Tomato spotted wilt virus, also known by the acronym TSWV, does not only affect tomatoes. More than 1,000 plant species can get it, both edibles and ornamentals, which makes it one of the plant viruses with the largest range of hosts.
In addition to tomatoes, hosts include peppers, potatoes, eggplants, squashes, lettuce, onions, spinach, watermelons, and also herbs like peppermint. Popular ornamentals susceptible to the virus are begonias and impatiens.
There is no treatment for plants with tomato spotted wilt virus. The only way to control it is to control the thrips, the tiny insects that transmit the virus. Here are the steps to take before, during, and after the gardening season.
How to Identify Tomato Spotted Wilt Virus
Tomato spotted wilt virus has many different symptoms but two stand out. Young leaves develop lots of yellow to brown circular spots on the upper side. These spots are so numerous that the leaves appear bronze in color, and the affected plant tissue later dies, which makes the spots look sunken.
The other telltale symptom is drooping leaves that make the plant look wilted.
Affected plants also have dieback of the growing tips, stunted growth, and leaves that roll upwards or downwards.
Tomato plants that have tomato spotted wilt are either stunted or only grow on one side. Depending on how early the plants caught the virus, they may not even produce any tomatoes, and if they do, the tomatoes are deformed and ripen unevenly, with chlorotic concentric blotches and bumps. In other words, your chances for making home-canned tomato sauce that summer will be slim.
How Do Tomatoes Get Spotted Wilt Virus?
The culprit that spreads tomato spotted wilt virus are thrips, tiny winged insects about one-sixteenth of an inch. At least ten different species of thrips spread the virus from plant to plant.
Thrips pick up the virus when feeding with their sucking mouthparts on an infected plant. The virus then replicates in the insect body and remains in there for the rest of the insect’s life, transmitting it on an ongoing basis to healthy plants when feeding.
The virus does not affect the insect itself. Infected female thrips do not pass the virus on to their progeny through their eggs, however, thrips that hatch on an infected plant easily contract the virus to become vectors on their own.
Thrips thrive in warm and hot weather, and that’s when they will spread the disease the most.
Treatment and Control
There is nothing you can do to save an infected tomato plant. The focus of controlling the virus therefore needs to be on controlling the thrips. And while you might not be able to eliminate the virus, you can reduce how much it impacts your tomato plants.
As always, good cultural gardening practices are key. First off, make sure to buy only virus-free seedings from a reputable nursery. Most nurseries grow their tomatoes in greenhouses, which is a controlled environment, so these transplants have the best chance of being virus-free. Still, always inspect the young plants carefully for signs of the disease as well as thrips.
Second, practice good weed control, which you should in any case. And don’t just weed in your garden but also around it to remove any weeds that are potentially alternate host plants for both the thrips and virus, such as dandelion, annual sowthistle, chickweed, buttercup, and plantain.
Also, thrips are repelled by silver reflective mulch. Note that it must be silver; dark or red mulch does not have the same glaring effect that keeps thrips away. Silver reflective mulch is available from garden supply companies, or you can make your own by spray-painting black plastic sheets with silver paint and secure them in the ground with u-shaped metal landscape staples.
Controlling thrips with insecticides is difficult because they move around. Spraying tomato plants early in the season may help to control thrips to a certain extent.
Insecticides available for home gardeners include different neem products, insecticides based on natural bacteria, summer or all-season oil, and insecticidal soap. As always when applying any insecticide, read and follow instructions on the label carefully.
Apply the insecticide in the morning when the thrips are the most active.
If the infestation persists, don’t use the same insecticide over and over again. Thrips are able to develop resistance to insecticides in a relatively short time. For this reason it’s recommended to rotate the insecticide—not just the brand, but the type.
Insecticides you should not use are organophosphate or pyrethroid insecticides because they kill beneficial insects that help keep the thrips population under control.
If you want to go one step further, purchase minute pirate bugs (Orius insidiosus) and big‑eyed bugs (Geocoris punctipes) from a company specializing in organic and biological pest control. Both insects are highly effective predators of thrips.
Removing Infected Tomato Plants
Remove an infected plant promptly, no matter its growth stage. After all the effort made to plant and get it growing, it’s hard to pull a young tomato plant but it won’t bounce back, and it will spread the disease to other healthy plants.
When you remove an infected plant, be careful not to spread thrips around even more. Don’t carry the plant across your garden but dispose of it in a garbage bag right on the spot.
There are resistant tomato cultivars, such as 'Health Kick'. In seed catalogs, their resistance is usually listed under the product details, either as SWV or TSWV resistance. Keep in mind though that it’s not a guarantee against tomato spotted wilt virus. You should still follow the other good gardening practices above.
These practices also include a thorough fall garden cleanup, removing and destroying all spent tomato plants, and any other host crops, as well as controlling host weeds in nearby areas.