Botrytis leaf blight is a fungal disease of onions caused primarily by Botrytis squamosa. It begins as small, whitish spots on the leaves, often surrounded by a pale green or silvery halo. As infections multiply, leaf tips begin to die back and entire leaves may eventually collapse.

The fungus thrives during cool, wet or humid weather and produces large numbers of spores that spread by wind. Extended periods of leaf wetness encourage new infections, and the fungus can survive on onion debris and other infected material between crops. Severe infections reduce the green leaf area the plant needs to develop and mature its bulb.

How to Fix It

Remove badly infected foliage and onion debris and dispose of it rather than leaving it in the garden. Rotate onions with unrelated crops and avoid planting new onions beside old onion beds or piles of discarded onion material.

Keep foliage dry as much as possible. Water at soil level, provide good spacing and airflow, control weeds and avoid excessive nitrogen, which can encourage dense, susceptible foliage. Water early enough that any wet leaves dry quickly.

Biological fungicides containing Bacillus species may provide some preventive suppression, but they will not cure infected leaves. For the home garden, sanitation, good airflow and reducing the amount of time leaves remain wet are the most useful controls.