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 destructive mildew attacks Apple, Peach, Quince, Cherry, and various other fruit trees belonging to the Rose family, but the Peach suffers most, as the fruit is so frequently attacked. When the fruit is about half-grown, one or more whitish patches appear on its pale-green surface. These patches gradually increase in size and present a powdery appearance, as if sprinkled with flour. These patches represent the summer fruit of the mildew, and this is the stage in which the disease is first noticed by the grower, judging from the numerous examples received for identification during an experience of twenty-five years. Nevertheless, in ninety-nine cases out of a hundred, the mildew first appears on the young leaves and shoots, and from thence passes on to the fruit. If a careful watch is kept soon after the leaves are developed, and the mildew arrested on its first appearance on the foliage, by spraying with sulphide of potassium, the fruit would be saved from attack. On the other hand, if the fruit once becomes infected, spraying is practically useless, as the dense mat of down on the surface of the fruit protects the fungus from the fungicide (see p. 51).