Text Size:   A+ A- A   •   Text Only
Site Image
Onion smut
Urocystic cepulae (= U. magica and U. colchici)

Plant hosts
  • Major hosts: Allium (onions, garlic, leek, etc.), Allium cepa (onion), Allium porrum (leek)
  • Minor hosts: Allium sativum (garlic)
  • Wild hosts: Antirrhinum (snapdragon)


Dark streaks first appear on cotyledons (the first true leaves). Many seedlings die during emergence or within a few weeks after emergence reducing plant populations. Those plants that live have tissues that are thickened and blisterlike. The blisters erupt and release masses of dark-brown powdery spores. As the infection progresses inward from leaf to leaf, plants become stunted and bulbs distorted. Mature bulbs less commonly have dark pustules. In storage, affected bulbs often succumb to infection by other pathogens.

Symptoms of onion smut
Symptomatic onion bulbs
photo courtesy of D. Ormrod

The fungus overwinters in the soil as spores and persists for 15-years or more. Smut spores are moved from one field to another on infested plants or bulbs, by surface drainage water, wind-blown soil, or soil contaminated tools and equipment. Although smut spores have occasionally been found on onion seed, spread on seed has not been considered important. Before the cotyledons emerge from the soil, the fungal spores germinate and infect the young tissue. The smut fungi can only infect onion plants in the young seedling stage. Dark lesions develop on the leaves and release spores into the soil. Healthy plants planted in infested soil remain free from the disease.

Geographic distribution
Onion smut is distributed worldwide.

Applicable regulations
Onion smut is a pathogen of concern to Oregon's interstate and international customers. Survey and diagnosis is available through the Commodity Inspection Division for this fungus.
Return to plant pathogen list.