Onion smut

Onion smut is a plant disease affecting the allium family, in particular, onions and shallots. Spring onions, leeks, garlic and chives are less susceptible to the disease.

Onion smut has been sporadically detected in South Australia since the 1950s and it is important that growers remain vigilant and adopt on-farm biosecurity practices that will protect crops and reduce the risk.

About the disease

Onion smut is a soil borne disease that attacks only the seedling stage, with most infected plants dying at 3-5 weeks of age.

Diseased plants that survive are often distorted, develop blisters on green leaves, remain stunted, and will not develop marketable produce. As a result, any produce from infected areas that reaches market is unlikely to be infected with onion smut.

Onion smut can be well managed through a range of measures including seed treatment to protect emerging seedlings, use of resistant varieties and cropping rotations.

Onion smut is not a public health risk and onions are still safe to eat. Onions infected with smut generally wither and die in the paddock and do not reach market.

What to look for

  • Onion smut has distinctive narrow long dark streaks, usually on the cotyledon (embryonic leaf) or the first true leaf, which become thickened.
  • The streaks are evidence of an infection inside the leaf which later bursts through the leaf surface and releases masses of dark-brown powdery spores.
  • Dark streaks are the most visible symptom on any part of the plant.
  • Infected plants that do survive become stunted, distorted, develop blisters on the green leaf tissue and the leaves may curve downward due to large lesions.

Read the onion smut fact sheet (PDF 1.3 MB)

How it can spread

Evidence suggests potential for natural dispersal is limited. Onion smut primarily spreads through contact with diseased seedlings and infected soil.

It can spread from one property to another on clothing, shoes, equipment and vehicles, if these materials have contaminants on them.

How to reduce the risk

Plant pests and diseases can easily spread from one property to another on clothing, shoes, equipment and vehicles. Maintaining good on-farm biosecurity practices will help protect crops and reduces the risk of spread.

Make sure any machinery is decontaminated and restrictions on movement are in place. Other on farm biosecurity measures you can put in place, include:

  • Make sure any onions or other Allium plants are free of soil prior to movement
  • Make sure bins are free from soil or plant material, or remove bins from your production system and harvest directly into trailers which are cleaned down between paddocks
  • Maintain biosecurity practices around people, vehicle and equipment movements
  • Display signage on your farm to alert people to the potential impact they could have entering your farm
  • Have a visitor register on your farm to track movements
  • Brief all workers on your biosecurity procedures.
  • Provide cleaning materials such as scrubbing brushes, disinfection footbaths, boot covers, rubber boots and protective clothing such as disposable overalls to people entering your farm
  • Make sure visitor and employee footwear, clothing and equipment is free of soil and plant material before entering or leaving your farm.

Read the maintaining on farm hygiene fact sheet (PDF 216.6 KB)

How to report

Industry and agronomists are encouraged to report any unexpected symptoms observed in the field. If you suspect onion smut, immediately call:

More information

Page Last Reviewed: 14 Oct 2019
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