Onion Stemphylium Leaf Blight

Onion Stemphylium leaf blight is a fungal disease of alliums caused by Stemphylium vesicarium. It causes oval-shaped, tan or brown lesions on the leaves, which may appear water-soaked and darker when sporulating. These lesions can enlarge and overtake entire leaves, as well as girdle seed stems. Blighted leaves can compromise the bulb, reducing yield and leading to secondary infections.

Primary Source: Infected plant debris
Spread: Airborne spores, often appears as a secondary infection of downy mildew lesions, herbicide or physical plant injury, and heat-stressed/drought-stressed leaves
Favorable Conditions: High humidity, moderate temperatures, excess moisture from precipitation or irrigation, high dew point

Infection & Disease Cycle

Stemphylium vesicarium overwinters in plant debris. Once temperatures warm in the spring, airborne spores called “ascospores” are released from this plant debris, infecting nearby leaves that have been wounded by other diseases (including downy mildew), insects, heat-stress, or damaging elements including weather, or chemical/mechanical injury. Subsequent lesions will produce airborne and waterborne spores called “conidia” that will travel, causing secondary infections. At the end of the cropping season, the pathogen will again overwinter in plant debris, continuing the disease cycle.

Cultural Control

Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:

  • Rotate away from susceptible crops ( 3-4 years)
  • Maintain proper nutrition & avoid excessive nitrogen application
  • Maintain proper moisture levels in the crop
  • Take care with use of herbicides to avoid phytoxicity or other injury
  • Destroy infested plant debris & culls
  • Manage onion downy mildew

Chemical Control

For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.


  • Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
  • UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.



Written by Amanda Gevens, Ariana Abbrescia, Russell Groves, and Ben Bradford. Last updated Nov 2023

Photo Credit: Dr. Amanda Gevens, Department of Plant Pathology, UW-Madison, WI
Close view of onion leaf damage due to Stemphyllium leaf blight. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Field of onions with Stemphylium leaf blight