Pacific Pests, Pathogens and Weeds

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Summary

• 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).