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.