This very destructive disease was first noticed in a vinery at Margate in 1845, and within a very few years it had invaded Europe, Syria, Asia Minor, and Algeria, and, as usual on the first introduction of a new disease, caused very serious injury for some years. For many years the summer fruit of the fungus alone was known in Europe, and was called Oidium Tuckeri. Of late years, however, the winter fruit has also been found, sparingly. The latter is common on native plants in Japan and the United States. This is one of those parasites where the spawn or mycelium does not enter into the tissues of the plant, but forms a thin white film on the upper surface of the leaves, young shoots, flowers, and fruit. After a time these mildewed patches become densely covered with the white spores, and look as if they had been powdered with flour. This powdery appearance serves to distinguish the present from another white mildew attacking the Vine. When young leaves are attacked, growth is checked, and they usually soon die. In the case of old leaves, only the parts attacked by the fungus turn brown and die, the remainder usually undergoing no change. Young shoots that are attacked turn black and die, as does also the inflorescence. Berries soon crack and become distorted (fig. 391).

Fig 391.   a, Mildew of Grapes, Uncinula spiralis, b, The summer form.

Fig 391. - a, Mildew of Grapes, Uncinula spiralis, b, The summer form (Oidium Tuckeri) with conidia germinating ( x 200).

As the fungus is quite superficial, it can readily be reached by fungicides, and should cause but little trouble if promptly attacked. Spraying at intervals of three days with a solution of sulphide of potassium is the most certain remedy. Flowers of sulphur dredged or blown over the affected parts is also efficient, but it is more difficult to cover every part with a dry powder than with a substance in solution.

The common practice of placing sulphur on the hot pipes is a practice that cannot be recommended. It is risky, and if the temperature exceeds a certain limit, scorching of the foliage is the result.