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.
Plastic pots must be washed and sterilized before reuse.
Sterilize metal or plastic stakes, etc. before reusing.
Sterilize all cutting tools before working on a second plant.
Have a clean work surface to use between plants.
Never reuse your old potting mix.
Wash your hands with soap and water before you start on a second plant.
Do not allow water to drain from one pot into another.
Have an isolation ward for newly purchased plants. Give them 3 or 4 weeks to display any problems.
Always think—How can I keep sap or debris from one plant from contaminating another.
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
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.