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.
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 strategies have not been developed for smut.
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 controls are most effective when combined with sound cultural practices. Only seed treated with a fungicide should be planted.
Product List for Smut:
Pesticide |
Product per Acre |
Application Frequency
|
Remarks |
|
EBDC and EBDC Mixtures—not all formulations listed |
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|
Dithane DF |
2.0 – 3.0 lb |
-- |
In-furrow drench |
|
Manex |
1.2-2.4 qt |
-- |
In-furrow drench |
|
Penncozeb 75DF |
2-3 lb |
-- |
In-furrow drench |
|
Thiram Seed Treatments |
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|
42-S Thiram |
6 fl oz |
Seed treatment |
Broad spectrum |
|
Thiram 50WP |
6 oz |
Seed treatment |
Broad spectrum |
|
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Onion, Disease, Smut
Date: 04/01/2007