Onion downy mildew is a disease of onions and related crops that damages foliage and bulbs, resulting in loss of yield or even a complete failure of worthwhile bulbs to form.
Onion downy mildew is caused by a fungus-like organism, which begins by infecting the leaves. In mild humid weather the bulbs of shallots and onions and garlic may also become affected. Perennial onions such as chives may also be affected. Downy mildew is worse in cool wet seasons and in wet areas. Closely spaced crops such as salad or spring onions are especially vulnerable.
There is no chemical available to control downy mildew on onions and other alliums.