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 is a white superficial mildew, appearing on the leaves, and sometimes - but rarely - passing on to the fruit. It grows on both surfaces of the leaf, and forms a very thin film which remains permanently white. Soon after the appearance of the mildew its surface becomes minutely powdery as if sprinkled with flour. This dusty appearance is due to the presence of myriads of the summer form of fruit, which are conveyed by various means to adjoining bushes, and thus the disease spreads apace unless checked.
At a later stage minute black bodies, smaller than the head of a small pin, may be seen scattered over the surface of the white mouldy patches. These are the winter form of fruit, which remain on the dead leaves until the following spring, when the spores are liberated and infect the young leaves.
This is not considered as a serious disease, at all events in Britain, and but rarely does real injury to the plants attacked; but as so many diseases, usually insignificant, suddenly, for some unknown reason, become destructive parasites, it is wise to arrest their development.
Spraying with half-strength Bordeaux mixture, or with a solution of potassium sulphide, on the first appearance of the mildew, will arrest its spread. Presumably the ground under the bushes- will be turned over during the winter. This will bury all dead and infected leaves, otherwise there will be a possibility, or almost certainty, of infection the following season.