Smut of Onion: Symptoms and Management (Control Measures)

Smut of Onion
Smut of Onion

Smut of Onion

Causal organism: Urocystis magica (syn.= U. cepulae) and U. colchici.

Origin: Smut was first observed on the onion in the Connecticut River valley of the USA in 1869.

Symptoms of Smut of Onion 

  • The earliest symptom appears on cotyledons and young leaves as longitudinal blisters that are blackish with a silver sheen. 
  • Dark pustules (sori) appear as slightly thickened areas at the base of seedlings near ground level. Blisters rupture to expose black, powdery spore masses (smut teliospores). 
  • The seedling often dies before emergence. A large lesion can cause leaves to curve downward and usually are shed prematurely.
  • The infection progresses inward from leaf to leaf, and infected leaves become stunted and may die within 3­-4 weeks after emergence.
  • If the plants survive, the disease becomes systemic, and they remain at a vegetative stage for the entire growing season. 
  • Elongated, raised black blisters can also be found in the outer scales of the developing bulb. 

Favorable condition of Smut of Onion

  • The optimum temperature for spore germination and growth is 13­-22C. 
  • Infection of the seedling can occur equally well at a temperature of 12-­25C. 
  • Above 29C, Onion seedlings grow without infection in even heavily infested soil.

Control Measures/Management of Smut of Onion

  • Prevent soil and plant debris movement among fields to prevent the smut pathogen from being introduced into other fields. 
  • Practice a three­year or longer rotation to non­hosts such as small grains and corn. 
  • Transplants should be used instead of seeds in infested soil. 
  • The disease can be controlled by seed treatment with Thiram @ 45 gm for 0.45 kg of seed.
  • Spraying formaldehyde solution in the furrow.

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