Plant Disease Management Handbook

Cause A fungus, Sclerotium cepivorum (teleomorph: Stromatinia cepivora), that produces hardy sclerotia, which may survive in soil for 20 to 30 years. Only Allium spp. such as onion, leek, and shallot are attacked. Sclerotia can infect plants from 12 inches below the soil surface. One sclerotium can infect a group of 20 to 30 adjacent plants.

Image related to Onion (Allium cepa)-White Rot
Note the development of black sclerotia within the mycelial mats on this bulb.
Photo by Paul Koepsell, 1982.

Small sclerotia form in and on the surface of affected bulb parts, often around the neck.

Melodie Putnam, 1994.

Cause Colletotrichum gloeosporioides (teleomorph = Glomerella cingulata) incites Twister disease, also known as anthracnose or seven curls. The fungus can be seedborne and survives on debris and alternate hosts. Rain and wind can spread disease, as well as insects and irrigation water. Infection is favored by high humidity and temperatures between 73°F and 86°F.

Cause Urocystis magica (syn. = U. cepulae) and U. colchici, fungi that survive many years saprophytically in infested soil. They attack only members of the onion family including onion, leek, and Welsh onion and are more a problem on muck soils. Susceptibility decreases as tissue ages. Affected bulbs are predisposed to other infections in storage.

Image related to Onion (Allium cepa)-Smut
Note the black, dusty lesions along the leaves and bulbs.
Photo by Paul Koepsell, 1980.

See:

Garlic (Allium sativum) - Rust

Rust pustules (orange-colored spots) on an onion plant. Photo by Cynthia M. Ocamb, 2013.

Rust pustules (orange-colored spots) on an onion plant. 

Photo by Cynthia M. Ocamb, 2013.

Several onion leaves showing the signs of rust, e.g., the orangish-colored pustules. The pustules on the middle leaf have matured and ruptured, exposing the masses of orange-colored spores that are contained within rust pustules. Photo by Cynthia M. Ocamb, 2013.

Several onion leaves showing the signs of rust, e.g., the orangish-colored pustules. The pustules on the middle leaf have matured and ruptured, exposing the masses of orange-colored spores that are contained within rust pustules.

Photo by Cynthia M. Ocamb, 2013.  

See:

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

Purple blotch on detached onion leaves. Note the brown-colored center on the top leaf, surrounded by a white to yellowish ring. The brownish black, powdery fungal growth is apparent on portions of the leaf spots. Photo by Cynthia M. Ocamb, 2013.

Purple blotch on detached onion leaves. Note the brown-colored center on the top leaf, surrounded by a white to yellowish ring. The brownish black, powdery fungal growth is apparent on portions of the leaf spots.

Photo by Cynthia M. Ocamb, 2013.

Onion plant affected with purple blotch. The surface of the spot on the left is covered with a brownish black, powdery fungal growth characteristic of this pathogen. Photo by Cynthia M. Ocamb, 2013.

Onion plant affected with purple blotch. The surface of the spot on the left is covered with a brownish black, powdery fungal growth characteristic of this pathogen.  

Photo by Cynthia M. Ocamb, 2013.

Onion seed plants showing symptoms of purple blotch. Note the collapsed necrotic portion of numerous leaves. Photo by Cynthia M. Ocamb, 2013.

Onion seed plants showing symptoms of purple blotch. Note the collapsed necrotic portion of numerous leaves.

Photo by Cynthia M. Ocamb, 2013.

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

Cause Phoma terrestris (syn. = Setophoma terrestris), a fungus that may survive several years in soil, and can also persist in plant debris.

Image related to Onion (Allium cepa)-Pink Root
Note the pinkish coloration of roots.
Photo part of OSU Extension Plant Pathology Slide

Slide is labeled Onion, white with pink rot on scale due to Phoma terrestris.

Melodie Putnam, 1999.

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.