Orchid Disease Transmission Question

[BrassavolaStars]

Hello All,

I had a quick question about orchid diseases. Which diseases are spreadable by splashing water? I know that water mold fungi like black rot/pythium are. Can fusarium be spread by water that splashes or drips from one plant to another?

What about viruses or bacteria?

I know that viruses are spread when dividing plants but can excess saucer water dripping on a plant below spread something like CymMV or ORSV or is that only spread by contaminated cutting tools?

[Ray]

There are a number of vectors and each pathogen will have its own “perfect storm” conditions, whether they be broad or narrow.

Fungi and bacteria are the biggies for “water transfer”, whether it be splash or drip, but can also be spread by insects, plant-to-plant contact and via handling. Viruses tend to spread more efficiently via sharing of juices via droplets, but that can happen courtesy of pests dining on an infected plant then another, during repotting or even splash and drippage if there is an open wound.

[SouthPark]

True! I had definitely been interested in trying to find out how a virus in particular can infect a plant ----- such as insect activity, or water transfer etc.

In particular, I had been wondering about whether a virus can just get into a plant if the virus sits on a leaf surface ----- just regular contact, with no cut or wound in or on the leaf.

For sure ------ it's sort of like a well, or 'water' eventually makes its way through or into something. Such as - even if the virus sits on a leaf (and assuming can't get in at that time), it can eventually get through. But also probably depends on how long a virus can survive if it sits on the outside of a potential host, but can't get into it yet.

[Roberta]

Lots of hypotheses about how virus infects a plant... and why some not others? No good data. One of the (big time) orchid hobbyists in my area even did a formal study, with some seedlings, actually TRYING to infect some of the plants. After two years, nothing. Yet some of our favorite plants get it no matter how careful we think we are. Maybe like people, some must more vulnerable than others. I wonder... with absolutely no science to back me up... whether age of the plant is a factor - so many of the legacy Catts from old collections are virused, even though (at least supposedly) good hygiene was practiced with them. But who can tell whether there was a lapse at some point? Certainly the odds of infection are better when plants exchange juices - like at repotting time, and/or if there is a cut. The evidence of spread through sucking insects is also inconclusive, but would certainly seem plausible. Also, viruses are less common in wild plants (I have seen statements of "none" but don't believe it, "less" is reasonable just because density and handling by humans greatly increases the odds) Certainly other diseases (bacterial and fungal) are more contagious. I think, best to just assume every plant is virused when you repot, and practice rigorous hygiene. And then hope.

[hypostatic]

Originally Posted by Roberta

Certainly the odds of infection are better when plants exchange juices - like at repotting time, and/or if there is a cut. The evidence of spread through sucking insects is also inconclusive, but would certainly seem plausible.

Assuming plants are like people (and I think I'm preaching to the right crowd for that haha), most viruses would have to spread through some sort of "contact". This would mean direct contact (ie rubbing against), fluid exchange (one plant leaking/dripping on another), or vector transmission (animals like humans/mosquitoes cutting into tissue).

I imagine that just like people, the toughest form of transmission would be direct contact. The virus would somehow have to penetrate through the waxy cell wall, or be lucky enough to make it through the stoma. The easiest transmission route would be through a vector, cutting through the waxy cell walls, and being able to move from plant to plant.

I'd imagine unless it's of huge agricultural relevance like TMV, most plant viruses wouldn't be worth researching, economically speaking. However a quick literature search seems to indicate that viral spread through plants is possible by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. (source: see this open access article -- worth a read if you're interested in the subject.)

I imagine plant viruses would be present in the environment similarly to animal viruses. If they're found less often, it's probably because they're harder to detect in general unless the infected individual is obviously sick, and because testing for viruses is generally harder. (source: the current covid-19 epidemic) (hello future googlers! wash. your. hands!)

Compared to fungi and bacteria, viral transmission is a bit more difficult, since viruses can't move by themselves. Bacteria and fungi can "crawl" along, but viruses need something else to move them from one place to the other.

[Ray]

Another concept brought up in the watering thread is that of time of exposure. If a pathogen-containing droplet lands on an adjacent plant and just sits there, the odds of that plant getting the disease are much greater than if that droplet is followed by a deluge of uncontaminated water, potentially washing it away.

[SouthPark]

I was also wondering whether or not ----- like in 'evolution' and the work of Darwin etc --- whether some orchids can handle a virus permanently, or whether some have immunity against some viruses - so that future generations just don't get affected by particular viruses.

Or ------ if some viruses don't completely destroy a particular kind of orchid (but causes deformities or irregularities, but not creating a path to total destruction - ie. not fatal, but always showing signs of irregularity in flower shape/colour) - then would permanent 'superficial' damage be acceptable? This obviously isn't what a lot of growers are after (ie. flowers that don't meet some expectation). But just wondering about whether some viruses lead to certain destruction for the orchid (or not).

[Roberta]

Generally, a virus-infected plant has less vigor - it weakens the plant but doesn't kill it. (One clue that inspires me to test a plant.. sometimes it's positive, sometimes the failure to thrive has other causes) There are exceptions... classic is Lc Porcia 'Cannizaro' - the grex, back to the original plant that was cloned, is virused, but the plants are vigorous, the flowers show no color break. Typhoid Mary... doesn't show the disease but can infect others. Some Lycastes are also notorious for being virused and asymptomatic. Practice good plant hygiene all the time, and it is much less likely to spread. (Sound familiar?)

[SouthPark]

Originally Posted by Roberta

Generally, a virus-infected plant has less vigor - it weakens the plant but doesn't kill it. (One clue that inspires me to test a plant.. sometimes it's positive, sometimes the failure to thrive has other causes) There are exceptions... classic is Lc Porcia 'Cannizaro' - the grex, back to the original plant that was cloned, is virused, but the plants are vigorous, the flowers show no color break.

Thanks Roberta. I didn't know the story about Ctt. Porcia 'Cannizaro' before, until now when you mentioned it.

I'll have to watch out for that one. I recently had one of these bloom. No colour breaks for sure. One interesting one with no dorsal sepal though ...... Click Here and Here.


On a different but related note ----- a pdf link here raises some concerns maybe ----- as it says on page 13:

"Growers please note: - Plants suspected of being infected with virus, disease or insect infestation will not be eligible for competition and such plants will be asked to be removed from the show."

And on top of page 16 of that pdf ...... we see that Ctt. Porcia 'Cannizaro' was in that competition.

So if it is indeed confirmed that all Ctt. Porcia 'Cannizaro' plants are virused, then maybe a bulletin needs to be sent out to all orchid societies around the world for some heads-up about this particular orchid.

[Roberta]

Folks ought to at least test them... the original FCC/AOS plant was virused, and so clones from it are too. It totally doesn't show it. Guess it's one of those tidbits that older growers know, but never got passed along. And the plant is so vigorous, that there are divisions of it all over the place. Maybe there are some that don't test positive, but have my doubts. Mine lives under a rose bush, well away from the rest of my plants... 'way too nice to discard, just gets handled with care.

The point is simply, that you can't tell from looking. (I have had some pretty nasty-looking plants that were negative...) Just never assume that it's absent, treat all plants accordingly when you do anything that might transfer juices.