Plant Problem Diagnostic
Tomato Spotted Wilt Virus: Diagnosis and Thrips-Based Prevention
The short answer
Tomato spotted wilt virus (TSWV) typically bronzes the youngest leaves first, then produces dark necrotic spots and concentric ring spots, distorted or one-sided stunted growth, and fruit with chlorotic or yellow rings and blotchy, uneven ripening (a distinctive calico look). Early infection can cause near-total fruit loss.
It is transmitted only by thrips, such as western flower thrips and tobacco thrips, which can acquire the virus only as larvae feeding on an infected plant and then spread it for the rest of their lives as adults. Importantly, TSWV is not seed-transmitted and not readily spread by handling, and it has an enormous host range including many weeds and ornamentals like begonias and impatiens that act as reservoirs.
There is no cure and no chemical that controls the virus itself, so rogue out infected plants immediately, start with virus- and thrips-free transplants, manage thrips before and after planting, control reservoir weeds, and where available use resistant varieties (carrying the Sw-5 gene) and reflective or metallic mulch. Because the ringspot symptoms mimic other viral and physiological disorders, confirm suspicious plants with an Extension diagnostic lab before removing a lot of plants.
Recognizing tomato spotted wilt
The early signal is bronzing at the top of the plant. UC IPM and NC State Extension describe bronzing of the upper sides of young leaves, which then develop distinct necrotic spots and concentric ring spots; leaves may cup downward and shoot tips die back, and growth is often stunted, distorted, or one-sided. On fruit, expect chlorotic spots and yellow ringspots with blotches of uneven color, a mottled calico ripening that is especially striking on Roma types. On pepper, TSWV brings brown necrotic or chlorotic ring spots and can cause a seedling shepherd's crook.
Those ring spots are distinctive but not unique, which is the diagnostic catch. TSWV symptoms can resemble other viruses, some nutrient and physiological disorders, and even herbicide effects, and they vary with plant age and variety. Because the management step, removing the plant, is irreversible, Utah State and others recommend confirming suspicious plants rather than roguing on a guess; a state Extension plant diagnostic clinic can test for the virus.
It only moves by thrips
TSWV has one delivery system: thrips. NC State and UC IPM note it is spread only by thrips, western flower thrips, tobacco thrips, onion thrips, and a few others, and that the insect can acquire the virus only in its larval (nymph) stage by feeding on an infected plant; once acquired, it transmits the virus as an adult for the rest of its life. It is not seed-transmitted and not effectively spread by mechanical handling, which sets it apart from the mechanically spread mosaic viruses.
The other half of the story is the huge reservoir. TSWV infects on the order of a thousand plant species, including common weeds (dandelion, sow thistle, chickweed) and popular ornamentals (begonias, impatiens) that carry the virus and feed thrips. That is why an infected flat of ornamentals or a weedy border can seed spotted wilt into a vegetable garden, thrips move the virus out of those reservoirs and into the tomatoes.
Managing it: rogue plants and target thrips
Once a plant is infected there is no cure and no chemical control for the virus, so remove and destroy infected plants promptly, ideally roguing them at the seedling or first-symptom stage before thrips can spread the virus further. Start clean by using virus- and thrips-free transplants, and avoid planting tomatoes next to infected ornamentals or weedy areas that harbor the virus.
Then go after the vector and the reservoirs. Manage thrips before and after planting, control weeds in and around the garden that host both the virus and thrips, and where available use resistant varieties carrying the Sw-5 gene (Tsw in pepper). UC IPM and NC State also note reflective or metallic mulch deters thrips from landing, reducing new infections. Insecticides alone rarely stop TSWV because thrips can transmit the virus quickly, so the program leans on resistant varieties, clean starts, roguing, weed control, and reflective mulch together. See the thrips guide for managing the vector itself, and the mosaic virus guide for the mechanically spread viruses this is often confused with.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- NC State Extension - Tomato Spotted Wilt Virus on Tomato and Pepper — TSWV bronzes the uppermost tomato leaves with small brown lesions and causes yellow ringspots and necrotic sunken areas on fruit; it is transmitted by thrips (tobacco thrips and western flower thrips) that acquire the virus only in the larval stage and transmit as adults; it infects over a thousand species including weeds and ornamentals like begonias and impatiens; and it is managed with Sw-5 or Tsw resistant varieties, reflective mulch, thrips control, weed control, and removing symptomatic plants.
- UC IPM - Tomato Spotted Wilt (Tomato Pest Management Guidelines) — TSWV causes bronzing of young leaves that develop necrotic spots, cupped leaves, tip dieback, and fruit with chlorotic spots or blotches often in concentric rings; it is spread by thrips (notably western flower thrips) where nymphs that acquire the virus transmit it for life; it has an extremely wide weed and ornamental host range; and it is managed with Sw-5 resistant varieties, virus- and thrips-free transplants, thrips management, roguing, weed control, and reflective mulch.
- Utah State University Extension - Tomato Spotted Wilt Virus — TSWV is a tospovirus spread by thrips (infection occurs when virus-carrying thrips feed on a healthy plant); it causes stunting, necrotic leaf spots, and fruit necrotic rings and calico patterns, and although it can discolor seed it is not seed transmitted; once a plant is infected there is no cure and no chemical controls, so management relies on removing infected plants, controlling thrips and weeds, resistant varieties, and healthy transplants.