Here’s a couple pics of captan burn (figure 1, 2, 3).
It doesn’t look to me like captan burn. I don’t think it’s the hot wind either. It has been hot here in KS, but I’ve never seen heat do things like that to newly planted apple trees. I was thinning apples yesterday and walked by some new apple trees and they don’t look like that.
Normally, mid April might be a tad late for planting for our springs, but as you know we had a really late spring this year so I don’t think that’s the issue either.
I think there is a good possibility it could be some root issue, but you mention you have check the soil daily. But, just to double check all the bases, have you taken a small hand spade and dug down beside the tree (say, at the edge of the wire cage) to a depth of where the roots would be? From the pic, the ground looks like it could be dry, and as you know, it’s been fairly dry here this year. Grass that close to the tree will suck up a lot of moisture. I lost some newly planted apple trees in 2012 when it was so hot and dry. But the ground was very very dry.
If the ground is still moist down where the roots are, then another possibility I can think of is perhaps some herbicide left in your sprayer? A very small amount of glyphosate (Round Up) can be very slow acting and would cause very wimpy growth/slow death. That’s probably unlikely though, because it looks like you have perhaps another older tree in the background in one of the pics, which was probably sprayed with the same sprayer?
Lastly, the new trees look really tall. I can see in one pic you tied off one side of a tree. If the trees are catching a lot of wind and whipping a lot, it can wallow out a hole and the roots won’t have a chance to settle in. When you scrape back the mulch around the trunk, are you seeing any evidence of wallowing out?
If it’s not one of those three things, then I don’t know what is causing your trees to stall and die, but I don’t think it’s the heat.