Peaches, nectarines and apricots flower in very early spring, and correct pruning is vital to successful pollination. If growth is congested, the emerging flowers are inaccessible to the very few pollinating insects abroad so early in the year. So we prune out the leafy growth in late winter to perhaps a few inches, or three or four pairs of leaf buds on each branch growing from the main fan-trained branches. The same is done in summer, but here the removal of most of the summer's leaf growth (which can be as much as 2' or 3' long if you take your eye off the game...) is to expose the ripening fruits to the sun. This pruning regime can appear recklessly excessive, even to one whose Felcos are never far from her hand.
Sometimes branches are left for a couple of seasons, then the loppers and pruning saw come out of the box and into action. The top picture shows this spring's growth of fresh new leaves bursting from the site of two sawn branches. The smaller branch growing underneath this sawn branch is being trained to replace the older, thicker branch above it, perhaps in a couple of season's. The third picture shows a similar remedially pruned branch producing new fruiting growth, and the red string in the fourth picture is a visual reminder that further work is required at this point.
In the cool, wet climate of the UK peaches and nectarines are vulnerable to peach leaf curl, Taphrina deformansa, a serious fungal disease prevalent in wet conditions. Fan-training peaches and nectarines in the glass house protects their early blossoms from frosts, and excludes rain so the spores of the peach leaf curl fungus lack the moisture required for germination. So throw out those chemicals, feed the soil not the plant, and plant a very large clump of marjoram or heather at the glass house door, to entice the early insects inside.