Why we should care about pathogenic viruses in our plants

Jacob Knecht jacobknecht@gmail.com
Wed, 21 Jul 2010 04:05:37 PDT
Dear PBS members,

Though I am not a professional pathologist, I am a passionate horticulturist
and conservationist.  Ben Zonneveld brings up a good point.  Not all viruses
reduce a plant's vigour nor are viruses responsible for many variegations
(especially foliar) found in ornamental plants today.  Some viruses are now
essential to modern biological research methods.  Still, there is so much
that we do yet not know about the viruses that affect plants.  Not only are
many undescribed, but we also lack comprehensive data on the biology of all
the species that have been isolated.  There is much to learn about their
full range of plant host families and information about in which plant
genera viruses can specifically do harm.  Research in microrganisms is more
often than not prioritised by the concerns of medical science and food crop
industries.  My guess is that because most geophytes have no market value
other than in trade as ornamentals, this is why there are comparatively less
studies on symptomatic plant viruses that especially target ornamental

Nhu Nguyen and I have compiled all the data that we have found on the
viruses that are recorded as affecting geophytes on the wiki.  Please see:

It is true that viruses are just as much a part of nature as humans or our
favourite bulb species are.  Thus a goal of complete eradication of the
problem viruses is not only highly unlikely but also possibly unnatural
(though I realise that humans do act as a part of nature).  In this age of
massive habitat destruction, unprecedented extinctions and threats to plant
life on this planet, including that of many of our beloved geophytes, our
thinking on how we treat what we cultivate takes on new importance.  Whether
or not the material that we grow will ever be relevant to in situ
conservation/re-introduction efforts, it is still very important to take
care of our species and keep good records of their provenance as if they
could be used for such efforts in the future.  Collectively, what we choose
to grow and how well we increase our species' by seed will determine what is
available for the next generation of horticulturists to choose from for
their gardens, particularly in the case of species that will go extinct in
our lifetime or have become impossible to transport across international

When we spot features in our plants that seem as though they may be
symptomatic of a virus, it is quite often very difficult to determine if our
plants are indeed infected with a harmful virus.  Lack of proper nutrients,
adverse weather conditions, damage from sucking insects and phytotoxic
reactions to chemicals may all mimic viral infection.  How can we know for
sure?  Labs offer ELISA testing services to orchid growers for the most well
known orchid viruses such as Cymbidium Mosaic Virus.  All one must do is pay
a small fee and send a tissue sample in the mail to have a definitive
result.  I have contacted many of these labs to see if they are set up to
provide testing for some of the viruses that affect geophytes (see above
link to the wiki page on virus for a list) but have not yet found any lab
willing to test for these.  I wish I could recommend a service.  I heard
that UC Davis may be able to do this but have not followed up on it.  I have
tested my orchid collection for infected plants and would love to be able to
do this for my geophytes.  Until then I keep an observant eye for symptoms,
vectors and keep plants we've grown from seed separate from the ones that we
haven't (bulbs grown from seed are almost always free of virus).

As emotional and painful as it may be to destroy a virused plant, it is much
better  do this than to have it spread to other plants in our collection.
It is my understanding that many beautiful and sought after geophyte species
in cultivation exist as single clones.  This is a serious concern if a
species is self-incompatible, refuses to produce seed and all clones in
cultivation become virused.  It can be tempting to tell ourselves that we
can perpetually isolate a virused plant because we can't bare to destroy it
and that we can keep it from infecting other plants forever.  But an
infestation of aphids, for example, could occur while one is on vacation,
ill, or too busy with work or family to spray the plants and in that time
virus can be transmitted to dozens of plants from one infected plant.  Why
risk it?

Remnant roots in potting media and the ground in which virused plants once
grew can serve as reservoirs for virus to re-infect susceptible plants if
they are planted in the same soil.  Virused plant material and its potting
media should not be composted because most compost piles/bins do not get hot
enough to kill viruses evenly.  Terracotta pots are very difficult (if not
impossible?) to sterilise and should not be re-used.  Plastic pots can be
reused after being soaked in a bleach solution, scrubbed and rinsed.
Cutting and cultivating utensils must be sterilised between each clone.
Smokers should wash their hands thoroughly before handling plants (see
Tobacco Mosaic Virus).  Pests can spread virus: aphids, thrips, mealybugs,
and scale.  Snails, slugs and some birds that eat flowers can also spread
virus mechanically. Finally, when pollinating between a virused plant and a
healthy plant, it may be better to use the virused plant as the receptor of
the pollen (mother of the seed), as sometimes pollen from a virused plant
may infect the healthy plant if applied sexually.

As bulb lovers, we all come to this hobby via different motivations and we
get different things out of it.  Our collections also change over time just
as we do as individuals.  So why care about viruses that may discolour our
plants' leaves and flowers but haven't killed them or noticeably slowed
their growth?  Virused Rembrandt Tulips (of old) were so gorgeous that they
once famously caused the Dutch economy to go boom and then bust - proof if
there ever was of the charisma of that even heavily infected plants can have
on us. Or why should we be educate ourselves about viruses than can cross
over from asymptomatic non-geophyte ornamental plants in the vicinity of our
bulb collections?  For some it may not seem to matter much or it may not
seem like what we do in our own gardens couldn't really make a big
difference.  However, I believe that how we manage the health of our
collections does matter and that aside the most important result of our bulb
hobbies is what legacy we will leave others from it.

Jacob U. Knecht

- http://www.flickr.com/photos/morabeza79/

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