I spotted a very strange looking tomato plant in a high tunnel on a farm in Pike County, it stood out like a sore thumb amongst a stand of very healthy tomatoes. It lacked any of the common symptoms I come across amongst the plethora of bacterial and fungal diseases found on tomato plants. There were also no signs of insects or insect damage. The plant was stunted but had still managed to grow to about 3 feet tall, it was short and stout with a thick stem, good plant structure and abundant foliage. It had only managed to produce two very small fruits, the ripening of which appeared to be stalled. The foliage was yellowed overall, and some was turning a strange purple color. Some leaves showed purple veination, some leaves showed purple mottling. The other strange thing was that the stem had an abundance of large adventitious roots. All that being said, it was not pulling its weight in the high tunnel, an expensive structure to build and maintain.

Tomato plant with Tomato Spotted Wilt Virus symptoms.

Purple veination of infected foliage.

Adventitious roots symptomatic of TSWV.

I hadn't come across this problem before, but these symptoms all align with infection by tomato spotted wilt virus, AKA TSWV. The even stranger thing, one day later a grower sent me an email with pics of large tomato fruit showing signs of TSWV. This grower's plants were large and looked rather healthy, with symptoms of a disorder only appearing in the fruit. The plants had abundant fruit that was sizing up nicely, however they had strange circular rings present on the fruit surface, characteristic of a TSWV infection. The unique thing about some plant viruses is that their symptoms can manifest in different ways in the same vegetable crop. My guess is that the first plant I saw was infected early, and the second wasn't infected until it was fairly large. There is no cure for TSWV, so the best thing to do is prevent infection. But how did they get infected?

Tomato fruit with concentric rings symptomatic of TSWV.

TSWV as with many plant viruses, is vectored by insects. In this case it is vectored solely by thrips. Western flower thrips are a problematic insect found in most high tunnels, their populations can explode quickly as it only takes 2-3 weeks to produce a second generation, and then a third… The virus itself exists in many common weeds but can only be transmitted to crop plants via thrips. Managing thrips is a challenge. One cultural practice growers should employ is to remove weeds both inside and outside of the high tunnel. You should create a buffer around your high tunnel by laying down landscape fabric and covering it with gravel if available. If this is not an option, keep grass mowed on a weekly basis, and weedwack those hard-to-reach spots. Manage weeds within the high tunnel during the growing season, and make sure you go into the fall and winter with a clean high tunnel.

There are also tomato varieties that have been bred for resistance to TSWV, resistance is a helpful tool to have in your toolbox. It is important to scout and monitor for thrips before applying an insecticide. You can use blue-sticky cards to monitor thrips or perform a visual inspection of plants. Many insecticide options are available to control thrips, make sure to rotate modes of action or resistance will build in populations. Use the online or print version of the Midwest Vegetable Guide to search for insecticides labeled for a given crop and insect. Plants infected with TSWV should be rogued from the planting and destroyed, as they will serve as a host for the disease. You can contact your local MU Extension horticulture specialist for assistance or submit a plant sample to the MU Plant Diagnostic Clinic.