Viruses are among the smallest organisms that cause plant diseases. They can be seen only when magnified thousands of times. Although they are simple organisms, made up only of nucleic acid and a protein coat, viruses cause devastating diseases.

More than 25 viruses have been reported to infect orchids. The two most common orchid viruses are cymbidium mosaic virus and odontoglossum ringspot virus.

Symptoms
The symptoms caused by a particular virus vary depending on the orchid species, the strain of the virus, and environmental factors such as temperature and light intensity. Orchids infected with a virus may show no visible symptoms. In addition, factors such as nutritional imbalance, excess salts, high light intensity, insect or mite infestation, fungal or bacterial disease, or genetic disorders can cause symptoms that resemble virus diseases.

There are, however, some symptoms that are frequently associated with virus diseases. Typical virus symptoms include yellow, brown, or black spots or line patterns on foliage, with discolored areas often sunken. Brown or yellow rings (ringspots) or a mosaic pattern of yellow and green coloration on foliage are characteristic of virus infections. Flowers may show brown streaks or color break. Color break describes a discoloration of flowers, usually seen as lighter intensity line patterns. (Some orchids show symptoms of color break from genetic mutations, not virus infection.)

Diagnosis
Each virus does not produce characteristic symptoms on orchids. This makes a precise visual diagnosis virtually impossible. Ringspot symptoms or sunken brown spots may lead to a preliminary diagnosis of virus infection, but identity of the virus cannot be determined. Virus infections can be confirmed only when certain tests are performed.

Methods used to diagnose orchid viruses include serology, plant bioassays, and electron microscopy. ELISA test kits, based on serological techniques, are now commercially available for many of the viruses that can infect orchids. These kits can be purchased and run by a grower. There are also commercial companies that will test plant samples for the presence of viruses. Plant bioassays involve inoculating a set of susceptible "indicator plants" with extract (ground tissue and sap) from a suspect plant and watching for characteristic symptoms to develop. Electron microscopy allows for the visualization of characteristic virus particles in plant cells. Unlike serological kits and plant bioassay techniques, however, electron microscopy methods involved specialized training and facilities.

Spread
There are several ways viruses can be spread from plant to plant. The means of transmission depends on the virus. Viruses may be spread in plant sap from infected to healthy plants when contaminated tools are used to divide plants or harvest flowers. Certain viruses can be spread through tissue culture. Also, aphids and thrips spread certain viruses.

Control
Orchids infected with a virus cannot be cured. Diseased plants should be discarded. (Unlike certain fungal and bacterial disease, there are no chemical controls.) Sanitation measures are extremely important in preventing spread of virus diseases. Tools can be sterilized by dipping them in alcohol and then flaming for several seconds or by dipping them in a 10% bleach solution for several seconds. It is also important to buy clean stock from growers that test their plants for viruses. Finally, aphid and thrip infesations should be controlled.

In the future, as genetic engineering techniques continue to be developed, orchids that are resistant to certain viruses may become available.

Further information
The book "Orchid Pests and Diseases" and other references on orchids can be obtained from the American Orchid Society, 6000 South Olive Avenue, West Palm Beach, FL 33405; telephone (407) 585-8666.

This article originally appeared in the April 12, 1996 issue, p. 54.