Plant Disease Management Handbook

Cause There are over 27 viruses reported to infect orchids. The two most important are cymbidium mosaic virus, which has been found to infect 56 genera of orchids, and odontoglossum ringspot virus (formerly the orchid strain of tobacco mosaic virus), which has been reported from 20 orchid genera. Mixed infections of both viruses are common. Both viruses are transmitted by means not involving a vector: mechanical inoculation, division of infected plants, and contact between infected and healthy plants.

Cymbidium orchid with Odontoglossum Ringspot Virus.

OSU Extension Plant Pathology Slide Collection, 1983.

These leaf samples reacted positively to an ELISA test for Cymbidium Mosaic Virus.

Photo by Meloide Putnam, 1997

A bacilliform (rod-shaped) virus was found in this orchid leaf. Note the ring spot pattern.

Photo by Melodie Putnam, 1994

See:

Greenhouse Plants, Ornamental - Gray Mold

Cause Acidovorax avenae subsp. cattleyae (formerly Pseudomonas cattleya), a bacterium. Seedlings are infected through stomata, older plants through wounds. Bacteria in exudate spread from orchid to orchid by splashing water and from place to place with infected plants. Contaminated propagation tools are another way to transmit the bacteria. Warm, moist conditions and high nitrogen fertility favor the disease.

This bacterial spot occurred on a Phalaenopsis orchid.

Photo by Melodie Putnam, 1997

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
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
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.

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.

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
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.