Purple Blotch

Elliptical brown lesion on onion leafElliptical brown lesion on onion leaf.

Multiple purple blotch lesionsOnions with multiple purple blotch lesions

Severe purple blotch infection with enlarged lesionsSevere purple blotch infection with enlarged lesions.

Onion crop infected with purple blotchOnion crop infected with purple blotch.

HOSTS

  • Garlic
  • Leeks
  • Onion
  • Shallot

BIOLOGY

Purple blotch is caused by by the fungus Alternaria porri. It is a concern throughout the entire growing season.

  • Infection occurs when fungal spores from nearby plants are blown by wind into onion fields where they infect wounded plants.
  • Overwinters in crop residue on or near the soil surface.
  • Optimum temperatures for infection are between 77-85°F.

SYMPTOMS

Early symptoms include small brown elliptical spots on leaves, similar to IYSV lesions, which enlarge over time and may result in brown, necrotic streaks. Lesions will eventually turn purple as fungal spores develop. In severe cases, lesions enlarge and coalesce to blight the entire leaf. This pathogen requires wounds caused by other diseases (e.g. botrytis), thrips feeding, or hail, to enter the plant.

GENERAL MANAGEMENT

This disease is occasional in Utah onion production. Older leaves are more susceptible.

  • Watch for symptoms starting on older leaves throughout the growing season.
  • Rotate crops.
  • Avoid excessive nitrogen applications.
  • Bury or dispose of culls and other plat debris.
  • Application of fungicides

Use fungicides as soon as the disease has been confirmed to prevent further spread. Treatment for purple blotch also controls Stemphylium leaf blight.
Click here for fungicide options for commercial farmers.
Click here for fungicide options for home gardeners.




















Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.