Worldwide distribution. On onion, and also shallot, garlic and leek. An important disease.
Mostly affecting leaves and causing parts above the blotches to collapse and die. Infections at harvest lead to neck rots.
Spores spread from blotches in wind and rain splash.
Older leaves damaged by thrips are more susceptible to the fungus.
Cultural control: disease-free seed; check plants in nursery for leaf spots; remove “volunteers”; use drip irrigation, not overhead; apply manure to increase plant vigour; harvest when dry and cure before storage; 3-year crop rotation; collect trash and burn or plough in after harvest.
Chemical control: chlorothalonil, copper or mancozeb (protectants); or triazoles (systemics).