Diseases of Vegetables - Onion

Basal Rot
Fusarium oxysporum f. cepae

What to look for?
This disease is worldwide and again caused by a specialized form of Fusarium. This disease can begin in the field and continue on in storage.

Photo: Ormrod

Picture description
Fusarium rot on onion.

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Management strategy
Growers must follow crop rotation and harvested bulbs must be thoroughly cured to reduce potential storage losses. Onions are very sensitive to low soil copper levels. In order to optimize crop production and disease susceptibility, additional soil copper fertility may be needed especially on mucky and sandy soils.

Downy Mildew
Peronospora destructor

What to look for?
This is a sporadic disease but very rapid spreading occurs under suitable conditions destroying most of the foliage.

Photo: Ormrod

Picture description

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Management strategy
Destroy cull onions and remove any perennial onion species from the vicinity of the crop. Fungicide sprays are sometimes effective. The disease affects all parts of the onion and other related plants such as the perennial welsh onion.

Leaf Blight (Blast)
Botrytis squamosa

What to look for?
Botrytis is the major disease of onions in cool climate areas. Light infections do not affect yields but heavy infections causing major yield reductions can occur.

Photo: Ormrod

Picture description
Botrytis leaf blight lesions.

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Management strategy
Fungicides may be applied every 5 - 7 days for disease control.

Pink Root
Pvrenochaeta terrestris. Fusarium spp.

What to look for?
This disease occurs sporadically in Canada. It can persist in the soil indefinitely.

Photo: Ormrod

Picture description
Infected onions are undersized.

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Management strategy
Infection is usually spread by onion sets. No other control procedures are known.

Rot
Penicillium spp.

What to look for?

Photo: Evans

Picture description
Penicillium on the skin surface.

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Management strategy
Several fungal and bacterial diseases are capable of causing storage rots in onions, particularly if the crop is inadequately or poorly cured following harvest. Adequate ventilation must also be supplied during storage.

Smut
Urocystis magica

What to look for?

Photo: Ormrod

Picture description

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Management strategy
Onion smut is a reason why onion growing has to move from one region to another. Stand reductions of over 50% have been reported. Spores can persist in the soil for 15 years or more. Onion sets are a major way of distributing this disease. Seed treatments have some success.

White Rot
Sclerotium cepivorum

What to look for?
A very destructive and wide spread disease of onions world wide. Primary infection occurs from long lived sclerotia in the soil.

Photo: Evans	Picture description Small damaged bulbs showing fungal infection.
Photo: Ormrod	Picture description Masses of long lived sclerotia.

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Management strategy
Crop rotation and clean seed or sets are the only effective control. The black resting sclerotia are the primary means by which this fungus persists in the soil for 5 - 6 years or more. They may be transported country-wide on infected onion sets and infect new growing areas.

Photographs and information assembled and prepared for ARD by Dr. Ieaun R. Evans Agri-Trend Agrology Ltd.

Diseases of Vegetables - Onion

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