Steps for Identifying Tomato Spotted Wilt Virus (TSWV) in Marigolds

Marigolds plants not infected with the tomato spotted wilt virus (TSWV)

‘Moonsong Deep Orange’ African marigolds

A recent e-Gro alert from Brian E. Whipker and Patrick Veazie looked at a recent crop of marigolds that displayed typical symptoms of a virus infection. With other agents being possible explanations, here’s how growers can properly identify the tomato spotted wilt virus (TSWV) as the origin of infection. 

Common symptoms displayed on the crop of marigolds were: 

  • Leaf distortion 
  • Leaf mottling 
  • White ringspots 

While fitting into the categorization of TSWV, these symptoms also correlate with other plant viruses, so how was the true cause narrowed down? 

Tomato Spotted Wilt Virus in Marigolds

Starting with the leaf distortion identified on the upper leaves of the marigolds, this symptom is common with feeding from Western flower thrips (WFT). Still, a limited insect presence was documented after scouting the crop. Additionally, a lack of stunting was observed, which is typical with TSWV infections but atypical with many other plant diseases and virus infections. Despite this being the first instance of authors Veazie and Whipker noting TSWV in marigolds, the above evidence raised it as a possibility. 

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The aforementioned symptoms and additional observations led to the marigolds being tested for tomato spotted wilt virus, for which it was “confirmed with an enzyme-linked immunosorbent assay (ELISA) test.” ELISA tests can be acquired through Agdia for in-house testing, or plants can be brought to experts for testing at a diagnostic clinic. However, for accurate testing results, it’s recommended “to test multiple leaves from the same plant that is exhibiting symptoms.  The total leaf area tested should be around 1 square cm (postage stamp size).”

Management of TSWV After Identification

Plants with tomato spotted wilt virus or impatiens necrotic spot virus (INSV), another common greenhouse production virus, cannot be cured. Therefore, the top priority for growers should be culling and discarding infected plants to prevent the spread of the virus. 

However, it’s important to note that some plants with TSWV or INSV may be asymptomatic and able to continue the spread of either virus without visual signifiers of its own infection. Therefore, short of destroying your entire crop, there’s no way to ensure complete eradication of the virus. So, what are growers to do? 

One of the primary infection vectors of TSWV is Western Flower thrips (Frankliniella occidentallis). Therefore, the propagation of thrips, and their subsequent feeding on your crop, must be kept under control, especially with warmer temperatures now settled in across much of the U.S. 

According to a 2020 report from Steven Frank and James Baker on NC State Extension, “Larvae of the western flower thrips can become infected with tomato spotted wilt virus (TSWV) or impatiens necrotic spot virus (INSV) by feeding on an infected plant for only 30 minutes. After a latent period of 3 to 18 days, these thrips can then infect new plants after feeding only 5 to 15 minutes.” 

For more information, details, and tips on identifying and managing marigolds with the virus, please read the full e-Gro (Electronic Grower Resources Online) alert “Marigolds: Tomato Spotted Wilt Virus (TSWV).” Additional and current e-Gro alert pieces from Volume 13 (2024) can be found online. 

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