California Pepper Commission


Proposal Gilbertson TSWV Proposal 2015

Project Title: Monitoring of thrips/Tomato spotted wilt virus (TSWV) in California peppers and the development of a regional IPM strategy for reducing the incidence and severity of TSWV (2015).

Project Leaders: Dr. Robert L. Gilbertson (Primary leader for correspondence) Plant Pathology Department, UC Davis. Davis CA 95616

Phone: 530-752-3163 FAX: 530-752-5674 e-mail:

Dr. Ozgur Batuman, Plant Pathology Department, UC Davis (

Dr. Neil McRoberts, Plant Pathology Department, UC Davis (

Dr. Brenna Aegerter, UC Cooperative Extension County Advisor, San Joaquin County (

Budget: $15,000/yr


Thrips impact the pepper industry by (1) causing direct damage to pepper fruit through feeding damage and (2) spreading viruses (particularly Tomato spotted wilt virus [TSWV]) that cause significant yield losses. In conventional production, thrips are managed with insecticides. However, this can be difficult due to lack of effective insecticides and difficulties targeting thrips. The most effective use of insecticides for thrips management requires accurate timing of sprays, usually in advance of population peaks, which limits feeding damage and virus transmission. Thus, the understanding thrips and TSWV development in peppers in the Central Valley is the first step for development of effective control strategies for this complex in California peppers.

Overall Objective:

The overall objective of our proposed research is the better understanding of viruses infect pepper in California with an emphasis on thrips-transmitted Tomato spotted wilt virus (TSWV). With results of surveys conducted in 2013 and 2014 in pepper fields in the Central Valley of California, we are starting to better understand thrips population dynamics and TSWV development in peppers. Our results indicated that, although peppers are a good host for both thrips and TSWV, the complex is not causing major losses and effective management is possible. However, if not controlled early in development, thrips and TSWV can cause economic losses in peppers in the Central Valley, especially in late-planted fields. By continuing to monitor thrips and TSWV development in pepper fields in 2015 and comparing the results with previous years we will get a better understanding of importance of this problem, which will allow for the development of a comprehensive and effective integrated pest management (IPM) strategy for controlling thrips and TSWV in peppers. Our long-term objectives are to reduce the impact of thrips and thrips- vectored viruses on peppers (and other crops) in California, and to provide growers the tools to do this. We hope that by reducing thrips population levels within each crop, we can achieve a long- term reduction in the incidences of thrips-vectored viruses.

Specific Objectives:

i. Continue to conduct surveys of selected pepper fields in the Central Valley to gain insight into when and from where thrips and TSWV enters into commercial fields

In 2015, we propose to continue to monitor thrips population dynamics and incidence and development of Tomato spotted wilt virus (TSWV) in pepper fields in the Central Valley. We are now starting to understand the association of thrips population dynamics and TSWV development in peppers. In 2014, TSWV first appeared in pepper fields in mid-April at or around the time when thrips populations increased. This was about a month earlier than in 2013 when the thrips populations increase was in mid-May. This suggested a correlation between thrips populations increase and TSWV appearance. In 2014, TSWV appeared in all monitored pepper fields and overall TSWV incidences in early-planted monitored pepper fields remained relatively low (1- 2%), similar to 2013. However, one monitored early-planted pepper field, which was planted adjacent to a processing tomato field with a high TSWV incidence (14%), had a TSWV infection rate that reached ~40% in one corner of the field. Pepper plants showed severe symptoms of TSWV infection, including wilting and necrosis on leaves and stems and shoot dieback. However, because the infection was relatively late and fruits were mature, TSWV did not seem to cause substantial economic losses in this field. In some late-planted green and red pepper fields monitored in 2014, TSWV incidences were higher (8-14%) and increased up to 14%-20% in parts of the fields by the end of the season. In some of these fields, TSWV came in late, probably via thrips from nearby processing tomato fields that were being harvested. However, even in these fields, damage from TSWV appeared to cause minimal economic loss, but clearly the potential for loss due to TSWV exists.

In 2014, Alfalfa mosaic virus (AMV) was more widespread than TSWV (or any other disease) in monitored pepper fields, with incidences as high as 40-60% in some fields. Interestingly, AMV did not seem to cause any visible damage to pepper fruits. Moreover, later in the season, most of the younger shoots that developed from these initially infected plants became symptomless and produced normal fruits. We believe that these pepper plants may be undergoing ‘recovery’ from AMV infection; this is a type of plant defense response (gene silencing) that specifically targets foreign nucleic acids (e.g., viruses). In 2014, we also became involved in a virus disease problem in peppers in the Coachella Valley. Here, peppers plants were infected with AMV and Beet curly top virus (BCTV), sometimes in mixed infection. In the cases of mixed infection, symptoms were more severe than AMV infection alone, and plants with mixed infections either died or became stunted, and did not produce marketable fruits. In plants with these mixed infections we also did not observe recovery. Fortunately, the incidence of mixed infections was relatively low (~2%).

Thus, we believe that monitoring of pepper fields in the Central Valley (and possibly in Coachella Valley) will continue to allow us to better understand how and when thrips/TSWV develops in peppers, which will ultimately allow us to develop an effective IPM strategy to control this complex in California. This objective will also allow us to have a better insight on the prevalence and impact of other virus diseases (i.e., AMV and BCTV) in peppers.

Key Findings: TSWV outbreaks are correlated with timing of thrips populations increase and field location (e.g., proximity to early-planted susceptible crops) and other viruses can impact pepper production.

ii. Continue to assess the applicability of our degree-day model and TSWV risk index, which were developed and validated for processing tomatoes, to predict thrips populations and TSWV in peppers

In the Central Valley, the population dynamics of thrips in pepper crops in the spring, both in terms of absolute numbers and association with TSWV, was determined for a second year. In 2014, thrips populations began to build-up in early April and reached very high populations (~1,500 thrips/yellow sticky card/two weeks) in mid- to late-May. As in 2013, the appearance of TSWV in pepper was correlated with thrips build-up. In 2013, thrips population built-up was later in pepper fields and TSWV appeared later. This suggested that TSWV outbreaks in pepper could be correlated with the initial build-up of thrips and that this maybe the critical time to implement control. However, it is important to get additional years data to prove this.

Fluctuations observed in thrips populations during the 2014 growing season indicated that thrips management with insecticides in monitored peppers were effective, but the effects only lasted a short period of time. This was because, after a drop in thrips populations, a rapid increase in thrips populations usually occurred in monitored fields. Therefore, we believe that if more precise timing of the thrips management in peppers were implemented based on thrips monitoring or our degree-day model, efficiency of the insecticide sprays could be more effective and delay appearance of TSWV.

In 2014, our degree-day model predicted generations of adult thrips development in peppers with 80% accuracy. Interestingly, both years, the degree-day model targeted the 2nd adult thrips generations in early-planted peppers, and the 3rd generation in late-planted pepper fields for thrips control. Thus, it is important to continue monitoring peppers over multiple seasons to confirm whether these results are typical.

Moreover, as we learn more about factors that play important roles in thrips and TSWV development in pepper fields in the Central Valley, we will continue to develop a TSWV risk index (TRI) to predict TSWV threat in individual pepper fields, which showed very good promise in 2014. Although, many of the factors used in the current TRI for tomatoes are relevant for peppers, it is clear that other factors may be involved. Thus, more pepper surveys need to be conducted in order to assess the applicability of the current TRI for TSWV prediction in peppers. This will allow us to modify or change the factors and/or their proposed point values so that the TRI can be developed as a tool for helping growers determine if TSWV poses a risk in a given pepper field. Thus, we propose to repeat these surveys in 2015 and compare the data that will be obtained in 2015 with the 2013 and 2014 data to better understand the factors involved in thrips population dynamics and TSWV outbreaks. We will also continue to test the reliability and accuracy of the degree-day model and the TRI for pepper crops. This knowledge will help target thrips populations early in the season, which will reduce TSWV development.

Key Findings: The thrips degree-day model accurately predicts appearance of thrips in pepper crops and has the potential to be a tool to time thrips control and reduce spread of TSWV. The pepper TRI can be used to predict TSWV threats in individual fields.

iii. Continue to evaluate and refine the IPM strategy for thrips and TSWV for the Central Valley peppers

By using the information generated in this project, we have developed the framework of an IPM program and were continued to refine the program for TSWV and thrips in peppers in the Central Valley of California (current IPM Program will be presented in our annual report). Thus, we propose to continue to refine the IPM program for TSWV in peppers, which we believe will allow for a reduction in the incidence and severity of TSWV, particularly if followed regionally.

Key finding: An IPM program for thrips and TSWV in pepper has been developed.

Work Plan

Similar to 2013 and 2014, we propose to survey 3-5 early- and late-planted pepper fields in the Central Valley with yellow sticky cards to determine thrips population dynamics and conduct regular inspections to visually monitor for TSWV and other viruses. We will also continue our efforts to identify inoculum sources for TSWV in pepper fields, including assessing the newly identified TSWV weed host, rough-seeded buttercup (Ranunculus muricatus), which is associated with walnut orchards. In 2014, winter and spring weed surveys revealed very low levels of TSWV infection (~2%). Rough-seeded buttercup was again widespread in walnut orchards and some plants were infected with TSWV.

We will continue to use our thrips degree-day model to predict when thrips populations begin to increase in pepper fields, and correlate this with yellow sticky card results and the appearance of TSWV. This showed promise in 2013 and 2104 as a tool to predict when to begin thrips management in pepper, which targeted the 2nd and 3rd adult thrips generations in early- and late- planted pepper fields, respectively. We will continue to develop the TSWV risk index (TRI) for peppers, which showed good promise in 2014. We will apply all the knowledge gathered through this project to develop the IPM strategy to control thrips and TSWV in pepper fields in the Central Valley. Finally, if necessary, we will also assess and test for incidence and importance of other viruses in peppers.


Tomato spotted wilt disease has the potential to cause serious losses in pepper production in California. In California, TSWV can be a very serious problem in pepper and the incidence of the disease has been increasing. It is now a serious disease of processing tomato and pepper in many areas of the Central Valley. The virus is spread by at least 10 different species of thrips, but the Western flower thrips (Frankliniella occidentalis) is most important in California. The symptoms of the disease can vary, but young leaves develop extensive necrotic spots and streaks; eventually, young shoots may dieback and entire parts of the plant may collapse. One of the most diagnostic symptoms is chlorotic or yellow ringspots in fruit.

Tomato spotted wilt virus and the thrips vectors have a very wide host range. Tomato spotted wilt has been a difficult disease to predict and manage. One of the main reasons is that both the virus and the thrips vectors have very broad host ranges. Economic hosts include tomatoes, peppers, celery, legumes, lettuce, radicchio and many ornamentals; whereas weed hosts include buttercup, sowthistle, prickly lettuce, nightshade, tree tobacco and jimson weed.

Understanding the biology of the thrips vector in critical to understanding the dynamics of tomato spotted wilt disease. TSWV is not seed-transmitted nor does it persist in debris; thus, the main means by which the virus spreads into and within pepper fields is by the thrips vector. The virus is acquired by larvae, but it is mainly spread by adults that must acquire the virus as larvae. Thus, adults can not acquire the virus, they can only transmit it.

Therefore, it is important to know when and from where adult thrips, carrying TSWV, enter pepper fields. As thrips can survive and reproduce on a diversity of plants, any of these can serve as potential sources of thrips/TSWV for peppers. Understanding the population dynamics of thrips in an area where TSWV occurs is the first step towards identifying the likely source of the vector.


Salary & Benefits $9,500 (10% Project Scientist salary $7,000 and benefits $2,500)

Supplies $3,000

Travel $2,500

Total $15,000

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