This disease (fig. 392) is said to have been introduced into Europe from the United States, where it is very destructive both to cultivated and native wild vines. Every part of the plant is attacked, although the foliage suffers most, where its presence is first indicated by the appearance of sickly yellowish-green patches on the upper surface of the leaf, corresponding in position with patches of a very delicate greyish mildew on the under surface of the leaf. Under favourable weather conditions - that is dull, warm, and moist - the patches of mildew rapidly increase in size, and often cover the greater portion, or the whole, of the leaf. After this stage is reached, the leaf turns yellow, then brown, and falls. Tendrils and flowers are as promptly killed. Even if the berries are not attacked they do not mature properly, owing to the loss of the foliage, and the entire plant suffers through lack of food, which tells upon it the following season. Resting spores or winter fruit are formed in abundance in the tissues of all diseased parts of the vine. These, if not removed, germinate the following spring, and set up the infection again.

Vine Mildew (Plasmopara (Peronospara) viticola).

Fig. 392. - Vine Mildew (Plasmopara (Peronospara) viticola).

1, Summer form of fungus on grapes (natural size). 2, Summer fruit (magnified 80). 3, 4, 5, Detached spores of summer fruit (magnified 350). 6, Winter form of fruit formed on the mycelium of the fungus present in the tissues of the diseased portion of the plant (magnified 350).

On the first indication of the disease, the Vines should be sprayed with half-strength Bordeaux mixture, the spraying being repeated at intervals as required, until the berries are set.