Departments of Plant Pathology and Entomology
North Carolina Cooperative Extension Service
North Carolina State University

FRUIT
AND
VEGETABLES

North Carolina Pest News
Volume 12, Number 7, June 6, 1997
Stephen J. Toth, Jr. and Thomas A. Melton, editors

The information and recommendations in this newsletter are applicable to North Carolina
and may not apply in other areas.

From: James F. Walgenbach, Extension Entomologist, Mountain Horticultural Crops Research and Extension Center, Fletcher

Tomato Spotted Wilt Virus on Tomatoes

Tomato spotted wilt virus is beginning to occur on tomatoes in at least one tomato field in western North Carolina. There appears to be some confusion as to the species of thrips vectoring the virus and the potential control options available. The only management strategies that I can recommend with any confidence at this time are: 1) make sure that transplants are free of tomato spotted wilt virus when planting; and 2) avoid early plantings.

The production of tomato spotted wilt virus-free tomato transplants is most easily accomplished by growing transplants in greenhouses that are used only for tomato transplant production. Production of certain flowering ornamentals that can harbor the virus (often symptomless infections) in the same greenhouse used for tomato transplants has been known to result in tomato spotted wilt virus infections in tomatoes. Also, is wise to avoid purchasing transplants from regions to the south, where the virus occurs in the field at higher levels than in North Carolina.

Cases where there have been relatively high incidence of tomato spotted wilt virus infections in field-grown tomatoes in North Carolina are almost always associated with early plantings of tomatoes (planted before June 10). Although marketing considerations usually dictate planting date, most growers usually have sequential plantings of tomatoes. Hence, their early plantings should not be placed in locations where the virus has occurred in previous years. These hot spots should be reserved for late plantings of tomatoes.

We are still in the process of determining the relative importance of different thrips vectors involved in the transmission of tomato spotted wilt virus to vegetables in North Carolina. However, at this time it appears that the tobacco thrips, Frankliniella fusca, is the most important vector. Of secondary importance (it is unclear how important), is the western flower thrips, Frankliniella occidentalis. This opinion is based on the overwintering habits of the two species and the fact that the tobacco thrips is the most common species occurring on tomato from April through June. Usually the western flower thrips does not appear on tomato until late June or July. These appear to be the only two vectors occurring in the field in North Carolina. The onion thrips, Thrips tabaci, once considered to be a vector, is no longer considered to be a vector of tomato spotted wilt virus. The tobacco thrips can be differentiated from the western flower thrips and the eastern flower thrips (which is not a vector) by the dark brown color of the tobacco thrips, while the western and eastern flower thrips have a yellowish appearance.

Insecticidal control of the thrips vector has not been shown to be effective in minimizing the spread of tomato spotted wilt virus. Virtually all tomato spotted wilt virus infections in tomato are due to primary spread (i.e., thrips vectors moving from borders of the field into crops). Secondary spread of the virus (i.e., spread from plant to plant within the field) is negligible. Unfortunately, thrips moving in from field borders can transmit the virus to healthy plants more quickly than insecticides can kill the thrips. However, insecticidal control of thrips is effective in avoiding direct damage to fruit. Western flower thrips feeding within new flowers, before stamens have been shed, will deposit eggs on the small developing fruit. These oviposition scars result in a dimpling effect on mature fruit. The western flower thrips, not the tobacco or eastern flower thrips, is the primary cause of this damage. Because the western flower thrips does not occur in all tomato production areas, it is helpful to have thrips identified to species to determine if insecticidal control is necessary. If western flower thrips is present in flowers, the most effective options for control are either Monitor, Lannate or Dimethoate.

Web page last updated on June 9, 1997 by Stephen J. Toth, Jr..

This Web version is a cooperative effort between the North Carolina Cooperative Extension Service and the Center for Integrated Pest Management