"K Barrett" <[EMAIL PROTECTED]> spaketh thusly:
>I wondered about how we judge 'by sight' if a plant has a virus. I
>know about color break in a bloom. I know about a trapeziodal mark
>on a leaf of a cymbidium. I know about longitudinal streaks in a
>leaf.Of these 3 the only one I believe to be pathognomonic is the
>trapezoidal mark on a cymbidium leaf. Color break can come from a
>variety of sources, heat, pesticides are two that come to
>mind. Streaked leaves from heat or other poor culture. In the back
>of my mind I recall that old cut-flower houses didn't really care
>about virused plants as long as the virus didn't affect the bloom,
>therefore there are several viruses housed in orchids, but since
>they don't affect the blooms they aren't considered a problem. Also
>in the back of my mind I recall that labs only test for a few
>viruses and you have to know which is virus a problem for your
>plant; like should you care about odont ring spot or cymbidium virus
>if you only grow cattleyas? Which brings up another possible myth:
>not all orchids are 'susceptible' to virus (Paphs aren't??
>something like that.)
All orchids are susceptible to viruses. I can't seem to find
the email right now, but I once asked this of Bob Wellenstein; as a
virologist who happens to run Antec, one presumes he is supremely
qualified to answer this question. His reply was to the effect that
none had been identified, but that they can not harbor plant viruses
is remote at best. Hopefully he'll correct me or confirm my
impressions if he has the time to read the list.
I did once hear from a newly-minted PhD in plant pathology
that orchids tend to have high false positive rates for viral
infection, thanks to issues with the ELISA test. I *was* able to find
this in my legendary email inbox from Bob when I asked him about his
impression on this subject:
"So yes, ELISA can yield false positives, and what I've seen
of the common orchid virus tests it is a problem. For ORSV and CyMV I still
advocate the low tech AGID. With a good antisera and a diagnostician who
knows a line of identity, it is more than sensitive enough for these two
high titered viruses, with fewer false positives. But it's not as sexy!"
Hmm. So- what to think about ELISA tests? Has there been a
good study on ORSV and CyMV to see if the tests correlate well with
actual viral loads in plants? Then one has to find a way to confirm
the plant really *is* infected; at that point, you're looking at
electron microscopy- needle and haystack stuff in some cases. Sure,
the virus may be there, but can it be found?
Making things worse is that there are, after all, more
viruses that infect orchids than just ORSV and CyMV. I have this
sneaky suspicion about impatiens necrotic spot virus (INSV) in phals;
I have absolutely no evidence, but if I wanted to keep myself
occupied with an orchid virus research project, that'd be it between
the purported "microfungus" issue and other assorted anomalies. And
then the Plant Viruses Online database notes that several viruses
have been confirmed as being present in orchids without commonly being known:
http://image.fs.uidaho.edu/vide/famly094.htm
Turnip mosaic potyvirus? Really?
Well, crap. ELISA is no good, the "I think it's infected"
eyeball test is junk, I can't find an AGID test for CyMV to save my
life, and even then you need to find the right test for the right
virus. May as well just incinerate your entire collection.
We sometimes forget that in these days of ELISA, we still
have indicator plants. Kind of. You need to find a strain of
indicator plant that is susceptible to viruses- a number of cultivars
have been developed specifically to avoid virus infection- and have
proper technique. To quote the master once again:
"I also really
don't recommend indicator plants for any but the experienced. The things the
AOS articles don't tell you! For instance, let's look for BYMV. First you
need to come up with plants that are susceptable, not easy when you realize
all commercial beans have been selected for generations for resistance. Then
you have to make sure they do not carry common bean mosaic, which is usually
asymptomatic but will interfere with BYMV infection. And then you have to
master the technique of sufficiently wounding and innoculating the plant,
without creating physical lesions that make detecting viral lesions
difficult."
May as well just pack up and quit, eh?
Discussing orchid viruses is like discussing how many angels
can dance on the head of the pin. Of the few people convinced they
know the answer, absolutely bloody no-one wants to run the
experiment. However, the discussion runs freely like vodka in Moscow.
I would like to be the first to profess vast and unfettered ignorance
of the subject. Beware anyone who claims a better understanding of
the subject than this.
-AJHicks
Chandler, AZ
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