A little over 100 years ago the first of a new type of a disease-causing agent in plants was identified--the virus. When I started studying plant viruses in the late 1960’s there were probably 300+ known. Depending on which authority you believe, there are now between 1500 and 2500 different viruses that infect plants. Fortunately only some 40 odd have been shown to cause disease in orchids, and of these, at this time, only 3 seem to be of major importance, Cymbidium Mosaic Virus (CymMV), Odontoglossum Ringspot Virus (ORSV) and to a considerably lesser degree Bean Yellow Mosaic Virus (BYMV) mainly in Masdevallias. Therefore the following discussion will be directed towards these three.
Symptom-wise the leaves of virus infected plants may show a mosaic pattern of light and dark green areas, yellowing, lines of brown necrotic spots along the veins, ring spot patterns of various kinds, and various necrotic spots often associated with pitting or sunken areas. Flowers may display color breaks, lines of necrotic spot following along the veins. Unfortunately, many of these symptoms are also generated by other pathogens, insect pests or physiological factors (chemical damage, nutrition, etc.). Some plants will show no symptoms at all!!
These viruses can easily be spread from plant to plant when sap from an infected plant comes in contact with a healthy plant and gains entry through some type (could be microscopic) of wound. The usual sources are unclean pruning tools or workspace when dividing plants, or even the worker’s hands—anything that will allow the transfer of the virus particles (smaller than bacteria) in the sap of an infected plant to a healthy one.
In the case of BYMV the virus can also be transmitted by aphids and via seed. CymMV and ORSV have not been demonstrated to be transmitted by either of these methods.
Sanitation in the green house and workspace are the keys to success in containing these viruses.
One of the worst ways in which these viruses got spread around during the 1960-70’s and unfortunately still today, is through meristemming an infected orchid. In the 60’s it was thought that meristemming would eliminate the viruses; we now know that this is rarely true, at least for CymMV and ORSV. Now that we do know, it would seem almost unethical to meristem without doing some basic testing first. I know for certain that many growers do test, but in some areas of the world where testing might be more difficult to obtain it is apparently not being done often enough.
So how does one test for the presence of virus? The electron microscope (EM) with techniques developed in the 1950-60s would be my first choice. Using negative stained dips gives one a chance to look for a broad range of viruses, although like any method it does have limitations. It is also time consuming, 3-5 minutes per specimen, and it requires an expensive piece of equipment, the EM. Because of this it is usually used to solve special problems.
A serological approach using a technique termed ELISA has become a commonly used test, widely applied in medicine as well as plant pathology. It allows large numbers of samples to be processed in a short time by one person making it inexpensive compared to the EM. It has essentially the same sensitivity and accuracy as the EM; the drawback is it only finds what it looks for—one virus at a time (there are some group tests). Since with orchids we are usually concerned with just two, CymMV and ORSV, or occasionally BYMV, this is not a problem.
There are other methods, use of indicator plants, RT-PCR, C-DNA probes, generally more expensive or onerous to use—usually more expensive.
In summary the most important principle in virus control is SANITATION—clean plants to start with, clean tools / work surfaces, and pots to keep them healthy.
Ideally start with uninfected plants through:
Buying from reputable dealer
Visual appearance i.e. no obvious symptoms of viral infection
Testing for presence of virus
Playing the percentages-
1) Vegetatively propagated plants are more likely to be infected than those produced from seed.
2) The longer a perennial plant has been in a green house the more likely it has picked up a virus.
Place newly acquired plants in an “isolation ward” to make sure no hidden pests or” not yet expressed” disease show up.
Space plants so they do not touch nor allow water drip through one pot into another.
Sometimes it may be useful to group plants so they may be treated as a single plant.
Sanitize all pruning tools before starting on another plant; this goes for harvesting flowers as well as trimming roots, too.
Wipe down your workspace with a sanitizing solution as well. This is especially important when repotting or dividing plants. Some growers when repotting only a few plants use a thick layer of newspaper for each plant (thick enough to prevent any sap/moisture from seeping through to the workspace) then gathering up all the plant and potting debris in it and start the next plant with a fresh pad of newspaper.
Wear latex gloves, and dip in a sanitizing solution between working on a new plant or new lot (group) of plants.
Never reuse old potting media.
If you are reusing plastic pots, they should be washed to remove all plant and potting material debris, and then soaked for an hour in a sanitizing solution.
This goes for metal or plastic stakes as well. Rinse before using or storing.
Unglazed ceramic pots could be sanitized by heating in an oven at 400-degree F. for two hours.
Sanitize your benches or tables on a regular basis.
Some viruses are vectored by insects or other animal pests. If these viruses are of concern, you may need to put screens on green house windows or grow in a screen house. Consider whether surrounding weed hosts may harbor an insect vectored virus of concern.