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Symptoms first appear as necrotic and/or chlorotic spots or streaks on the foliage, leaf petioles and stems. Plants start to turn a pale green color and become very distorted. Stems may also develop purplish-brown streaking. These symptoms on the stem lead to wilting and stunting in the plant.
Green immature tomato fruit are mottled and have slightly raised areas with faint concentric rings. On mature fruit, these faint rings turn into very distinct red or yellowish-white rings. Blotches with sunken areas can also form. Necrotic ringspots form on tomato fruits with Tomato chlorotic spot virus.
Pictures of symptoms caused by Tomato chlorotic virus (top left; top right) and Tospovirus (bottom left; bottom right).
Tospoviruses overwinter in dead plant debris, such as tomatoes and weeds. Susceptible hosts become infected through mechanical transmission through thrips feeding and vegetative propagation. Low seed transmission is known to occur. Thrips larvae feed on infected plants and acquire the virus, which is propagatively transmitted. Adult thrips do not acquire the virus. Once ingested, the virus travels to the midgut of the insect, where it multiples and remains infective there for the remainder of the insect’s lifetime (30 to 40 days). This virus has three single-stranded negative sense RNAs that replicates within the thrips host. The virus can also be passed from generation to generation and are transmitted by ten different species of thrips worldwide. Symptoms appear first as necrotic and chlorotic lesions on leaves, leaf petioles, and stems. The stems develop purplish-brown streaking. The tomato plant then begins to wilt, and new growth is stunted. If tomato fruits are infected, they develop blotchy orange and yellow discoloration patterns and light green rings with sunken centers. If they become infected with Tomato chlorotic spot virus, very distinct necrotic ringspots will form on the fruit.
Favorable Environmental Condtions
Dry and warm conditions (approximately 75ºF) are favorable for thrips reproduction.
Often Confused With
- Fusarium wilt – Symptoms begin with a wilt and yellowing of the leaves of older leaves. When the disease progress, the plant becomes very chlorotic and dies.
- Verticillium wilt – The older leaves are first affected and develop large yellow lesions. The plant becomes very stunted and depending on disease severity, there will be a reduction in fruit size and quantity.
- Pepino mosaic virus – Early symptoms include light green areas on the leaves and overall stunting of new growth. Chlorotic spots form on the foliage and the leaf veins turn yellow. Fruits also develop a yellow-red discoloration pattern, similar to the discoloration formed by tospoviruses.
Since Tospoviruses are transmitted by fourteen different thrips species, it is very important to look for any signs of thrips damage on tomato plants. Sticky traps can also be hung in the high tunnels to monitor thrips. If any plants exhibit viral symptoms, remove them in order to prevent further infections. More information can be found at University of Florida IFAS Extension website.
Economic action thresholds have been described by the University of Florida IFAS Extension website. Management needs to take place if one thrips adult is found per tomato flower. If at least two larvae are found per fruit of any size on average, management actions should also be taken.
- Monitor Thrips Populations – Tospoviruses are transmitted by fourteen different thrips species in a persistent propagative manner, meaning once an immature thrips insect acquires the virus, it remains infective for the remainder of its lifetime and passes the virus onto its progeny. Thrips populations can multiply exponentially so it is very important to monitor the populations.
- Use Resistant Varieties – There are several commercial tomato varieties resistant to tospoviruses. These tomato varieties are being used to reduce losses to tomato spotted wilt diseases. A list of these varieties can be found in the publication Host Plant Resistance to Tomato spotted wilt virus (Bunyaviridate: Tospovirus) in Tomato (Riley and Joseph, 2011).
- Use Virus-Free Tomato Transplants – Tospoviruses can be technically transmitted to new healthy plants through vegetative propagation and grafting.
- Remove Weeds – May weed species harbor tospoviruses and may serve as reservoirs for the new season. It is important to remove these weeds while tomatoes are planted in the high tunnels to prevent infection.