HPIPM:Onion Smut

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Ustilaginomycotina
Class	Ustilaginomycetes
Order	Urocystidales
Family	Urocystidaceae
Genus	Urocystis

Scientific Name

Urocystis colchici

Common Name

smut

Author: Howard F. Schwartz, David H. Gent, and Michael E. Bartolo

Identification and Life Cycle

Smut is caused by soil-borne fungi, Urocystis colchici or U. cepulae, which primarily affect seedling plants. These pathogens attack only members of the onion family including onion, leek, and green bunching onion, and are a problem on seeded onions grown in muck soils. Susceptibility decreases as tissue ages, and transplant seedlings are rarely attacked. Affected bulbs are predisposed to other infections in storage. The smut pathogens survive in the soil, and can be disseminated among fields by the movement of soil particles or infested crop debris.

Plant Response and Damage

Symptoms appear on cotyledons and young leaves as longitudinal blisters that are blackish with a silver sheen. Seedlings often die before or within six weeks after emergence. Onions are susceptible to smut infection only during germination and emergence, after which plants become resistant. If plants survive, the pathogen becomes systemic in embryonic tissue and plants remain vegetative for the entire growing season. Developing bulbs become covered by blackish lesions, and are predisposed to infection by other organisms that cause secondary rots. When the skin of blisters splits, fungus spores are released onto soil where they remain infectious for many years. Infection is favored by cold, damp weather in the spring which delays emergence and favors infection. The disease is rarely of concern in the High Plains.

Biological Control

Biological control strategies have not been developed for smut.

Cultural Control

Practice a three-year or longer rotation to non-hosts such as small grains and corn. Transplants should be used instead of seed in infested soil. Prevent the movement of soil and plant debris among fields to prevent the smut pathogen from being introduced into other fields.

Chemical Control

Chemical controls are most effective when combined with sound cultural practices. Only seed treated with a fungicide should be planted.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.