Downy mildew thrives in moist damp conditions and loves young plants. The upper leaf surfaces develop yellowy discoloured patches which can extend across large areas of the leaf. These patches are matched on the underside of the leaf by areas of fuzzy white or greyish, fungal growth.
None other than as described above.
Treatment and control
Spray with a suitable fungicide: Spray with a suitable fungicide containing Mancozeb, for example Dithane.