Some Virus Diseases of Greenhouse Crops

Introduction
As more greenhouse crops are produced by vegetative cuttings, greenhouse growers are more concerned about the potential of viruses to infect their crops. Some of the viruses that may infect greenhouse crops include (but are not limited to): calibrachoa mottle virus, cucumber mosaic virus, tobacco mosaic virus, tobacco ringspot virus, tomato ringspot viruses and tospoviruses including impatiens necrotic spot virus and tomato spotted wilt virus.

There is no control for plants infected with a virus. It is important to have the virus disease accurately identified. Serological techniques such as ELISA (enzyme-linked immunosorbent assay) are now available to accurately identify a wide range of viruses. On-site grower kits using this same technology are also available to test for viruses such as calibrachoa mottle virus, cucumber mosaic virus, impatiens necrotic spot virus and tobacco mosaic virus as well as others.

Symptoms
Virus symptoms can be easily confused with nutritional disorders, chemical spray injury, fungal or bacterial pathogens or injury from fumes from a faulty furnace.

Symptoms can also vary depending with the type of virus, the host plant, how long the host plant has been infected, the strain of the virus, and the environmental conditions. Symptom expression can be temperature sensitive – some viruses are expressed at high temperatures whereas others are expressed at lower temperatures. Viral symptoms can also be masked when the plants are growing vigorously. Sometimes, symptoms may only be apparent when multiple infections are present or when plants become stressed. So, don't rely on visual diagnosis to determine whether or what type of virus is present. You may not become aware of a problem until it is widespread. Routine testing of plants showing symptoms and those not showing symptoms is needed, especially before taking cuttings.

Viruses rarely kill their hosts but they alter the host plants appearance. Some virus-infected plants are propagated because of their attractive appearance! As an example, the variegated foliage in flowering maple is due to the abutilon mosaic virus.

Mosaic (a variable pattern of chlorotic and healthy tissue on the same leaf), distortion of leaves or flowers, yellow or chlorotic streaking, yellow veins, ring spots, dead brown areas (necrosis), and unusual line patterns may be symptoms of viral infections. Stunting is common. Infected plants may also show only mild symptoms or symptoms may be latent. For example, cultivated geraniums infected with pelargonium flower break virus usually show no symptoms.

Causal Agents and Disease Development
Viruses are ultra-microscopic particles that infect living cells and alter their host's development. They consist of nucleic acids surrounded by a protein coat. Viruses usually begin infection through a wound, often from insect feeding. Once a plant is infected, the virus spreads systemically within the plant.

Plant viruses are often named on the basis of the symptoms they cause on the host first detected. For example, a virus causing light and dark green areas (mosaic patterns) that was first seen on tobacco was named tobacco mosaic virus.

Viruses can be transmitted by insects, primarily aphids, leafhoppers, and thrips. Sometimes, whiteflies can transmit viruses. Mites, fungi and nematodes can occasionally transmit viruses. Viruses are often spread by the propagation of infected plant parts (cuttings, bulbs, and sometimes seeds) and some can also be spread by mechanical means including contact (rubbing, abrasion, or by handling).

Many different weeds can also become infected with viruses without showing symptoms. Weed control is so critical because the weeds can be both reservoirs of a virus plus an alternative host of the insect vector.

Calibrachoa Mottle Virus (CbMV)
Calibrachoa Mottle Virus has been recently identified in calibrachoa. Infected plants may show streaking in the flowers, interveinal chlorosis and mottling or blotching on the leaves. Some cultivars will not show symptoms unless they are stressed. Calibrachoa mottle virus is transmitted mechanically in plant sap. Currently, researchers are investigating if a vector plays a role in its spread.

Cucumber mosaic virus (CMV)
Cucumber mosaic virus has a wide host range of over 400 species of plants. CMV has been reported on ajuga, aquilegia, campanula, delphinium, dahlia, lilium, petunia and phlox. Infected plants may show mild mosaic patterns and mottling, flower color breaking, flecking, and fern leaf distortion.

CMV is primarily spread by aphids that can acquire the virus in as little as 5 to 10 seconds. Aphids then move the virus from plant to plant for a few hours. CMV is also spread mechanically in the plant sap when cuttings are taken from infected stock plants. CMV is also both seed and pollen transmitted in petunia where symptoms develop in very young plants.

Rogue diseased plants. Control aphids. Eliminate weeds such as common pokeweed, chickweed, field bindweed, yellow rocket, and bittersweet nightshade that may be reservoirs of CMV.

Tobacco mosaic virus (TMV)
Tobacco mosaic virus has a wide host range but is especially a concern on solanaceous crops. In recent years, TMV has been reported on ajuga, calibrachoa, cyclamen, epimedium, gerbera, helianthus, impatiens, lisianthus, lysimachia, New Guinea impatiens, nicotiana, pepper, petunia, penstemon, tomato and torenia.

Symptoms include yellow mottling, upward leaf curling and overall stunting. Some infected plants may not show any symptoms at all. Different strains of TMV exist, too. A specific strain known as odontoglossum ringspot virus (TMV-O) causes ringspots or line patterns on orchids.

TMV is not transmitted by insects! TMV is a very stable virus that can be spread by contact. Workers can easily spread this virus when they handle plants or when cutting tools become contaminated. TMV can persist in dried tobacco leaves, so tobacco products can also be a source of TMV.

Discard infected plants including roots. Disinfect hands by washing with milk, or tri-sodium phosphate and then thoroughly with soap and water. Smokers need to wash their hands before entering the greenhouse so they do not infect plants. In greenhouses, hard surfaces such as doorknobs, or flats can become contaminated after handling virus-infected plants and remain a source of infection. Thoroughly disinfect the growing area with a commercial disinfectant. See references for more information. Control perennial weeds in the solanaceous family such as ground cherry and horsenettle that could be reservoirs of TMV.

Tobacco ringspot virus (TRSV)
Tobacco ringspot virus has a narrow host range. It has been reported on begonia, geranium, impatiens, iris, phlox, petunia, portulaca, and verbena. Ringspots are the primarily symptom. Tobacco ringspot virus is spread mechanically, by nematodes (Xiphinema) and by seed in petunia. It is not spread by contact.

Tomato ringspot virus (ToRSV)
Tomato ringspot virus also has a narrow host range, and has been reported on begonia, chrysanthemum, geranium, impatiens and orchids. Ringspots are the primary symptom. It can be transmitted mechanically, by nematodes (Xiphinema), in the seed or pollen.

Tospoviruses
Tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) have a very wide host range of over 600 hosts. It has been reported on begonia, campanula, centranthus, cyclamen, garden impatiens, monarda, New Guinea impatiens, pepper, phlox, primula, tomato and many others. Infected plants may show stunting, necrotic and chlorotic spotting, stem cankers, line patterns and ringspots.

Tospoviruses are spread by thrips. Thrips cannot transmit the virus unless they acquire it as first instar larvae when they feed upon infected plants (including weeds). Winged adults are primarily responsible for viral spread. Tospoviruses are also spread in the sap when cuttings are taken from infected plants.

Rogue infected plants. Use yellow or blue sticky cards to monitor for thrips and promptly begin a strict thrips management program. When tospoviruses are present, the threshold level for thrips is zero.

Prevention
The best way to control viruses are to keep them out of production areas. Prevention is the grower's first line of defense against virus infection.

Purchase clean, virus-free seed, cuttings and stock plants from a reputable supplier. Virus-indexed plant material may be available for certain crops. If unsure, isolate incoming plants in quarantine type area until you have determined that they are virus-free.

Do not take cuttings from infected stock plants. Many viruses are spread mechanically in the sap that contaminates worker's hands or cutting tools. To remove contamination of most viruses from tools, they can be soaked in quaternary ammonium compounds or hydrogen dioxide. Soak the tools for at least one minute. Propagators need to soak their cutting tools on a regular basis, after use on each stock plant or defined area.

Control insect vectors. If designed and maintained properly, the use of insect screening in propagation areas can help to reduce insect pressure.

Keep growing areas weed-free. Weeds can be reservoirs both of viruses and their insect vectors. Discard virus infected plants.

Leanne Pundt, Extension Educator, University of Connecticut
Photos by Leanne Pundt

References:
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Agdia website. Source of on-site test kits and list of viruses recently detected on ornamental plant species.

Boucher, T. J. and R. Ashley. 2001. Northeast Pepper Integrated Pest Management (IPM) Manual.

Daughtrey, M. 1999. Tobacco Mosaic Virus: A Different Enemy from the Tospoviruses. Northeast Greenhouse IPM Notes. 9(8): 1-2.

Daughtrey, M., R. Jones, J. Moyer, M. Daub, and J. Baker. 1997. Tospoviruses Strike the Greenhouse Industry. Plant Disease. 81(11): 1220-1230.

Dreistadt, S.H. 2001. Integrated Pest Management for Floriculture and Nurseries. Statewide-Integrated Pest Management Project. University of California Division of Agriculture and Natural Resources. Publication No 3402. 422 pp.

Horst, K. R. 2001. Westcott's Plant Disease Handbook. 6th edition.

Nameth, S. 1996. Virus Diseases of Greenhouse Floral Crops. Ohio State University Extension Fact Sheet. 4 pp.

Pundt, L. 2002. Halt viruses. GrowerTalks. July 2002. 138-140.

Scott, S.W. 1996. Hosts of Tomato Spotted Wilt Virus and Impatiens Necrotic Spot Virus. Plant Diagnostics Quarterly. 17(2): 27-41.

Smith, T. 2002. Disinfecting the Greenhouse. University of Massachusetts Extension Fact Sheet. 5 pp. http://www.umass.edu/umext/floriculture

White, J. D. 2003. Calibrachoa mottle virus: Are your plants in danger? FloraCulture International. February 2003. 12.

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

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