new growth every winter to keep plenty of bearing wood on the tree.

Don't forget dormant sprays in the winter. Study what you want to do as far as organic or not, but you will have problems with your plums and peaches if you do not spray. I use light oil which I hear is OK for organics as it is a mechanical assassin, not a chemical assassin. I supplement it with fixed copper, which may not be as environmentally friendly, but I don't use Malathion or other similar insecticides. Dormant sprays several times over the winter are critical to controlling peach leaf curl and bacterial infections.

Congratulations. I'm sure you will enjoy your trees and the fruit they provide.

Prune when you have time. Over the years I have become completely sanguine about removing even large branches most any time of the year, although early fall is supposed to be the most taxing on the trees because they need all their resources for the hardening off process.

Years ago when I first learned that espalliers are primarily pruned in summer I began this liberation from the early spring only mindset. Dormant pruning is primarily to reduce the dwarfing affect of pruning. Once trees have reached a desired size this is no longer an issue. Dormant pruning is also best for trees without adequate vigor.

I actually believe beginners can benefit from pruning a tree entirely when in full leaf in the mid fall and using it as a point of comparison while dormant pruning so they can see how open the tree needs to be once it leafs out.

There has been some research that indicates that summer pruning can reduce sugar content of fruit, I have no idea what this pruning consisted of. Espallier fruit tends to be very high quality so there has to be some conditional aspects to this that I don't yet know about.

I renovate hundreds of huge neglected apple trees and have found that if I prune them in late summer the bark is much less apt to be scorched by major branch removal for reasons too involved to go into now.