virus testing - false positive?

[Jmoney]

I wanted to ask people here if they have heard of any false positives with virus testing. (I did a search but didn't find anything on this). Let me expand a bit:

I recently bought a fairly expensive phalaenopsis stem prop from a well-known vendor in the US. The plant originally came from Taiwan.

I don't grow a lot of phals, but I do like to collect heirloom cattleyas, at least the ones that I can afford, and try to keep my modest collection virus-free. I use the Agdia ImmunoStrips which seem quite popular.

Anyway I test this new phalaenopsis and it comes out positive for ORSV and CymMV. pic attached. I email the vendor (without any accusatory language, mind you), and was basically told it was a false positive, and that there's a ton of information about how unreliable these virus tests are, etc.

Now as far as I've seen, and granted I'm not a microbiologist specializing in orchid viruses (and as far as I know this vendor is not either), people in the cattleya community seem to regard Agdia as a reliable test, and often use it to advertise their verified virus-free plants. A search did reveal possible issues with a batch of Agdia tests from 2012 causing false positives, but that's like ancient history as far as I'm concerned.

Basically I'm not buying the story that this is a false positive, and think it's much more likely that this plant is somehow truly positive, and I'm not too pleased that I was not offered a return on this plant. But I want to perform my due diligence and see if anyone else might think this is not a true test result.

[Toadwally]

Reading looks legit to me.

[Roberta]

If this were a Rega test, I'd say maybe the weak like could be a false positive with a too-large sample size (too much chlorophyll can give a weak response, it's really sensitive) But this is Agdia, I have not heard anything about those giving false positives. And the strong line, that's positive for sure. (CymMV I think) It's stronger than the control line. It might as well have flashing lights. It's loaded.

[Dimples]

Even if false positives were a known and accepted fact for these tests, they’re still going to be fairly rare. Running a second test would be a good way to rule out a false positive and would give you a better chance at resolving the issue with the vendor (assuming they’re a reputable vendor that cares).

[Louis_W]

Agreed, i know the tests arent free but two positives would be pretty watertight. If you are right, your refund will be the least of the vendors problems but it would ultimatly be a huge favor to them

[Jmoney]

thanks for the input! I did end up testing this plant 3x (I often "batch" tests - for my supposedly "low risk" plants I might test 4-7 samples in the same assay).

so in this case I tested 3 new phals I acquired in the past month, along with a couple phrags I repotted, and got the same double positive much to my chagrin in the "pooled" sample, and after several more tests I finally narrowed it down to this one phal. (I batch tested the other plants and confirmed negative).

I'm perplexed since this vendor is supposedly "highly regarded", or at least I thought she was. Maybe it's because there aren't that many nuts like me testing their phalaenopsis? And I'm really not a phal grower, I only dabble in these newer "novelties"!

[Louis_W]

So after the 'false positive' comment did you get back to her again? She might need to get a bunch of money back too from whoever the supplier is

P.S. i like your clever testing strategy. Its a great idea. Since im fairly sure my collection is clean, i could potentially test them all at once. Or is there some reasonable limit?

[Jmoney]

I emailed her back expressing my experience with members of the cattleya community who regarded this test as reliable, and then as she suggested I keep the plant and not toss it for no good reason, I suggested that I send it back to her in exchange for a refund. So far radio silence. I want to do some investigation before calling her out, but I have yet to find any reliable information calling the validity of this test into question. I mean, nothing is impossible, and no test is 100% reliable, but in my mind it seems far more likely that this plant is virused rather than the entire hobbyist testing platform being bogus. (which is almost verbatim what this vendor said)

[nhbeek]

I am understanding when it comes to virused plants. At this point, if the plant I receive looks healthy and tests positive, I do not bother the vendor and bin it.

This vendor response however is such bullshit. Name and shame!

There is no evidence published that these tests give false positives. Even if it does occur, what could possibly make this vendor so sure its a false positive? Electron microscopy?

Anyway that is a STRONG positive. Sometimes (especially with rega tests) the plant juices will bind to the reagent and produce "a line", however in these cases with a bright light and a magnifying glass one can confirm that the line is green/yellow/grey and not red.

In your picture though, it looks like the plant is 1/2 virus by mass - a "dye stealer"

[Roberta]

Originally Posted by Louis_W

P.S. i like your clever testing strategy. Its a great idea. Since im fairly sure my collection is clean, i could potentially test them all at once. Or is there some reasonable limit?

There is a limit...first, you need to have enough of each plant so that the titre is above the detection limit. The Rega test is actually more sensitive than Agdia (it requires a much smaller sample) A member of one of my clubs, who has approached the issue pretty scientifically, has found that the ideal number of bits for one test is around 6, not just for the sensitivity, but efficiency. If you do more than that, and hit a positive, then you are back to individual tests, and if you go more than that you're likelier to end up having to burn more tests than if you just did them one at a time I don't recall the details of the graph that he produced, but that was, as I recall, the result. He ran pretty extensive statistics (hundreds, possibly a couple thousand plants got tested). This was with the Rega test, so there would be just a sliver of a sample needed for each plant.