It’s been a long winter and now temperatures have warmed to the point that spring emphemerals in my backyard are beginning to emerge and bloom. As their leaves begin to appear, I am on the lookout for symptoms of tobacco rattle caused by Tobacco rattle virus (TRV).
I know I have TRV in my garden and I am reasonably certain that I introduced the virus via a bluebell that a fried gave me many years ago. The plant showed interesting line patterns on the leaves and a bit of leaf distortion. If I had been a “good” gardener, I would have thrown the plant away as it had obvious symptoms of a viral disease. Instead, I was a “good” plant pathologist and plopped the plant into one of my beds and let it do its thing. Over the years I have seen symptoms of TRV in numerous plants in my backyard flowerbeds. I have volunteer Canada goldenrod plants that have lightning bolt (think Harry Potter’s forehead scar), yellow line patterns on their leaves every year. About 10 years ago, I noticed a similar line pattern on leaves on my bleeding heart, odd blotchy color and crinkly of leaves on my bloodroot, and dimpling and distorted leaves on my twinleaf. I had my visual diagnosis of TRV confirmed by diagnosticians at the Wisconsin Department of Agriculture, Trade and Consumer Protection, who had just started testing for the virus in nursery/greenhouse stock. My bleeding heart in particular was quite positive for the virus.
The diagnosis and symptoms of TRV fascinated me as a plant pathologist. However, they horrified me as a gardener because virus-infected plants often decline over time and typically stop blooming as the virus redirects plant energy and nutrients from producing more plant tissue and setting new flower buds to producing more viral particles. Interestingly though, after that one year of dramatic symptoms 10 years ago, my plants (other than the volunteer goldenrod) have been conspicuously lacking in any symptoms of TRV. Several years ago, I began testing for TRV in my own clinic and last year I noticed that the stock of positive control material for my test was getting low and I needed TRV-infected tissue to generate more. My bleeding heart was huge, lush, blooming profusely and totally asymptomatic, but I thought, “What the heck,” let’s test the plant again for TRV. Lo and behold the test lit up like the proverbial Christmas tree as positive for TRV. So did the symptomatic goldenrod from my yard. I had my positive control material.
All of the blathering above about TRV is well and good, but what are the take home messages?
Beware of plants showing viral symptoms
No matter what the virus, these plants can be bad news because they can serve as a source of a virus that eventually end up in other plants. Interestingly, TRV is transmitted by stubby root nematodes, microscopic worm-like organisms that feed on roots of infected plants, pick up the virus, and then transmit the virus once they feed on the roots of healthy plants. TRV can also be transmitted mechanically via contaminated tools (e.g., shovels, knives, etc.) used to divide plants. Nematode-transmitted viruses are somewhat unusual, but mechanically-transmitted viruses are very common. Another common way that certain viruses (but not TRV) can be moved about is by insects (aphids and thrips are notorious movers of plant viruses). Some viruses can even be transmitted by touch!
Even healthy-looking plants can be infected with TRV
As my bleeding heart demonstrates, plants that look healthy and bloom profusely can be have a viral problem. TRV has been a real issue in the perennial plant industry as the virus has a wide host range (including but not limited to the plants I have already mentioned as well as peony, astilbe, coral bells and relatives, and columbine) and often the plants show no symptoms. The onus is on plant propagators to supply healthy virus-free plants, but often they do not. So consumers buy TRV-infected, asymptomatic perennials, and happily plant them in their gardens only to have the virus rear its ugly head in other plants as it spreads. Asymptomatic plants can particularly be a problem if you plant them near a commercial potato field. Potato is a host for TRV. The virus does not cause foliar symptoms, but leads to necrotic (i.e., dead) flecks and arcs in potato tubers. If these tubers are sliced and fried, you end up with potato chips with black spots. Thus commercial potato producers (FYI, Wisconsin is the third largest potato producer in the US) are very worried about this virus.
Proper sanitation is critical for managing this (and other viruses)
Watch for any symptomatic plants and immediately remove and destroy them (by burning, burying or hot composting). Unfortunately, you may still have asymptomatic plants and the only way to check them for TRV is to have them professionally tested. This is not an inexpensive test (my clinic currently charges $35 for TRV-testing). Also, be careful to decontaminate anything (e.g., tools, working surfaces) that may have come in contact with infected plants. Soapy solutions work best. I typically recommend a solution that is 10% shampoo (make sure the label says the shampoo contains sodium lauryl sulfate) and 1% Alconox® (a laboratory detergent) in water.
All of this said, you may decide you think TRV-infected plants look cool (I do!) want to leave them in place. TRV-infected plants are actually quite beautiful. But be aware that I DO NOT recommend this if you live near a commercial potato field. And even in urban areas, your neighbors may not be happy with you if the virus spreads to their plants. Luckily I have neighbors who are tolerant of my plant pathological eccentricities. You may not be so lucky! As always, if you have questions about plant diseases and their management, feel free to contact me at (608) 262-2863 or pddc@wisc.edu.
P.S.: Happy Belated Robigalia (April 25), the Roman festival of the god of rust!