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.