Onion downy mildew is a common fungal disease that attacks onions, chives, shallots and salad onions. It is widespread especially in cool, damp, humid weather and wetter parts of the country.
Affected leaves take on a greyish appearance, then turn yellow, start to wither and collapse from the tips. In moist conditions a white mould which turns purple develops on the dead foliage. The bulbs are usually soft and shrivel in store.
Onion downy mildew is a difficult disease to control as there are no suitable fungicides available. Lift and destroy the affected plants as soon as the symptoms appear. Spacing the plants further apart and keeping weeds under control will aid air movement around them, reducing the risk of attack. Avoid growing the crop in a damp, sheltered position. Where the disease has occurred do not replant onions or related plants in the same area for five years.
Grow onions in a different area each year and ensure the soil is well drained. At the end of the season clear up any remaining debris.
The variety Santero shows good resistance to this disease.