Neck Rot of Onion

Neck rot of onion, garlic and shallot is one of the major bulb destroying diseases of these crops. It is caused by a fungus which usually infects mature plants through the neck tissues or through wounds in the bulbs.

SYMPTOMS

The disease is most common on the bulbs after harvest and is generally associated with the neck tissue. Occasionally, infections will occur through cuts or bruises. The affected scale tissue first becomes sunken and soft. The older diseased tissue becomes grayish in color and eventually a dense gray mold develops between the onion scales. Sometimes hard, black bodies (sclerotia) will form around the neck. Usually soft-rot bacteria invade causing the bulb to break down into a soft mass; often accompanied with a foul sulfur odor.

CAUSAL ORGANISM

This disease is caused by the Botrytis fungus. There are three species of the fungus known to cause slightly different symptoms; however, control measured are the same for all species.

CONTROL

The most common point of infection is through the exposed succulent tissue when plants are topped before they have dried sufficiently. A combination of several of the following cultural procedures should reduce losses.

1. Plant varieties that mature properly so neck tissues are dry before storage. Generally colored varieties are more resistant than white varieties.

2. Do not apply nitrogen fertilizer after mid-July and be sure that slow release nitrogen fertilizers are not applied too late in the season.

3. As harvest time approaches discontinue irrigation to allow tops to dry down.

4. Allow tops to mature well before harvest.

5. Undercut and windrow onions until inside neck tissues are dry before topping and storing. Do not store improperly cured bulbs.

6. Avoid injury during harvest. Bruised or damaged tissue is very susceptible to Botrytis. Therefore, do not crush the tops to expedite drying; allow natural ripening. Do not roll the onions in the soil during windrowing unless the tops are well dried. Hand topping causes less rot than mechanical topping.

7. Fungicide applications during the season and especially prior to harvest may reduce the incidence of neck rot. However, fungicide applications cannot overcome improper cultural or storage practices. Use chlorothalonil (Bravo), or maneb plus the zinc ion (Manzate 200, Dithane M-45 or Mancozeb). Follow label directions.

8. Cure onions with forced, heated air at 80-95 F (27-35 C) for a few days at the beginning of the storage period.

9. Store at temperatures slightly above freezing at a relative humidity of about 64%. Do not circulate warm air over cold onions since this will cause sweating with resultant mold problems. Open the storage doors when the air outside is cool and dry to exhaust warm moist air.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

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