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Ministry of Agriculture, Food and Rural Affairs

onion smut

Onion smut Onion smut Onion smut Onion smut Onion smut
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Beginner

Scientific Name: Urocystis cepulae

Identification

  • Infection initiated in the flag leaf as it emerges from the soil
  • Characteristic black streaks and blisters appear in the plant stem and developing bulbs
  • Many seedlings are killed by the disease; those that live produce distorted bulbs with dark streaks and lesions

Often Confused with
Onion maggot damage

Period of Activity
A cool, wet spring increases the incidence of smut infection because onion seedlings grow slowly and the flag leaf is in the soil for a longer period.  Infection occurs at 10-12°C (50-54°F).  The disease attacks seedlings, with most dieing within 3-5 weeks of infection.   

Scouting Notes
Regular field scouting methods are the best management practices to assess disease levels. 

Thresholds
None established.       

 

 

 

Advanced

Scientific Name: Urocystis cepulae

Onion smut is a disease that persists for long periods of time in the soil.  Smut spores survive in the soil for many years, and even long crop rotations may not reduce disease incidence.  The spores spread incredibly fast and with ease. Plants that survive infection often produce little to no bulb; bulbs that do form are often covered in black lesions. 

Identification

  • Infection initiated in the flag leaf as it emerges from the soil
  • Characteristic black streaks and blisters appear in the plant stem and developing bulbs
  • Many seedlings will be killed by the disease those that live produce distorted bulbs with dark streaks and numerous lesions

Biology
Smut survives in the soil as a spore for extended periods of time.  Spreading of smut spores occurs in surface drainage water, on equipment, via wind borne soil and human activity.  Onion sets grown in infected soil and transplanted into smut-free soil can also transmit the spores. 

Onions are primarily susceptible to smut after germination and during the development stage of the first true leaf.  After these stages, there is reduced risk of infection. 

Often Confused with
Onion maggot damage

Period of Activity
A cool, wet spring increases the incidence of smut infection because onion seedlings grow slowly and the flag leaf is in the soil for a longer period.  The disease attacks seedlings, so the greatest risk is early in the season.  Most infected seedlings die within 3-5 weeks of infection.   

Scouting Notes
Most commercial producers purchased treated seeds or apply an in furrow fungicide at time of seeding to help reduce the incidence of onion smut. Most infected seedlings die, but those that survive remain in an unhealthy vegetative state.  Some plants may recover from the infection by shedding their leaves in the second month of growing.  Bulbs of infected plants will still have back streaks and are susceptible to passing on the disease to other bulbs in storage. 

Thresholds
None established.      

Management Notes

  • The only practical means of controlling onion smut is the use of a seed treatment or fungicide in furrow
  • Planting onion seeds too deep may increase risk of infection
  • Seed treatments can reduce losses to the disease and growing onions from transplants avoids the disease
  • Currently there are no commercial varieties of onion available that are resistant to smut