Virus in Orchids
Watering
your Orchids
It is hard to know
what to do with your orchids for example when to feed and when to water what to
spray with etc., as every one and every book has a different way. After 40
years of experience I feel I can give the amateur and expert some guidelines to
help you understand a little better about your plants and in turn you should be
able to make better decisions when, how and what to do what you have to.
One of the most
important things is to remember that nearly all popular cultivated orchids are
epiphytes, that means they live on the air and only attach themselves to a host
for support. The host takes the form of trees or rocks etc., and with the use of
leached out salts from these droplets that are mixed with rainwater or droplets
of mist to form food that splashes on the leaves and roots. The leaves and
roots absorb most of the moisture, during rain or when the humidity is at it's
highest. Orchids that are generally cultivated are found predominantly in a
wide variety of conditions ie., forests and all of these forests have one thing
in common like the rest of the places that orchids grow. The humidity increases
during the night and is much lower in the day, but this can vary when it's
raining. Where orchids are plentiful in their natural habitat a light to heavy
mist or fog can be readily seen almost every night.
Most orchids have
adapted to use this higher night humidity to absorb moisture through their
leaves and root. Where plants have extended drought or dry seasons, species
such as Dendrobium bigibbum, Dendrobium canaliculatum and Dendrobium bifalce
regularly endure then they rely on this night rise of humidity to survive for
many months. Often if a wet season is missed they can last for up to two years
as has just happened in Papua New Guinea. Almena or as in Pidgin English the
Big Sun.
How do plants maintain
the moisture without loosing it during the day?
If you look closely at
an orchid leaf or new root you will see lots of small dents, which are holes,
called stomata. These holes open as the sunlight dwindles to darkness or as the
light is greatly reduced because of heavy cloud and as the sunlight becomes
brighter they close so reversing the process. Commonly known as Stomatal
Rhythm.
This process helps the
plant hold moisture within itself when in strong light and replenish what
moisture is lost at night.
It is this reversal of
the normal Stomatal Rhythm, which enables Crassulacean Acid Metabolism (CAM),
in orchid plants to be drought tolerant. CO2 diffuses into the
leaves and is fixed into organic acids during the night, when temperatures are
low and humidity is high, so minimising the loss of water by transpiration
(Arditti and Sinclair).
What does this mean?
To get the best
results from your watering and feeding you should water at daybreak before the
stomata close.
Only use insecticides
or fungicides that are not oil based such as white oil as the will seal over
the stomata. Use an agricultural wetting agent to help spray cover the leaves
and roots that reduces the meniscuses on the water. If none is available use a
little dishwashing detergent but only use a few drops and mix well in.
Not many plants CAM
and Orchids are one of the few groups of plants that do another group is
Succulents.
Orchid roots can tell
you a lot if you have a little understanding of them also. Keeping in mind and
applying the above information the roots will tell you if they like being
potted in a fine, coarse mix or even mounted on a wet or dry host just hanging
free in the air.
If the new root ends
are white they can be potted in a medium that is not very open, but if the ends
are green there is a need for a fresh supply of oxygen and the darker the green
those tips are the more open the mix should be.
For example
Phalaenopsis and Vanda plants have a few roots that secures the plant to the
tree and all the rest hang in the moist air these root tips are dark green and
the roots are thick.
Spathoglottis have thin
roots and like a close medium.
Dendrobiums have white
green root tips that are medium thickness and nearly all species have dark
green root tips. They do best on a host but will do fairly in a pot that has
plenty of holes in to let in the air along with an open potting mix that does
not become soggy but maintain a small amount of moisture.
Just remember research
your orchids and where they come from, or what species are in your hybrids this
will give you the information on what your orchids require as to building a
microclimate and to assist you in growing better orchids.
Viruses are a group of
minute infections agents characterised by a lack of independent metabolism, and
the ability to replicate only within the living cells of the host organism. As
such they are obligate parasites.
There has been some
debate about whether viruses should be considered as truly living organisms. On
one hand they are constructed of organised compounds, and they are able to
multiply with genetic continuity (hereditary characteristics are transmitted by
nucleic acids) with the possibility of mutation.
On the other hand,
however, once viruses are removed from their host they are entirely inert and exhibit
none of the characteristics attributed to the living state. Even within their
host, they do not carry on respiration or synthesise any materials for energy
and growth. Even multiplication, which can occur only within host cells,
differs from reproduction in other organisms. The viruses particles do not grow
and divide, and particles do not arise from pre-existing particles.
Viruses are composed
of a protein sheath surrounding a core of nucleic acid, either DNA or RNA,
which carries the genetic code of the virus (as in all organisms, the DNA or
RNA carries the hereditary information).
Their method of
replication is to invade a host cell and to convert the activities of that host
cellto producing more virus particles. In effect, the switch the genetic code
of the host cell, although it is not clearly understood how this mechanism
works. In doing this, the host cell is destroyed and the newly formed virus
particles are released to invade surrounding host cells.
Viruses are extremely
small in size and a re invisible even under the most powerful light
microscopes. An electron microscope is used to study them.
In shape they may be
oval, spherical, polyhedral (with numerous facets), rigid rods or flexible
threads. They are not given scientific names, like truly living organisms, and
are usually named after their main host or the disease symptoms produced.
Viruses are generally
separated into subgroups, bacterial viruses, plant viruses, and animal viruses.
Animal Viruses
Many infectious diseases of humans and other animals are caused by viruses from
common childhood diseases like chicken pox, measles and mumps, through to the
common cold, influenza, herpes, warts, viral pneumonia, poliomyelitis, to the
real nasties such as Yellow Fever, Rabies, Small Pox, AIDS (HIV virus) and
Marburg's Disease (a highly fatal haemorrhagic fever caused by the Ebola
Virus).
Most viruses are
highly infectious and are readily transmitted from diseased to healthy hosts by
various means. Many are spread by direct contact or by droplets in the air.
Mosquitoes and ticks are also common transmitters.
We control virus by:
* Natural species or race immunity (not affected at all to mildly affected).
* Developed immunity due to previous exposure and subsequent production of
antibodies.
* Immunisation by a specific vaccine ( a suspension of killed or altered micro
organisms administered to produce antibodies).
Antibodies attack and neutralise viruses by isolating and or metabolising them.
Plant Viruses
Over 400 plant viruses have been identified. Among economically important
plants affected by virus are potatoes, tomatoes, sugar cane, corn, wheat,
peaches, beans, rice, cucumbers, strawberries, raspberries, apples, and many
ornamentals including of course?orchids.
Viral diseases are
generally most serious in plants that are vegetatively propagated by man. Virus
may kill localised areas, entire plants, or most commonly, reduce plant vigour
and thus yield.
Methods of plant virus
transmission are numerous, including direct contact, sap drip and insect
vectors such as aphids, thrips, white fly, mealy bugs and grasshoppers.
In cultivation the most common form of transmission is sap transfer from and
infected plant by cutting tools, which occurs commonly during grafting,
pruning, and cutting flowers. Mechanical and insect transmission may also
occur.
The mosaic group of
viruses is important; the effect they produce is a mottled or irregular
patchwork appearance caused by chlorophyll destruction. Foliage may also be
wrinkled. Another symptom is colour breaking in flowers especially in gladioli,
pansies and wallflowers. This response, is best known in tulips, which feature
streaks or lines of contrasting colours. Many have been in cultivation for
years and show no signs of cumulative weakening. In bygone days some strains
attracted high prices.
Viruses in Orchids
A number of viruses are known to affect orchids.
For many years growers have been aware of Tobacco Mosaic Virus, Cymbidium
Mosaic Virus, Cymbidium Necrotic Ringspot Virus and Odontoglossum Ringspot Virus.
In recent times a new group of viruses known as Rhabdovirus, has been
identified in collections in Austraila.
In a series of
articles in some of the orchid journals including the "Orchadian",
and Orchids Australia, over the past 18 months or so, Don Gowanlock of the
University of Queensland has detailed the symptoms associated with these
"new" viruses. They are excellent articles, and all growers should
read them and take note of the pictures.
However, for the
average grower, precise identification is not really important. All we really
need do is recognise the symptoms of virus infection generally, and take steps
to prevent its spread.
The presence of virus
in orchids disrupts normal growth, and may cause weakness, distortion and
malformation. The effects are especially noticeable in stressed plants. Leaf
symptoms include irregular chlorotic or mosaic patterns, necrotic streaks and
spots, often in circular or mosaic patterns and erosion of the leaf surface.
Other symptoms include reduction and or distortion of stems, leaves and
inflorescences, colour-break in flowers and bud drop. Plants weakened by virus
are also more susceptible to attack from other diseases and pests.
In the collections of
most growers, it will be leaf symptoms that will indicate the presence of
virus. While leaf symptoms are many and varied, and may be quite different from
genus to genus, the main differential diagnostic features are the more or less
circular arrangement of the necrotic spots, and the presence of chlorotic
areas, usually in association with the necrotic spots and patterns. Chlorosis
is an absence or reduced amount of chlorophyll, giving a yellow or bleached
appearance.
Even plants with no
apparent symptoms may be infected, with good culture masking the effects, but
eventually the signs will show through.
As with other plants,
sap transmission will carry virus from an infected orchid to a noninfected
orchid. This commonly happens with cutting implements such as scissors, secateurs,
knives and the like. Plant leaves rubbing together or even just touching, on
the nursery bench, during transport or at shows may also transmit virus.
Touching your plants, especially running your fingers along the leaves as many
growers do to kill aphids, may also transmit virus. Pollen transfer is also
known to result in virus transmission.
Virus particles are
very small and can enter through the tiniest of wounds on a plant, wounds that
may be quite invisible to the naked eye.
After mechanical transmission,
which is essentially the result of cultural techniques and practices, the next
main method of virus transmission is by sucking and chewing insects. Aphids in
particular are known carriers of virus and other suspects must include thrips,
mealy bug, grasshoppers, beatles (especially the Dendrobium Beetle) and scale
insects.
Treatment
You cannot do anything about a plant once it is infested with virus. There is
no treatment and no cure, so you must at least isolate an infected plant from
no-infected plants, and preferably destroy it by burning it.
Do not put it in the
bin, or take it to the tip, as someone will pick it up and take it hope with
them, thereby perpetuating the problem. Remember also that all parts of of the
plant will be infected, and divisions and Keiki?s will carry and perpetuate the
virus. Even meristem propagation?s (ie. mericlone) will usually be infected.
Plants can be tested
for virus if they are valuable, but even if you only suspect the presence of
virus, you must isolate these plants until they can be checked. There are
commercial laboratories where this testing can be done at reasonable rates.
It sounds hard to say
but you really MUST DESTROY infected plants. While they remain in your
collection they are a potential source of further infection, and you run a real
danger of contaminating non-infected plants, not only your own but other
people's as well if you take your plants to meetings and shows.
Control
Do not use any cutting instrument from plant to plant without sterilising such
instrument in between - by flame or boiling, by alcohol, or by soaking in a
saturated solution of Trisodium Phosphate. This means any cutting of plants,
including rhizomes, stems, leaves, inflorescences and flowers.
Do not unnecessarily
touch or handle your plants, and especially keep your hands off other people's
plants at meetings and shows.
When working on your
plants (repotting, potting-on, grooming etc) keep your work surfaces clean and
use fresh sheets of newspaper between each plant. When you have finished with a
particular plant, wrap up all plant debris, old potting material, pots, stakes
and the like in newspaper sheets you have been working on, and dispose of the
package. Then wash and dry your hands, lay out fresh sheet of newspaper, and start
on the next plant.
This may seem like an
elaborate and time consuming procedure, but it is simply a habit you must get
in to, and it will certainly be worth it in the long run.
Do not re-use pots and
potting mixes, stakes, ties and anything else that has been in contact with
your plants. The big advantage of using plastic pots is that they are cheap, so
you don't have to re-use them.
Do not introduce new
plants directly into your collection. Keep them segregated and under
observation, especially adult plants and divisions, until you are satisfied
that they are clean. Likewise, do not give away, swap or sell any plant,
division Keiki?s growth that is infected or shows any suspicious signs.
Do not allow a build
up of insect pests within your orchid house. An effective insect eradication
program is an important part of creating and maintaining a virus-free
collection. As soon as you see something that needs attention treat it
immediately. General orchid house hygiene will assist in this area.
Conclusion
To maintain a virus-free collection, you must adhere to guidelines detailed
above. Even if your plants appear to be free of virus-like symptoms an you
believe them to be uninfected, still follow the procedures. In this way you
will completely rule out the possibility of transmitting virus through your
collection. In the past couple of years there has been considerable amount of
research work done on viruses in orchids, and growers should be aware of the
signs and symptoms. Contrary to popular belief, native orchids are not somehow
less susceptible to virus infection, and native orchid growers cannot afford to
be complacent.
References
Gould Medical Dictionary 4th Edition 1980
Gowanlock D. Orchid Virus - Orchids Australia Vol. 6 N05 Oct. 1994
Gowanlock D. and Wolf JSome New Viruses In Our Native Orchids - The Orcadian
Vol.11 No.8 June 1995
Gowanlock D. Rhabdoviruses In Orchids in South-East Queensland - Orchids
Australia Vol.7 No.3 June 1995
Gowanlock D. Virus Diseases of Orchids - Orchids Australia Vol.8 No.1 Feb. 1996
Wilson C and Loomis W. Botany 4th Edition 1972
Winchester A. Biology and Its Relation to Mankind 4th Edition 1969
How often should I
water? This question would with out doubt be the most asked question by the new
or novice grower. The easy answer is of course water your plants only when they
need it, but how does one go about determining when they need it?
Plants need water for
the process of photosynthesis. They need water when they are in active growth
and they need water when they are transpiring heavily. From this it could
possibly be said that for plants under shade-house conditions it is almost
impossible to over-water in summer and conversely it is almost impossible to
under water in winter. This is assuming of course that the plants have adequate
drainage. Generally during the very hot summer months when the air is extremely
dry most growers find it necessary to water twice a day. In winter once a week
or even once a fortnight may be sufficient. It is something of a paradox that
the further we are away from the equator the more watering is required. This is
due to the fact that the humidity is much lower in the temperate regions and
must be compensated for by using more moisture retaining material and more
frequent watering.
This then leaves
autumn and spring as the seasons that give most trouble. In spring a good
indicator of watering needs is the condition of the root tips. If you can see
green tips on the ends of the roots then the plant is commencing active growth,
so it is time to begin regular watering. If the roots are not active the plant
will not be able to, no will it require much water. In such cases water can not
only rot roots and set the plant back but the excess water may encourage fungal
disease.
In autumn as the days
shorten and the nights grow longer most plants move slowly into dormancy.
Decreasing the frequency depends not only on the season but also the potting
medium or type of mount, the size of the container and the plants? position in
the shade-house relative to sunlight and air movement. A fine potting medium
will retain more moisture than a course mix, a cork mount will dry quicker than
a treefern mount, a plant suspended near the roof will need more water than
those in a cooler position.
Experienced growers
are usually able to tell by their nose when a shade-house has sufficient
humidity. A humid shade-house has a particular smell about it, what the old
books referred to a "a buoyant atmosphere". There is no substitute
for experience in this. Until your nose develops this sense of smell you should
be particularly attentive when visiting the shade-houses of other growers and
take note of the smell and feel of the atmosphere within the house.
Some further points to
bear in mind are that no two shade-houses are identical and what works for one
grower will not necessarily work for another. Different combinations of temperature,
light and humidity produce a wide range of differing requirements. Don't follow
other people's advice slavishly and don't be afraid to experiment