Downy Mildew is a leaf blight that can affect mature plants and seedlings. Infection in a seedling almost always is fatal. Easily confused with powdery mildew but not even closely related. In fact, downy mildew is not even a fungus but a fungi-like organism.
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.
White-greyish/purple mould on underside of the leaves.
Yellow spotting on the upper surface of the leaves.
Leaves curling up and falling.
Planting resistant cultivars. Treating with hot water on Roses cuttings can be effective, the water must be 44 degrees and treated for 15 minutes.
Some copper fungicides can be useful in treating downy mildew. However, these are being phased out and so you will have to check what is legal and available.
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.
Good hygiene around plants eg. Removing dead leaves from the base of the plant, burning infected material etc. Having good soil drainage. Good airflow around and through the plant. Not planting in areas that have had a recent infection. In a glasshouse, avoid prolonged leaf wetness by watering in the mornings and not watering over the foliage during Winter. Rotating crops can also reduce the risk of infection. Sowing into fresh compost.