Downy Mildew is a foliage blight that produces a white-grey/purple mould that grows on the underside of the leaves. Beneath the mould will be yellowing which may go through to the top surface of the leaf. It can attack mature plants and seedlings. Leaves often curl up and fall from the plant. In onions, the mould appears all around the leaves and on the dying tips. In grapes, the shoots, leaf stalks, tendrils, and even the fruit can present with mould.
Cool damp conditions with poor ventilation.
Spores germinate on the exterior of a wet leaf and enter through the stomata where they grow and impregnate the tissue. Once mature, they will produce a sporangiophore out of the stomata and release new spores into the air.