Plant Disease Management Handbook

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.

Cause Verticillium dahliae, a fungus widespread in soil and that affects a wide range of herbaceous and woody plants. The fungus gains entry through the root system; progress in the plant is primarily through the vascular system. Cool, moist soil favors the disease.

Symptoms Leaves turn yellow and dry up. Wilting progresses up the plant until the plant dies. A cross-section of a node near the plant base may reveal discolored vascular tissue.

See:

Fungicide Effectiveness as Seed Treatments for Cereal Diseases

See:

Wheat (Triticum aestivum) - Common Root Rot (Seedling Blight)