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. This pathogen survives on overwintered infected plant residues, nearby crops, or host weeds, spreading via airborne spores or insect vectors like thrips. It often is a secondary infection, colonizing lesions caused by stronger pathogens like downy mildew or insect-damaged, heat-, or water-stressed leaves.

Favorable conditions include high moisture and moderate temperatures. Cultural management involves planting disease-free seed and transplants, managing host weeds and thrips, and rotating away from susceptible crops allium crops for at least three years.


  • 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.


Last updated Aug 2023

Photo Credit: Dr. Amanda Gevens, Department of Plant Pathology, UW-Madison, WI