Smut of Onion: Symptoms and Management (Control Measures)

Table of Contents

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.

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.

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.

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.