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 MosaicVirus (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, ringspot 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 virus infected 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.
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 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.
reprinted with permission from NHOS April 2005 guest
speaker
Dr. Arthur Allison, Ph.D. Plant pathology
Critter Creek Laboratory
Ideally start with
uninfected plants through:
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
and trimming roots, too.
Wipe down your workspace
with a sanitizing solution as well. Consider using several layers of
newspaper and then gathering up all the plant and potting debris in it and
start the next lot with fresh paper.
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.
Some reagents for
sanitizing/sterilizing:
Bleach such as Clorox—10%
solution (1 cup of bleach in 9 cups of water)
Sodium Hydroxide (lye) – 1% solution (1 pound can in
99 pounds of water [~12 gallons]
TSP (trisodium phosphate)-- saturated solution—add
solid to (warm) water with stirring until no more will dissolve and then a
little bit more so there is always undissolved solid on the bottom of
the container. It requires approximately 1½ pounds of TSP per
gallon of water.
Remember to wear eye protection, gloves and water
repellant apron when preparing and using these materials.
Soak cutting tools for 3 minutes before using again.
Even better give them a brushing in the solution at the start of the
“soak” Give workspaces 20 minutes of soak time.
A propane torch is good for sterilizing cutting tools.
Heat the blade to just before it turns red. I know that this is hard on
secateurs, so buy cheap ones and throw them away when they become unusable.
Single edge razor blades are great when their use is
feasible. Use once, drop them in a can and then sterilize in an
oven—2 hours @ 400 degrees F.
reprinted with permission from NHOS April 2005 guest
speaker
Dr. Arthur Allison, Ph.D. Plant pathology
Critter Creek Laboratory
