Pacific Pests, Pathogens and Weeds - Online edition

Pacific Pests, Pathogens & Weeds

Onion purple blotch (151)

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Photo 1. Purple blotch, Alternaria porri, causing spots on leaves of shallots.

Photo 2. Purple blotch, Alternaria porri, on leaves of shallots showing zoned spots. Spores are produced in large numbers on these spots.

Photo 3. Leaf-tip dieback on leek, caused by the purple blotch fungus, Alternaria porri.

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).

Common Name

Onion purple blotch

Scientific Name

Alternaria porri

Photo 4. Purple blotch caused by Alternaria porri, on onion. The large spots are oval, purple and up to 150 mm long.

Photo 5. Leaf-tip dieback on shallot, caused by the purple blotch fungus, Alternaria porri.

AUTHOR Grahame Jackson
Information (and Photo 3) from Gerlach WWP (1988) Plant diseases of Western Samoa. Samoan German Crop Protection Project, Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) Gmbh, Germany; and Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, et al. CSIRO Publishing; and from Madeiras A (undated) Alliums, purple blotch. The Center for Agriculture, Food and the Environment. University of Massachusetts Amherst. (https://ag.umass.edu/vegetable/fact-sheets/alliums-purple-blotch). Photo 2 Howard F. Schwartz, Colorado State University, Bugwood.org. Photos 3&5 Kohler F, et al. (1997) Diseases of cultivated crops in Pacific Island countries. South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia. Photo 4 Howard F. Schwartz, Colorado State University, Bugwood.org.

Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.

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