Purple blotch of onions

Purple blotch has been seen on onion transplants in the Holland Marsh. This disease is often found on leaves damaged by other diseases, insects or environmental stresses. Infection occurs when warm weather, 18-30°C (64-86°F), coincides with prolonged dews or leaf wetness.  Tipburn has been seen on transplanted onions and can serve as an entry point for diseases like purple blotch and stemphylium leaf blight. For more information on purple blotch, see http://www.omafra.gov.on.ca/IPM/english/onions/diseases/purple_blotch.html#beginner

Purple blotch on onion. One to 3 cm brown spots with reddish-purple margins. As in this example, there may be a yellow zone around the lesion.

There are a few fungicide options for purple blotch, including Manzate Pro-Stick, Pristine WG, Lance WDG, Cabrio EG, Inspire, Aliette WDG, Quadris Top, Fontelis (suppression) and Scala SC. As an aside, some of these products will control or suppress other foliar diseases of onions. For specifics, please refer to labels for directions for use. Pest control products should be used in an integrated pest management program and in rotation for resistance management.

Other ways to control purple blotch?

  • Reduce inoculum by destroying infected crops, remove or relocate cull piles
  • Rotate to non-host crops (e.g. carrot, celery, lettuce)
  • Monitor and control onion thrips levels
  • Harvest onions during dry weather and the tops are dry to prevent introduction into storage


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