March: Viral Villains – Gruesome Guests for Indoor Gardeners and Greenhouse Growers

Recently, I have seen an increase in conservatory and greenhouse-grown plants arrive at the PDDC.  It’s certainly the time of year that greenhouses gear up their plant production in anticipation of spring sales (assuming that spring is going to arrive this year – I have my doubts).  There are several viral diseases that I routinely see in home and greenhouse-grown plants that, if undetected, can spread easily and pose challenges for both indoor and outdoor gardeners.

CymMV Orchid
CymMV on an Orchid

I recently received several orchid samples from a local conservatory.  As it turned out, several of the plants were infected with Cymbidium mosaic virus (CymMV), an orchid specific virus.  In some orchid species, CymMV causes few, if any symptoms.  In other orchid species, a typical symptom is the appearance of necrotic (i.e., dead) leaf spots, symptoms that in most other plants I would attribute to fungal or bacterial pathogens.  Over the years, I’ve learned that with orchids, testing right away for viral pathogens like CymMV, particularly when there is leaf spotting (and oftentimes even when there isn’t), is a good idea.  Luckily, I have a quick, easy-to-use serological dip stick test (the plant virus equivalent to a home pregnancy test) to test for CymMV. When plants test positive, my recommendation is to throw out the infected plants.  It’s too easy to accidentally move viruses around in plant sap that you get on tools or even your hands when you are trimming leaves or deadheading flowers.  Once infected plants are removed, it’s important to decontaminate items that may have come into contact with the plants.  For details on what to use, check out the recipes in the University of Wisconsin Garden Facts on Hosta virus X (HVX).  HVX is another common plant virus, albeit on hostas rather than orchids.

INSV Begonia
INSV on Begonia

I also recently received a Lysimachia sample with a viral problem that turned out to be a bit more of a challenge to diagnose.  The plants came from a commercial greenhouse.  I noted that the edges of the leaves were dead and also noted damaged areas elsewhere on the leaves.  Some of the damage seemed to be consistent with that due to thrips feeding.  This sent up a red flag, as thrips can carry plant viruses like Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV).  These viruses can infect a wide range of plants, can spread quickly (it doesn’t take a lot of thrips) and can cause significant economic loss.  I used dip stick tests for INSV and TSWV, as well as for two other common plant viruses [Cucumber mosaic virus (CMV) and Tobacco mosaic virus (TMV)], but all of these tests were negative. To double-check that my thoughts about possible thrips damage were reasonable, I showed this sample to PJ Liesch, the UW-Madison/Extension insect diagnostician.  PJ verified the presence of a small number of thrips in the sample, but indicated that the brown leaf edges were not a typical symptom of thrips feeding.  PJ was on vacation when this sample arrived and he wasn’t able to look at the sample for about a week after submission.  By that time, I had noticed that the plants were developing additional symptoms including growth distortions and botchy color.  Everything was pointing to a viral problem.  Given PJ’s verification of thrips, I again tested the sample for INSV and TSWV, and lo and behold, this time the sample tested positive for INSV.  At that point, everything fell into place and I reported back to my client that I thought INSV was the primary issue with the plants, and recommended plant removal and decontamination.  The conflicting results that I got with this sample point out a difficulty in confirming viral pathogens.  Dip stick tests require a certain amount of a virus to be present in a sample to get a positive reaction and the amount of a virus in a plant can vary both in terms of the age of the plant part being tested (old vs. young leaves), as well as how long the plant has been infected.  Testing symptomatic tissue of different ages, as well testing more than once over a period of several days, can be critical in making an accurate diagnosis.

TMV on Tobacco

Finally, the virus that I haven’t yet seen this year (and that I don’t really want to see) is TMV.  This virus has a particularly wide host range and is particularly nasty given how easily it can be moved around.  You can pick up TMV on your fingers as you handle infected plants and transmit the virus by touch as you handle health plants.  TMV is a very stable virus.  It not only can be found in live plants, but can also be found in dead/dried plant tissue, including dried and processed tobacco.  If you smoke or use chewing tobacco, you are at increased risk of picking up this virus and spreading it around.  TMV can also hang around on inert items (e.g., clothing, boxes, work surfaces, tools) and can eventually be moved from these items to plants.  TMV, given its stability and easy transmission, is one of the most problematic and destructive viruses that I know of.  The growth distortions and blotchy color (i.e., mosaic) caused by TMV make infected plants unmarketable.  Destroying infected plants, and carefully and systematically decontaminating anything that has come into contact with infected plants is a must to get this virus under control.  And if you are thinking of quitting smoking or chewing tobacco, and need another reason, this virus is it.

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