Downy mildew (fungus) on grapevines develops on all herbaceous parts of the plant, with a particular fondness for growing (water-rich) vines.
The facies (oil spots) of downy mildew on grapevines, often observed on young leaves, is characterized by the appearance of discolored, yellow, oily patches on the upper surface, followed by the formation of a fairly dense white down on the underside.
The altered tissue turns brown and dries out. The "mosaic" appearance of mildew is usually observed at the end of the season on older leaves. The mildew attack manifests itself in numerous small yellow to reddish-brown spots, bounded by the veins.