This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This very destructive disease is present wherever the Peach tree is cultivated. The foliage and the young shoots are the parts affected. Diseased leaves are thicker than healthy ones, and are very much puckered and more or less curled. The colour is at first a pale sickly green, gradually changing to rosy or sometimes bright red. Eventually the diseased leaves become covered with a delicate white bloom, resembling the bloom on a grape. This bloom consists of myriads of spores of the fungus growing in the tissues of the leaves, These spores are conveyed by wind and other agents to neighbouring young leaves and shoots, and by such means the disease is spread. In some instances the leaves only are attacked, and usually form a sort of rosette at the end of the shoot. In many instances, however, the shoot is also attacked, which is indicated by a more or less pronounced swelling or thickening near the tip. Now when a shoot is attacked the mycelium or spawn becomes perennial in the tissues; that is, it lives in the tissues from year to year, and each season passes into the new leaves, which become infected. The injury caused by this disease is a loss of foliage which impoverishes the tree, and tells very materially on the quantity and quality of fruit, also on the general growth of the tree, as when shoots become infected they never form any more wood.
So far as preventive methods are concerned, my advice is to prune all shoots showing leaf curl about 6 in. behind the point where the leaves are produced. The mycelium of the fungus present in the shoots never extends backwards towards older wood, but invariably advances along with the new shoots, where the richest store of food is present to supply food for the growth of the new shoot, into which the mycelium of the fungus passes. Where the disease has existed the trees should be sprayed the following spring with self-boiled lime-sulphur mixture, first just when the leaf buds are expanding (see p. 50).