[sirpsycho]

I noticed a lot of my onions have white spots on their leaves. After googling, it appears I have Botrytis leaf blight.

Can anyone recommend an over-the-counter cure?

[michael brenock]

more likely you got Cladosporium leaf blight which is always worse in wet weather and overwintered onions are usually the carriers of the disease.There is no real cure for it as it is more weather related and avoid growing overwintered onions. Botrytis dries up in dry weather but Cladosporium does not. Cladosporium also develops a type of black eve from the small white spots coalescing.
michael brenock horticultural advisor iretired)