Tomato spotted wilt virus (TSWV) is usually spread by Western flower thrips. During the past several years, the virus has caused severe damage to tomato, tobacco and peanut crops along the Gulf Coast and in Tennessee and Kentucky. Extension specialists from New Mexico State University and the University of Georgia have commented on disease transmission, life history, disease symptoms, control methods and how to scout for Western flower thrips, as is noted below:
TSWV is transmitted from infected plants to healthy plants by at least nine species of thrips. Thrips are tiny (approximately 1/16th of an inch) winged insects that feed on plants through rasping-sucking mouthparts. Thrips transmit the virus in a persistent manner – once the insect has picked up the virus, it is able to transmit the virus for the remainder of its life (about 30 days). The virus is not passed from adult to egg; however, progeny that develop on infected plants will quickly pick up the virus and be effective disease vectors.
The adult female thrips can deposit 50 to 60 eggs within plant foliage. Hatching occurs seven days after deposition. After hatching, larvae feed six to seven days before entering a three- to four-day quiescent stage. Afterward, the insect emerges as an adult and continues to feed. The period from egg to adult is approximately 16 to 21 days. The lifespan of an infected thrip is thought to be less than 30 days.
Thrips are the primary vector of TSWV. Only thrips larvae can acquire TSWV, but both larval and adult thrips are capable of virus transmission. Larval thrips ingest TSWV particles (virus acquisition) from infected plants while feeding. Larvae must feed at least 10 to 30 minutes for virus acquisition. After acquisition, the virus circulates and replicates within the thrips’ bodies. Virus transmission begins seven to 10 days after acquisition and can occur after 15 minutes of feeding.
Tomato plants infected with TSWV are stunted and often die. Initially, leaves in the terminal portion of the plant stop growing, become distorted and turn pale green. In young leaves, veins thicken and turn purple, causing them to appear a bronze color and develop numerous small dark spots. The leaves often droop on the plant, creating a wilt-like appearance. Necrotic spots or ring spots frequently occur in infected leaves. Stems of infected plants often have purplish-brown streaks. The plants may develop a one-sided growth habit or may be stunted completely.
Plants that are affected early in the growing season often do not produce any fruit, while those infected after fruit set produce diseased fruit with striking symptoms, including chlorotic ringspots, raised bumps, uneven ripening and deformation. Infected plants produce poor quality fruit and reduced yield.
Controlling TSWV is difficult. The wide host range, which includes many perennial ornamentals and weeds, enables the virus to successfully overwinter from one crop to the next. Control weeds that are adjacent to fielda. Efforts to control the insect vectors in crop fields has little effect on TSWV because large populations of thrips may fly or be blown into treated fields from non-treated areas nearby, however. Obtain current chemical control information from your County Cooperative Extension Service.
Scout for thrips at the same time you would for tomato fruitworms and armyworms, which is when there is a significant number of green fruits at least one inch in diameter, or at the beginning of flowering. Sample fields at least once per week (preferably twice per week). Sample in a pattern that covers the entire field (X, V or zig-zag pattern). Check field edges, but walk at least 10 rows or 25 paces into the field to begin sampling.
Select one tomato flower from each of the plants sampled (five plants per location in the field). Place flowers in a jar with alcohol. After a few minutes, thrips will sink to the bottom of the jar. Count the number of thrips; a spray is recommended if an average of one thrip per flower is found.