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 disease is caused by one of the superficial mildews, related to the Hop Mildew, American Gooseberry Mildew, etc. It is widely distributed in this country, and probably exists wherever the Apple is cultivated. The young leaves clustered at the tips of the shoots present the appearance of having been thickly dusted over with flour, and the action of the fungus is to arrest the growth of young shoots, consequently the formation of new wood is prevented when the parasite is present in abundance.
When the fungus is present in small quantity, spraying with Bordeaux mixture, or with a solution of liver of sulphur, may arrest its progress; but when present in its worst form the only certain means of restoring the tree to health is to cut off and burn all the diseased rosettes of leaves. During the following spring the pruned trees should be sprayed with Bordeaux mixture. Trees thus treated throw out healthy shoots.
As the winter fruit is rare, and not in sufficient quantity to account for the enormous development of mildew each season it is supposed that the mycelium of the fungus persists over year in the tips of the shoots. Hence the necessity for removing these.
All suckers should be removed, as the mildew often commences on them, and it is there where the scanty amount of winter fruit is mostly produced. [g. m.]