These two peach trees are about five years old. As you can see, they're about the same size, but one is flourishing and the other is dying. They (as far as I can tell) get the same amount of water, sunshine, same soil, etc. etc. etc. (They don't fruit, but that's due to late frosts here...)
The dying leaves turn pinkish-red and drop off, as if it were autumn....but it isn't.