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 pest has long been known as an enemy of Strawberries, both cultivated and wild plants suffering equally. This same fungus is also the cause of the still more serious " Hop Mildew ", and is also more or less common on about twenty kinds of British wild plants, hence the opportunities for cultivated plants becoming infected are ample. Fortunately, in the case of Strawberries, only the leaves, as a rule, are attacked, and that usually somewhat late in the season. When the leaves are attacked the edge bends upwards, exposing a considerable amount of the under surface of the leaf, which, on careful examination, is seen to be more or less covered with a very delicate, whitish mildew, which is powdery here and there, due to the accumulation of the summer spores of the fungus. When the fungus attacks the foliage early in the season, before the flowers expand, the mildew passes on to the fruit, which usually becomes entirely covered with a white mildew. The fungus does not usually retard the growth or ripening of the fruit, but it renders it absolutely useless - being devoid of all sweetness, and having an insipid and watery taste. It is not unusual to meet with fruit on sale that has been rinsed in water to get rid of the mildew; but it presents a dull, waterlogged appearance, and, as stated above, is worthless for eating or jam making.
Where the disease has previously existed the leaves should be mowed and burned, as described under " Strawberry-leaf Spot". Early the following spring, when the leaves are expanding, spraying with sulphide of potassium should be commenced, and continued at intervals, if a trace of mildew is observed, until the blossom is ready to expand. The under surface of the leaf is the part attacked by the mildew, hence the nozzle of the sprayer should be of such a pattern as will allow of its being placed close to the ground, under the leaves. [g. m.].