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| Onion (Allium cepa) -- Neck Rot |
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| Cause:
Botrytis allii and B. aclada, fungi that overwinter on plant debris in soil, on infected bulbs, and as sclerotia in soil. The pathogens also can be seed-borne. The disease is seen primarily in storage. Most common causes of severe losses are excessive nitrogen, which delays crop maturity; irrigation and/or rain late in the season; inadequate or improper curing; and improper storage. Infection is through neck tissue or wounds in bulbs. Some bulb infections may arise from symptomless leaf infections, but often the fungus directly enters the neck via airborne spores when onions are topped before soft, susceptible top tissues dry properly.
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| Symptoms:
Disease is more apparent after harvest, while bulbs are in storage. At first, soft neck tissue looks water soaked, and a yellow discoloration moves down the neck into the scales. Bulbs break down to a soft mass. A gray mold develops between the onion scales, later producing small to large black bodies (sclerotia) which develop as a solid layer around the neck. |
Cultural
control: Bulb onions:
- Allow tops to mature well, then lift or undercut the onions. In dry weather, cure onions on the ground 6 to 10 days.
- Increase the length of onion necks at harvest.
- Minimize bruising and mechanical injury in topping and storing.
- Store in well-ventilated houses at 32°F or slightly higher. Use higher temperatures if humidity cannot be held below 75%.
- Practice a crop rotation of at least three years.
Seed onions:
- Bury culls and onion debris from onion crops in nearby fields.
- Avoid planting new seedlings within several miles and/or downwind from current seed crops.
- Practice a crop rotation of at least three years.
- Use seed treated with commercial fungicide.
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Chemical
control: - Dithane DF Rainshield at 2 lb/A on a 7- to 10-day interval can be used on Allium seed crops. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
- Nevado 4F at 1.5 pints/A on 14-day intervals. Do not apply within 7 days of harvest. 24-hr reentry.
- Presidio at 3 to 4 fl oz/A on 10-day intervals as a tank-mix. Preharvest interval is 2 days. 12-hr reentry.
- Pristine at 14.5 to 18.5 oz/A on 14-day intervals. Do not make more than two (2) sequential applications of Pristine before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days. 12-hr reentry.
- Quadris Opti at 1.6 to 3.2 pints/A on 7- to 14-day intervals. Do not apply more than one foliar application of Quadris Opti or other Group 11 fungicide before alternating to a
fungicide with a different mode of action. Do apply within 7 days of harvest. 12-hr reentry.
- Switch 62.5 WG at 11 to 14 oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
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References:
Chilvers, M. I., and du Toit, L. J. 2006. Detection and identification of Botrytis species associated with neck rot, scape blight, and umbel blight of onion. Online. Plant Health Progress doi:10.1094/PHP-2006-1127-01-DG. |
| Content edited by:
C. M. Ocamb and D.H. Gent on
January 1, 2009 |
