Viruses in Dahlias
Most dahlias in the US are infected by at least one virus. In the 2022 American Dahlia Society virus testing program 87% of the 1334 samples analyzed were positive for at least one virus. In this program, participants are instructed to send their healthiest looking plants. Viruses have been infecting dahlias for a long time, likely prior to their domestication (Geering et al. 2022). Because dahlias can be asymptomatic or mildly symptomatic, viral infection is often undetected. Dahlias have not been shown to be cured after virus infection. Culling sick stock and sourcing clean stock are best practices for dealing with viruses in dahlias.
Virus Symptoms
Viruses can weaken plants and cause many symptoms: stunted growth, yellow mottling on leaves, vein clearing, color breaking or lighter streaking on petals, and curling, asymmetrical leaves. Vein clearing has been an early and common indicator of virus infection in my plants. Sometimes plants are asymptomatic or symptoms are overlooked.
In the 2022 ADS virus testing program, over three-quarters of infected samples were reported to have “Excellent” foliage. Virus symptoms are most pronounced in early summer and late fall when temperatures are cooler. Dahlias grow more slowly during cooler, shorter days and viruses may get the upper-hand. However by summer the symptoms may ameliorate, and a grower may mistakenly believe that the plant has recovered from the infection (Dr. Keith Hammett, Quince Flowers Podcast Ep. 6: Min. 48). While fertilization, Epson salts, etc. may mask the infection, the virus persists. Asymptomatic dahlias can infect other plants. There is no known cure for viruses in dahlias. Culling infected stock — and more elaborately, tissue culture— is the only way to remove the virus.
Vein clearing and mosaic
Fan leaf is a clear indicator of viral infection.
Most Common Dahlia Viruses
DAHLIA MOSAIC VIRUS
DMV is the most common virus infecting dahlias. Over the last 5 years the commercial testing lab Agdia in Elkhart, Indiana has reported a positivity rate of 84% for DMV. The Washington State plant pathology laboratory reported in October 2022 that due to a virus mutation, they did not detect any positives during that time period. However, after retesting 2022 samples, WSU/ADS reported a positive rate of about 75% (personal correspondence with Ron Miner 4/3/23). This virus only infects dahlias, and its known vectors include aphids and cutting tools. Pollen could be another vector.
DMV was described to be transmitted through seeds to seedlings (Pahalawatta et al. 2007). However, Dr. Pappu has since clarified that a specific strain of DMV, DMV-EPRV (Endogenous Pararetrovirus) was transmitted. DMV-EPRV fka DMV-D10 is likely* a non-infectious strain, embedded in the dahlia genome, likely* lacking a mechanism to make the plant sick (Geering et al. 2022). *Ongoing study at Utah State University by Dr. Claudia Nischwitz suggests DMV-EPRV can be activated and infect dahlias. The more I learn and ask about DMV, the more I understand that the science it still young, and we have more to learn.
Although DMV test kits became available from Agdia in early 2022, lab testing is still the most economical way for most growers to test for DMV. WSU and Agdia offer testing for DMV/DCMV (infectious strains only). As of January 2023 Agdia charges $74 for the first sample and $34 for each additional sample.
TOBACCO STREAK VIRUS
Approximately half of the dahlias I have tested have been positive for TSV. Similarly, 48% of samples submitted by dahlia vendors to the 2022 ADS Virus Testing were positive for TSV. TSV is spread by thrips and floral snips/pruners. The virus can infect more than 200 other plant species. Anecdotal evidence suggests that TSV is not passed from parents to seedlings.
Field testing for TSV is available from Agdia. Pricing for 25 field tests is $155 ($6.20/test) as of January 2023.
Above Agdia immunostrips were used to test for TSV. The top test shows a negative result (1 red line); whereas the lower result is positive (2 red lines).
TOMATO SPOTTED WILT VIRUS
TSWV in US dahlia stock grew to fame in spring 2022 when many dahlia cuttings infected with TSWV were distributed widely in the US. While Agdia reported a lab positivity rate of 1.2% from 2017-2022 and ADS reported 2.8% infection rate in vendor samples, my infection rate was tenfold higher and from multiple domestic sources in incoming stock in 2022. Thankfully TSWV's most common symptom — yellow mottled leaves with concentric circles — is fairly easy to spot. TSWV is known to infect over 1000 plant species and is spread by thrips and cutting tools. If field testing, note that TSWV is unevenly distributed in dahlia plants. Testing the petiole might yield the best results (Asano et al. 2017). Agdia TSWV test strips can be purchased in bundles of 25 for $155 ($6.20/test) as of January 2023.
Impatiens Necrotic Spot Virus
Agdia reports a lab positivity rate of 0.3% for the past 5 years for INSV. WSU reports much higher rates. 26% of the vendor samples submitted to WSU in 2022 were positive for INSV. WSU/ADS also reports plants testing positive one year are free of the virus the following year. Like TSWV, INSV is a tospovirus, and the virus’s concentration varies throughout the plant.
OTHER PATHOGENS
In total, Agdia offers testing for 10 pathogens known to infect dahlias. In addition to those listed above, those pathogens (and their positivity rate at Agdia’s lab) include:
Arabis mosaic virus (<0.1%)
Cucumber mosaic virus (<0.1%)
Chrysanthemum stunt viroid (1.2%)
Potyvirus group (1.0%)
Potato virus Y (<0.1%)
What NOW?
Do what you can to limit the spread of viruses in dahlias.
Be selective of where you buy your stock. A few vendors and some dahlia clubs offer clean stock. Most experienced growers know what to look for in regards to virus but may be such skilled growers that symptoms are masked in their beds. If you find what you think is virus, kindly let the grower know what variety(s) was affected, but generally do not expect a refund unless it is explicitly guaranteed. Only 13% of the samples submitted in the ADS clean stock initiative in 2022 tested clean. Dahlia virus is part of doing business. Quarantine new stock, keep notes, mark the source on your plant tags, and support growers who have provided you good stock.
Sanitize cutting equipment. Virus can spread anytime sap from one plant is transferred to another. Consider carrying around a caddy of bleach solution or a spray bottle of commercial anti-viral disinfectant for snips when pruning and harvesting. The American Dahlia Society provides a list of disinfecting options.
Likewise manage pests, namely aphids and thrips, that spread disease. While aphids haven’t taken a liking to my dahlias yet, I manage them on my other crops with insecticidal soap. The soap solution is not a preventative or a deterrent, so applications must be repeated until the infestation is gone. The solution must be sprayed directly on aphids to be effective. The soap breaks up the waxy coating on the soft-bodied insects. I use 1 tsp of Dawn dish soap per gallon of water. Thrips are a more recent pest for me. I am treating them weekly with Spinosad (sold under the labels Captain Jack’s Deadbug Brew, Monterey Garden Insect Spray), a bacteria that is effective at killing thrips. Do not apply this product to flowering plants as it is lethal to bees for about one day after application.
Inspect your plants regularly for signs of virus. Remove any plants that you suspect are diseased. Test for viruses, if you can.
Truly, if you have a doubt, toss it out.
My goal is to grow virus-indexed dahlia stock through comprehensive testing and culling. I do not want viruses from my dahlias to spill over into other plants. While I believe that it is the right fit for me, my intent is not to evangelize. It is a very costly endeavor that may fail. Viruses may spill over into my clean stock, and even if I succeed, my collection will likely be quite small for some time.
If you want (AND CAN) test your dahlias for viruses
Here are my recommendations:
Test only plants that look healthy. I spent a lot of money to learn this, but Daisy Duke, above, with her curly, distorted leaves is not worth testing. While nutritional deficiencies are possible, plants that appeared diseased in my beds tested positive for disease.
Test for Tobacco Streak Virus first. It is the most prevalent virus that can be tested without lab equipment. Agdia’s TSV field tests are currently $6.20/test ($155 per 25 tests).
If the plant is free of TSV, consider testing for Tomato Spotted Wilt Virus. TSWV can be field tested, which is less costly than lab testing. Note also that TSWV generally has pronounced symptoms. Agdia’s TSWV test strips are $155 per 25 tests.
If the plants are clean of TSV and TSWV (opt.), submit leaves for lab testing for Dahlia Mosaic Virus. DMV currently costs $74 for the first sample and $34 for each additional sample at Agdia. Because the virus if so common, submit each variety or plant as a separate sample. Expect dismal results.
Finally if you want peace of mind, with any DMV-negative samples, pool them into composite samples (up to 10 leaves/composite) for TSV, TSWV, and INSV lab testing. The cost in 2023 is $91 from the first sample and $23.90 for each additional sample at Agdia.
There is a chance that one of the less common pathogens will be present and additional testing can be done to detect those.
Finally, this is completely unsponsored. However, if you have questions about virus testing, my interactions with Agdia (testing@agdia.com) have been very helpful.
