Plant Disease Management Handbook

Cause Paratrichodorus allius and P. minor, ectoparasitic nematodes that overwinter in soil. The largest nematode populations and conspicuous plant symptoms occur in cool, damp springs. P. minor affects a wide range of plant species.

Symptoms Roots are extremely short with yellow to brownish cast; plants are stunted. Root tips become darker, stubby, and more branched.

Image related to Onion (Allium cepa)-Nematode, Stubby-root

Note the short, deformed roots.

Photo by Kathy Merrifield.

Plants are stunted (left) and root tips become darker, stubby, and more branched.

Phil Hamm

Cause Pratylenchus penetrans, a migratory endoparasitic nematode that overwinters in plant debris. P. penetrans affects a wide range of plant species.

Symptoms Plants are stunted and lesions can be found on roots, which also lack fine roots.

Biological control

By C. M. Ocamb and D. H. Gent

Neck tissue develops a yellow discoloration that moves down the neck into the scales.

Phil Hamm

Slide is labeled Botrytis allii on onion.

Melodie Putnam, 1995.

By D. H. Gent and C. M. Ocamb

Image related to Onion (Allium cepa)-Iris Yellow Spot

Onion leaves with characteristic ring spots are shown.

Photo by M. Putnam, 2001

An onion plant with a diamond-shaped lesion is shown.

Photo by L. Jensen, 2001

This is an onion field affected with IYSV.

Photo by L. Jensen, 2001

Iris yellow spot on onion scape.

Photo by Cynthia M. Ocamb, 2006.

Cause Fusarium proliferatum, a fungus. This fungus can persist in soil and plant debris. Other crop plants can be hosts to this fungus, including garlic and sweet corn. Fusarium species are good saprophytes, which enables F. proliferatum to persist more than several years in soil in spite of not producing chlamydospores (durable survival structures).

By C. M. Ocamb and D. H. Gent

Image related to Onion (Allium cepa)-Fusarium Basal Rot

Photo by Melodie Putnam

Sometimes, a white moldy growth develops on the stem plate or between affected scales.

Melodie Putnam

See:

Onion (Allium cepa) - Stemphylium Leaf Blight and Stalk Rot

Image related to Onion (Allium cepa)-Downy Mildew

Note the gray growth on the upper leaf.

Photo by Melodie Putnam, 2001.

By D. H. Gent and C. M. Ocamb

When an onion with slipper skin is cut, one or two inner fleshy scales are seen to be soft and have a water-soaked appearance. This was due to the bacterium Pseudomonas.

Phil Hamm

Cause The fungus, Botrytis squamosa (syn. Botryotinia squamosa), overwinters in cull piles, in field debris, or in soil. Wind disperses the spores. Cool temperatures (55°F to 75°F) and long periods of leaf wetness favor infection. Older leaves are more susceptible. There are no resistant varieties.

Image related to Onion (Allium cepa)-Botrytis Leaf Blight

Note the black sclerotia forming on this bulb.

Photo by Paul Koepsell, 1976.

Lesions due to Botrytis are at first tan and, as seen here, alternately develop black sclerotia as is progresses day by day along the leaf.

Melodie Putnam

By C. M. Ocamb and D. H. Gent

Cause Aspergillus niger,a fungus common in soil and crop debris, affects a large number of fruit and vegetable species. The fungus may be seedborne. The disease attacks colored and white onions in storage and transit. It may do a lot of damage in storage if bulbs were at high temperatures for long periods (such as high-temperature drying in storage more than 2 to 3 days). No resistant varieties are known.

Image related to Onion (Allium cepa)-Black Mold

Note the black sporulation of this fungus.

Photo by Paul Koepsell, 1989.

Infected bulbs are discolored black around the neck, and masses of powdery black spores generally are arranged as streaks along veins on and between outer dry scales.

Melodie Putnam, 1996.