Project title: Assessment of thrips populations and Tomato

Project title: Assessment of thrips populations and Tomato spotted wilt virus (TSWV) incidence and development of a TSWV risk index and IPM strategy for processing tomatoes in the Central Valley of California (2011)

Principal investigator: Robert L. Gilbertson

Department of Plant Pathology, University of California, Davis

Cooperating personnel: Ozgur Batuman, Postdoctoral Researcher, UC Davis

Li Fang Chen, Postdoctoral Researcher, UC Davis

Michelle LeStrange, Farm Adviser, Kings County

Tom Turini, Farm Adviser, Fresno County

Scott Stoddard, Farm Adviser, Merced County

Gene Miyao, Farm Adviser, Yolo County

Neil McRoberts, Department of Plant Pathology, UC Davis Diane E. Ullman, Department of Entomology, UC Davis Summary

The goal of this ongoing project is improved understanding of thrips population dynamics and Tomato spotted wilt virus (TSWV) incidence in processing tomatoes in Central California and application of this knowledge to the development of an IPM strategy for TSWV. Population densities of western flower thrips (WFT; Frankliniella occidentalis) and TSWV incidence were monitored in processing tomato fields in the Central Valley of California for a fifth year in 2011, which turned out to be abnormally cool early in the season. Thrips were monitored with yellow sticky cards and in tomato flowers, whereas as TSWV incidence was assessed visually and infection was confirmed by testing selected plants. In 2011, monitoring of representative tomato fields in, Kings, Merced, Yolo and Colusa Counties revealed a delay in the build-up of thrips populations (started in May) compared with years in which temperatures were normal or above normal early in the season (March-April). However, by June 2011, populations reached high levels, which persisted into September. Populations gradually declined in October. In late-planted fields, thrips populations continued to increase through October and then slowly decreased in November. This situation was slightly different in Fresno County, as thrips populations built-up earlier (mid April), and then populations fluctuated during the season. In 2011, the first detection of TSWV in tomato plants was in Kings County in late April, but this was in a relatively early transplanted fresh market tomato field and the virus likely was introduced with the transplants. TSWV was first detected in a fresh market tomato field in Merced about two weeks later (early May). However, in Fresno, Yolo and Colusa counties, TSWV was not detected until mid-May, which also represented a delay relative to other years. Although, TSWV was eventually detected in

most monitored fields, overall incidence was relatively very low in 2011 (i.e., 0-10%, 0-2%, 0-7% and 1-4% in monitored fields in Merced, Yolo & Colusa, Fresno and Kings Counties, respectively). In small number of monitored fields in Colusa and Merced Counties, incidences reached up to 10-20%. However, these levels of TSWV did not cause yield losses in the monitored fields. Inoculum sources for thrips/TSWV vary depending on the county and agricultural production area. Thus, there is no single main source of the virus throughout the state. As in the last 2-3 years, fall crops (lettuce and radicchio) were monitored during fall/winter seasons. In general, these potential TSWV bridge crops had low thrips populations and TSWV incidence, although some fall-planted lettuce fields in Fresno had high incidences of TSWV infection. As in previous seasons, winter and spring weed surveys revealed very low levels of TSWV infection (<0.1%). In Merced, the implementation of effective thrips and TSWV management practices in radicchio has helped to reduce the importance of this thrips and TSWV inoculum source, resulting in low TSWV incidences in monitored tomato fields through out the season. In Yolo County, thrips and TSWV were not detected in winter-planted fava beans or daikon radish, or in most of the weeds sampled. Furthermore, results of inoculation experiments in our laboratories to assess whether daikon radish, a proposed cover crop, can be a host for TSWV were negative. RT-PCR testing of thrips revealed that most thrips were not carrying the virus in early in the season, however, many of the thrips samples collected from tomato flowers after mid-June/early July were positive for the TSWV. This is consistent with the notion that TSWV builds-up in tomato fields later in the season. Thrips-transmission efficiency experiments revealed that male adult thrips transmit TSWV more efficiently than female adult thrips. Remarkably, the overall transmission efficiencies of colonies from Fresno and Yolo were different, with thrips from the Fresno colony having a higher transmission rate (35.6%) than thrips from the Yolo colony (19.4%). Results of our greenhouse experiments for the assessment of the role of the soil-emerging adult thrips as an inoculum source for early season tomatoes revealed very low levels of thrips emergence from soil samples and no TSWV in these thrips. In insecticide trials in 2011, materials that reduced thrips numbers included Radiant either with or without Pre-Am, Athena with Beleaf and Venom. Results of the 2011 trial generally support results of earlier trials in that Radiant is among the top performing materials, and that Beleaf either alone or tank-mixed with a pyrethroid can provide some control. In 2011, we developed a TSWV Risk Index (TRI), for predicting potential losses due to TSWV in Central Valley processing tomatoes. Based on gathered information for each monitored field, the TRI for all monitored fields in 2011 was moderate. We also developed a phenology model, based on degree day accumulation, for predicting thrips

population dynamics in the Central Valley. This model was able to predict the timing of adult thrips generations with overall accuracy of up to 80%. Therefore, integrating the thrips predictive model with TRI may enhance the capacity to predict and manage thrips and TSWV without extensive sampling. In conclusion, research efforts have revealed a good understanding of the biology of thrips and TSWV in Central California and IPM strategy based on these findings appears to be helping reduce TSWV incidences. We now hope to be able to validate the model and risk index to improve the IPM strategy and make it sustainable and grower friendly. Objectives

The objectives of this project were 1) to determine thrips populations and TSWV incidences associated with processing tomato fields in 2011, 2) to gain insight into potential sources of thrips and TSWV for tomatoes in the Central Valley, 3) to assess the role of soil emerging thrips in TSWV epidemiology, 4) to evaluate transmission efficiency of TSWV for thrips populations from different geographic origins, 5) to assess various thrips control methods, 6) to develop a phenology model and a risk assessment system for thrips and tomato spotted wilt disease, respectively, in processing tomato fields and 7) to develop and validate an integrated pest management (IPM) strategy for TSWV in the Central Valley.

Materials and Methods Thrips monitoring in representative fields. Table 1 lists the fields (25) that were monitored for thrips and TSWV in 2011. In 2011, an additional 16 tomato fields (referred as secondary fields) were regularly surveyed for TSWV incidence but without monitoring thrips populations. Furthermore, 16 fields planted with other crops (4 wheat, 4 onion, 3 radicchio and 5 lettuce fields) in the same areas were monitored for thrips and/or TSWV. Yellow sticky cards were placed in each of the four corners of each field, just above the canopy. For the tomato fields, cards were changed weekly or biweekly beginning in March and up to harvest (August-October). Population densities were estimated by counting thrips on yellow sticky cards in the laboratory with a dissecting microscope at 40x magnification. Population densities of thrips were also estimated weekly or biweekly by randomly collecting flowers, placing these into vials with 70% ethanol and returning vials to the laboratory for counting thrips. Flower samples were collected from the same sites where yellow sticky cards were placed (four sites per field and 10 flowers per site). Total numbers of thrips adults and larvae were counted and identified to species.

TSWV incidence and detection. Percent TSWV incidence in tomato, lettuce and radicchio fields was determined by visually examining plants at the four locations in each field. At each location, all plants in 10 yards (meters) of each of 5 randomly selected rows (each separated by 5 rows) were examined. An overall incidence of tomato spotted wilt at each site of the field (four per field) was calculated (presented as number of infected plants per 100 row feet and % incidence). Disease incidence was assessed weekly and selected plants tested with ImmunoStrips (AgDia) and RT-PCR by using N gene-specific primers to confirm TSWV infection. Assessment of the potential role of the soil-emerging thrips as sources of early TSWV infection for processing tomatoes. We collected samples early in 2011 (January-March) by taking the upper 4 inches of the soil from selected locations where high incidences of TSWV were observed in 2010. Soil was collected (4 samples/location) from a total of 16 sites, 4 each from Fresno, Kings, Merced and Yolo/Colusa counties (Table 3). In addition, a control soil sample (sterile greenhouse soil) was also included. Fig. 1 depicts the procedure used in these experiments. Briefly, these soil samples were placed in containers (100 liter) with insect-proof mesh covers. These containers were maintained in greenhouses at UC Davis for 3-4 months to follow thrips activities. Thrips that were emerging from these soils were detected by yellow sticky cards that were placed inside the containers. Then, the emerging thrips that were captured from these soils were tested for TSWV with the RT-PCR test. Additionally, fava bean indicator plants were placed in these cages with these soil samples as another way to detect emerging viruliferous thrips. Isolate collection and genetic diversity of TSWV. In 2011, selected crop and weed plants with TSWV symptoms, or in a few cases without obvious symptoms, were confirmed to be infected with TSWV by immunostrips or RT-PCR and then used to further investigate the nature of the TSWV isolate. To assess the genetic diversity of these TSWV isolates from the Central Valley, the fragment of RNA encoding the N gene was amplified by RT-PCR and the sequence of the N gene determined and compared with sequences of isolates previously collected from tomato and other crops. Comparison of thrips transmission efficiency for the TSWV-California isolates between two thrips colonies from Fresno and Yolo. In 2009, we found that thrips populations were relatively high in Yolo County processing tomato fields, but that TSWV incidence was similar or lower than in other monitored fields in other counties

where thrips populations were substantially lower. These results suggested that thrips populations in different areas may have different virus transmission efficiency. To test this idea and develop a system to study thrips biology more thoroughly, we established a thrips rearing system in our laboratory and established colonies from Fresno and Yolo Counties. These two populations of western flower thrips were collected from tomato plants in Fresno and Yolo counties, and maintained on fresh green bean pods in the laboratory. Two samples of TSWV-infected tomatoes from Fresno and Yolo counties were also collected and the virus from these samples was maintained on Datura stramonium (Jimsonweed) plants via sap-inoculation or thrips transmission. For these experiments, first instar larvae were allowed to acquire TSWV from infected Datura plants for at least 3 days and then reared to adulthood on green beans. Transmission assays were performed with 1-2 male or female adult thrips in 15 ml conical polystyrene tubes with a 1-cm-square wet filter paper and single Datura cotyledon leaf for a 48-hr inoculation access period. Leaves were incubated on the surface of water in multiwell plates for 4 days and TSWV titer was determined with the enzyme-linked immunosorbent assay (ELISA). Comparison of insecticides for control of thrips on tomato. The 2011 evaluation of insecticides for thrips control was conducted at the UC West Side Research and Extension Center. The processing tomato variety H8004 was transplanted and irrigated with buried drip (depth of 10 in.). The selection of materials tested was based on results of previous thrips insecticide trials, and communication with Pest Control Advisors and chemical company representatives (Table 5.). The experimental design was a four replication randomized complete block. Materials were applied with a CO2-pressurized backpack sprayer at 30 psi with equivalent of 25 gallons of water per acre with surfactant (Induce 0.25%). 25 flower samples per plot were collected and kept in 70% EtOH, then thrips nymph and adults were counted with a dissecting scope. Development a phenology model and a risk assessment system for thrips and tomato spotted wilt disease in processing tomato fields in Central Valley of California. To obtain a risk index (RI) value for a particular field, point values were assigned to critical components of tomato production practices according to their relative influence on spotted wilt incidence (Table 8). For establishing the TSWV risk index (TRI) we have emphasized factors that have been found play important role on disease development in processing tomatoes, based on our research conducted thus far. Examples of these factors are shown in Table 8.

The phenology model is based on a “degree day’ model that utilizes the effects of weather (i.e., temperature) on thrips biology. Currently, this model generates the degree-day accumulations for each year from January 1 to October 31 in a map format (heat availability). We are comparing results of this model with actual population data collected from monitored fields in Fresno, Kings, Merced, Yolo and Colusa counties in 2009-2011. To do this, weather stations were selected from these Counties, and weather parameters were used to map climate conditions for each county. We then used this data and the phenology model to predict appearance of thrips generations in a particular area and year. The predicted data were compared with actual thrips population dynamics that were documented during our surveys in 2009-2011.

ViruliferousThrips

Virus-freeThrips

Are they carrying

the virus?

Fig. 1. Flow chart depicts our experimental procedures for assessment of the potential role of the soil-emerging thrips as sources of

early TSWV infection for processing tomatoes.

RESULTS Field Monitoring

In 2011, all of the monitored fields were established with transplants as there were few direct-seeded fields. Because of delays in tomato planting due to cool weather conditions early in the season, field monitoring for thrips and TSWV in tomato fields was initiated in mid-April in Fresno and early May in the other Counties. Table 1 lists the 25 fields that were monitored in 2011 and includes their locations, sampling dates, harvest dates and TSWV incidence. Fresno County

Early in the 2011growing season, the average thrips populations in Fresno County were considerably higher than those detected in 2007-2010. However, subsequent thrips populations fluctuated considerably, with mid-season (June/July) populations relatively high, but lower than those detected in 2008. Towards the end of the season (August/September) additional population peaks (presumably new generations) were detected, and then populations decreased dramatically in early September (Fig. 2). These late season peaks also were observed in 2010 another year where early season temperatures were lower than normal (Fig. 2). Thrips populations continued to be detected into October in these locations, but then began to decline (Fig. 2). However, note that this allows for overlap with fall-planted lettuce fields.

Kings County

In 2011, there was a striking delay in the build-up of thrips populations in Kings County, with early season populations considerably lower than those detected in 2007-2010 (Fig. 3). However, populations quickly increased starting around 12 May (a month later than in 2007-2010), and then remained high in August and early September. Furthermore, these populations were much higher than those detected in 2007-2010. Populations remained high (2,300-5,000/card) through September and into October before eventually dropping in November. This was similar to the situation in 2010, and was probably due to the delay of growing season in these years due to cool spring weather (Fig. 3).

Merced County

In 2011, the build-up of thrips populations was also delayed due cool weather, and was not detected until mid-May in Merced County (Fig. 4). A similar delay was

observed in 2010, another year in which early season temperatures were below normal. This contrasted with 2008 and 2009, years in which the early season temperatures were typical, and populations increased in late March/early April (Fig. 4). Thus, depending on season weather conditions there was a month difference in the timing of thrips population build-up. Interestingly, in 2011, thrips populations increased steadily from mid-May through June, eventually reaching populations that were considerably higher than those detected in 2008-2010. The peak populations in 2011 were detected around 21 July, similar to 2010, but were two-fold higher (2,000 vs. 4,000 thrips/card). Furthermore, peak thrips populations in 2010 and 2011 were detected two months later than those in 2008 and 2009, again revealing the influence of the cool temperatures early in the season (Fig. 4). In most locations, average thrips populations remained at high levels (1,000-4,000/card) through September (Fig. 4). In all years, thrips populations remained high until early October and then quickly dropped through November.

Yolo and Colusa Counties

In 2011, the thrips dynamics in Yolo and Colusa Counties were similar to those detected in 2009 and 2010 (Fig. 5). However, in 2009, the average thrips populations were higher than those detected in 2010 and 2011, especially in August-September (Fig. 5). In 2011, similar to 2010, populations in early-planted fields started to increase in late May, which was three weeks later than those in 2009 (Fig. 5). Again, this can be attributed to the cool early season temperatures in 2010 and 2011. Peak populations were detected in mid- to late August. In 2010 and 2011, peak populations were similar (4,000/card), whereas these were 2.5-fold lower than the peak detected in 2009 (Fig. 5). In 2011, average thrips populations in most fields remained at high levels (2,000-4,000/card) through September (Fig. 5). Thrips populations began decreasing in late September and by mid-October had decreased considerably.

Flower sampling, another method to assess thrips populations, was initiated at the bloom stage of development. Overall, thrips populations in flowers were detected soon after flower emergence, and thrips continued to be detected in flowers for the rest of the season. In 2011, average thrips populations in flowers in July was consistently lower than those detected in May, June or August in all Counties (Fig. 6a). In 2011, the average thrips population in flowers was higher in early season and then dropped in Merced; however, in the other Counties, average thrips populations were highest in August. In 2011, similar to other years, most of the monitored fields had populations of 2-5 thrips per flower throughout the blooming stage (Fig. 6a). Regardless of field location, average

thrips populations in flowers were generally high in early season (consistent with extensive blooming in developmental stage of the tomato plants) and low during fruiting stage in July as flower numbers decreased (Fig. 6a, b, c and d). Overall, it appears that the populations determined by yellow sticky cards may be more informative than populations from flowers.

In 2011, thrips larvae were commonly found in flowers, similar to 2007-2010.

This indicates that thrips are reproducing on tomatoes, and that there is the potential for secondary spread of TSWV within fields by viruliferous adults coming from juveniles that fed on TSWV-infected plants within a field. Evidence for this came from the observation of a significant increase of late season TSWV infections in which only one or a few shoots on a plant developed spotted wilt symptoms in 2010 and 2011. This was also observed in 2008 and 2009 but to a lesser extent.

All thrips captured in the monitored fields in 2011 were identified as western flower thrips and, consistent with previous results, female thrips populations were about three-fold higher than male populations.

Table 1. List of monitored fields and their: locations, survey and harvest dates and TSWV incidence in 2011. Yolo/Colusa Counties 5.5.11 5.19.11 6.2.11 6.16.11 6.30.11 7.14.11 7.28.11 8.15.11 8.25.11 9.8.11 9.22.11 10.6.11 TSWV %

RO (Winters) Harvesting Harvested 0

AO (County Line) Harvested 0

BF (County Line) Harvested 0DB (Rd 14/ Hwy 113) Harvested 0

EG (Sutter Co.) Harvested 20

TP (Wilsonbend Rd) Harvested 1.3

MS (Lone Star Rd & Meyer Rd) Harvested 1

Merced County 5.12.11 5.19.11 6.9.11 6.23.11 7.7.11 7.21.11 8.4.11 8.18.11 9.1.11 9.15.11 9.29.11

GC (Gun Club Rd- Gustine) Harvested 7.7

HT (Hunt Rd-Gustine) Harvested 16

BC (Bert Crane Rd) Harvested 0

FR (Franklin Rd) Harvested 0BU (Burchel Rd) -Fresh Market To Harvested 0

PT (Rogers Rd-Paterson) Harvested 9.5

Fresno County 4/19/2011 5/6/19/11 6/3/8/11 6/19/2311 7/7/15/11 7/2/28/11 8/3/18/11 8/31/2011 9/23/2011 10/13/11North -Firebaugh area Harvested 6.5Oakland -Five Points area Harvested 1.2Shields -Firebaugh area (Fresh Market To) Harvested 1Mt. Whitney -Five Points area Harvested 0Tranquility -Tranquility area Harvested 0.5Nees -Firebaugh area Harvested 0

Kings County 5/12/2011 5/20/2011 6/3/2011 6/16/2011 7/1/2011 7/15/2011 7/29/2011 7/30/2-11 8/6/19/11 9/2/2011 9/16/2011Tomato #1 Gale & Jameson Harvested 1.5Tomato #2 Lassen & Phelps Harvested 3.5Tomato #3 Dorris & El Dorado Harvested 1

Tomato #4 Lassen & Jayne Harvested 3.8

Tomato#5 Hwy 198 & Avenal Cutoff Harvested 1

Average Thrips Populations per Card Fresno Co. 2007-2011

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Build up Peak Drop Highest in the peak2007 6-Apr 25-May 20112008 9-Apr 29-May 20-Oct 20082009 19-Mar 22-Jul 11-Aug 20102010 3-May 21-Jul 21-Oct 20092011 11-Apr 18-Aug 23-Sep 2007

Fig. 2. Average thrips counts per yellow sticky card in monitored fields in Fresno County in 2007-2011. Note the thrips population

build up, peak and drop dates are indicated below graphs. Highest thrips populations during peaks are ranked (high to low) from top to bottom.

Average Thrips Populations per Card Kings Co. 2009-2011

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Build up Peak Drop Highest in the peak2007 31-Mar 29-Jun August 20112008 3-Apr 1-May October 20082009 14-Apr 11-Aug October 20102010 23-Apr 26-Aug November 20092011 12-May 16-Jun November 2007

Fig. 3. Average thrips counts per yellow sticky card in monitored fields in Kings County in 2007-2011. Note the thrips population


Average Thrips Populations per Card Merced Co. 2008-2011

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Build up Peak Drop Highest in the peak2008 4-Apr 16-May 10-Oct 20112009 6-Apr 15-May 2-Oct 20092010 14-May 23-Jul 5-Oct 20082011 12-May 21-Jul 15-Oct 2010

Fig. 4. Average thrips counts per yellow sticky card in monitored fields in Merced County in 2008-2011. Note the thrips population


Average Thrips Populations per Card Yolo/Colusa Co. 2009-2011

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Fig. 5. Average thrips counts per yellow sticky card in monitored fields in Merced County in 2008-2011. Note the thrips population


Fig. 6. Average thrips populations in flowers in monitored fields (A) in 2011, (B) in Fresno and Kings Counties in 2007-2011, (C) in

Merced County in 2008-2011, and (D) in Yolo and Colusa Counties in 2009-2011.

Average Thrips Populations per Flower Fresno & Kings Counties 2007-2011

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In 2011, the first detection of TSWV in tomato was in Kings County in late April. However, this was in an early season fresh market variety and, based on the severe symptoms in the young plants, the virus was probably brought in with transplants. In Merced, TSWV was also first detected in a fresh market tomato field, about two weeks later (early May) than in Kings County. In Fresno, Yolo and Colusa counties, TSWV was detected in processing tomatoes in mid-May, two weeks later than in Merced. The first detection of TSWV in Fresno in 2011 was similar to 2010, but was much later than in previous years (2007-2009). For example, TSWV was first detected on 24 April in 2007, and in the first week of May in 2008 and 2009. The first detection of TSWV in 2011 in Yolo and Colusa Counties was two months later than in 2009 and 2010.

In 2011, the overall incidence of TSWV in processing tomato fields remained,

with a few exceptions, very low (e.g., 0-10%, 0-2%, 0-7% and 1-4% in monitored fields in Merced, Yolo & Colusa, Fresno and Kings Counties, respectively [Fig. 7]). In Merced, TSWV incidence was very low in monitored fields throughout the season, with the exception of the HT processing tomato field (16%). Similarly, the TSWV incidences in monitored fields in Yolo and Colusa Counties were very low except for a single field in Sutter County that had higher incidences (20% incidence in parts of the field by the end of the season [Fig. 7]). The high TSWV incidences in these two fields in Sutter and Merced Counties were observed at certain areas, typically near the edge of the field, suggesting the presence of a nearby inoculum source. Higher rates of TSWV infection were reported or observed in some fields that were not monitored, including some of our secondary fields. For example, a 15 acre late-planted processing tomato field in Yolo/Colusa had a TSWV incidence as high as 35%. However, in Fresno and Kings Counties, TSWV incidences in few of the monitored and secondary fields reached 4-7%, whereas in the rest of the fields, incidences remained very low (0-2% and Fig. 7).

Although TSWV eventually appeared in all monitored fields in 2011, the overall

incidence was less than 5% in most monitored fields, similar to 2008 and 2009. In 2011, higher levels of infection developed later in the season, similar to in 2010, but even tehse levels were not sufficient to cause economic losses. Thus, the overall incidence of TSWV in monitored fields in 2011 (0-20%), similar to 2010, was slightly higher than in 2007-2009. However, the overall pattern of disease development was similar in all of these years: low TSWV incidence in early-planted fields and higher incidences in late-planted fields.

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Fig. 7. Final percent TSWV incidences in monitored fields in 2011. Detection of other tomato-infecting viruses

In 2011, the incidence of other viruses in processing tomato fields was also determined. These viruses included the new tomato-infecting ilarvirus, Tomato necrotic spot virus (ToNSV), curly top disease (caused by Beet mild curly top virus [BMCTV] and Beet severe curly top virus [BSCTV]), Alfalfa mosaic virus (AMV) and Pelargonium zonate spot virus (PZSV). This activity is very important because it prevents misdiagnosis of TSWV (some of these viruses cause TSWV-like symptoms), and gives an idea of the overall importance these other viruses in processing tomatoes in California. Early in the growing season in 2011, we detected more ToNSV than in 2010. In some tomato fields in Merced, ToNSV was widespread. Furthermore, we have now proven that ToNSV is readily transmitted by infected pollen via thrips feeding activities, and that is likely enter to processing tomato fields via pollen from near-by asymptomatic reservoir plants. However, overall incidence of ToNSV was <1% and it did not appear to spread within the field. In addition, tomato plants seem to recover from infection by this virus. This supports our previous conclusion that the virus does not pose a serious threat to tomato production.

The presence of these viruses complicates disease diagnosis, especially early in

the season when symptoms induced by TSWV and some of these other viruses (especially curly top viruses) can be very similar. In addition, some fungal diseases (e.g., Verticillium and Fusarium wilt and Fusarium foot-rot) were common in some fields, and

symptoms of these diseases sometimes made it difficult to accurately identify TSWV infection in tomatoes.

We have now developed specific primers for all of these other tomato-infecting

viruses, which allows us readily identify plants infected with these viruses with PCR (curly top viruses) or RT-PCR (AMV, PZSV and ToNSV) assays. This is important because in some fields, these other viruses can be more prevalent that TSWV. For example, in some parts of the PT field in Paterson, a transplanted processing tomato field in Merced County, the incidences of AMV and ToNSV were 3%, whereas no TSWV infections were detected. Consistent with the results, the PT field was planted next to an old alfalfa field (a source of AMV). Interestingly, this field also had high populations of thrips (based on yellow sticky cards monitoring and feeding damage on leaves), which persisted throughout the season. We believe these thrips came from alfalfa and were not carrying TSWV.

Finally, mixed infections of more than one virus can also occur in the field and

this can complicate diagnosis. In 2007 and 2008, during the course of testing many plants with virus symptom, we found infections by either TSWV, BMCTV/BSCTV or AMV; however, we never observed mixed infections of these viruses. However, as we have tested additional samples in subsequent years we have found a small number of plants with mixed infections of TSWV and BMCTV/BSCTV, TSWV and AMV, BMCTV/BSCTV and AMV, or TSWV and ToNSV. Together, these results indicate the need for testing for multiple viruses to confirm the identity of the virus(es) in a particular field and to allow for the selection of appropriate management strategies.

Survey of potential hosts for TSWV and thrips

To search for reservoir hosts of TSWV and thrips before, during and after the processing tomato season in 2011, we again surveyed spring- and fall-planted lettuce in Fresno, spring- and fall-planted radicchio in Merced, and numerous weeds collected in the winter and spring.

In 2010-2011, TSWV levels were relatively high in a number of surveyed fall-

planted lettuce fields (2010). Importantly, no TSWV was detected in the spring-planted lettuce (2011). Thus, thrips populations in monitored spring-planted lettuce fields in Fresno County were low (10-200 thrips/card), and TSWV was not detected in these fields. This is very important as it appears that spring-planted lettuce was not an inoculum

source for early season tomato fields. The high incidences in fall-planted lettuce were attributed to high incidences of TSWV in late-planted tomatoes, which were in the field after than usual due to the delayed planting of the 2010 tomato crop because of cool early season temperatures. This resulted in greater overlap between late-planted tomatoes and the fall lettuce crop, allowing viruliferous thrips to move from tomato to lettuce and spread TSWV. Furthermore, this overlap occurred over a long period of time: August to early-October. In 2011, where there was again a delay planting due to cool spring temperatures, thrips populations in some fields were still at very high levels in August and September (>5,000 thrips per card, and Fig. 2), and many late-planted tomato fields had high incidences of TSWV (>20%). While these late infections did not appear to cause yield losses in tomatoes, they appeared to be sufficient to provide inoculum for fall-planted lettuce. Fortunately, in 2010/2011 the TSWV from fall-planted lettuce did not jump into the spring-planted crop, possibly due to very low activity of thrips in winter, thereby not making spring lettuce a potential inoculum source in 2011.

We are currently surveying three fall-planted lettuce fields in Five Points and

Huron locations, to see if high TSWV incidences are associated with the delay planting of tomatoes in 2011. In these fields, thrips populations have been moderately high (500-1,000 thrips/card) in October, and the TSWV incidence was already 5-10%. Some isolated August-planted lettuce blocks have even higher percentages of plants with TSWV symptoms. It will be very important to monitor spring-planted lettuce to see if TSWV develops in these fields and whether these fields may serve as TSWV bridge crops.

In the winter of 2011, radicchio fields in Fresno and Merced, which were initially

under plastic, did not have plants infected with TSWV, and thrips populations were very low (0-5 thrips/card, and data not shown). In spring 2011, TSWV was detected in low incidences in some monitored radicchio fields in Merced, but the incidence was sporadic and did not cause economic losses in this crop. More importantly these fields did not appear to serve as sources of inoculum for tomatoes in Merced County. Furthermore, thrips management efforts combined with effective sanitation of harvested radicchio crops has reduced the role of radicchio as a TSWV bridge crop in Merced County, and this has been reflected by the low incidence of TSWV in tomatoes in 2008-2011 (Fig. 7, and data not shown). Thus, through these efforts the importance of radicchio as a TSWV inoculum source for tomato in Merced has been substantially reduced. In Fresno, a single

radicchio field was surveyed for TSWV in spring 2011. A low incidence (<1%) of TSWV was detected in this field.

In 2011, in areas with recent outbreaks of TSWV, weeds (and plants other than

tomato) were collected and tested for the virus (Table 2). Most samples were negative for TSWV; however, some radicchio, cardone, lettuce, pepper, prickly lettuce, sowthistle, groundsel (pineapple weed), bindweed, Malva, Datura (jimsonweed) and black nightshade plants tested positive for the virus. Furthermore, most of the symptomless weeds, collected before and during 2011 tomato growing season, were negative for TSWV. Only a few symptomatic weeds were found and these were found to be infected with TSWV. In addition, most of the weeds that tested positive were collected during the tomato growing season and came from fields of tomato or other TSWV-susceptible crops in which symptomatic plants were present. For example, the symptomatic Datura plants were collected from fields with TSWV-infected tomatoes. Therefore, it appears that the virus infecting these weeds may have actually come from the infected tomato plants rather than vice versa. Thus, the overall incidence of TSWV infection in weeds was very low (28/363 or <0.1%) and this is similar to results from previous years. To date, we have not found evidence of any weed that is extensively infected by TSWV in the Central Valley of California.

Table 2. Weed survey results for TSWV incidence during 2011.

Weed a Tested (+) Weed a Tested (+) Black nightshade 12 (4) Lambs quarters 10 (0) Bindweed 30 (4) Malva 67 (1) Bur clover 10 (0) Datura -Jimsonweed 3 (3) Pineapple weed 54 (3) Goose foot 19 (0) Sowthistle 48 (11) Shepherd purse 4 (0) Prickly lettuce 49 (2) Fiddler neck 19 (0) Russian thistle 5 (0) Wild radish and Mustard 23 (0) (+) number of plants tested positive for TSWV by immunostrips and RT-PCR. a, Total weed samples

from all counties

Early in 2011, surveys were conducted in Yolo and Colusa Counties (January-

March) in order to try to identify inoculum sources of thrips and TSWV for processing tomato fields. We found a couple of fava bean fields in one location, but surveys revealed

very low thrips populations and no TSWV infections. Daikon radish, a recently proposed a winter cover crop, was assessed as a potential TSWV in 2011. Here, we sap-inoculated plants with TSWV. Despite numerous attempts, daikon radish did not become infected with TSWV, whereas positive control N. benthamiana (a type of tobacco) and Datura plants did become infected (indicating that these method was successful in delivering the virus to the inoculated plants). This indicated daikon radish is not a host. Consistent with these results, daikon radish crops inspected in the field did not show symptoms of TSWV infection and representative plants also tested negative for TSWV. Thrips populations associated with daikon radish in these fields were also very low.

Before the growing season in 2011, wheat and onion fields were monitored for

thrips and collected thrips from these cards were counted and tested for TSWV with the RT-PCR assay. Until late May, thrips population densities on wheat were low (average 100-300 thrips/card) on yellow sticky cards placed in these fields. In onions, thrips population densities were also low (average 100-300 thrips/card) until in early April, and then populations rapidly increased in late April (1,000-3,000 thrips/card). To date, TSWV has not been detected in thrips collected from wheat and onions, which are non-hosts of the virus. Assessment of the potential role of the soil-emerging adult thrips as inoculum sources of TSWV for early planted tomatoes

In February 2011, we collected the upper 4 inches of the soil, with minimal disruption, from locations known to have had late-season TSWV outbreaks. Table 3 indicates when and where these soil samples were taken, and provides summaries of the results of these experiments. In Yolo and Colusa Counties, we collected soil samples from a late-planted processing tomato field that had very high TSWV incidences (20-100% in affected parts of the field) in 2010. Also, this location (sample # 3) is a known TSWV hot-spot. Sample #2 was collected from a relatively weedy area in this hot-spot (olive orchard, near sample #3). Samples #1 and 4 were collected from a fall-planted radicchio crop having TSWV outbreak and a large field of fava bean, respectively. In Merced County, a soil sample was collected from the corner of a late-planted fresh market tomato field with a very high TSWV incidence (~100%). This location (sample #7) is also a TSWV hot-spot. Soil from a neighboring radicchio field also was collected (Sample #6). In Fresno and Kings Counties, soils were collected from late-planted tomato fields with relatively high late-season TSWV incidences. These locations also have had history of TSWV (samples #9, 11 and 13). In these Counties, we also collected soil

samples from fall lettuce fields with very high TSWV incidences in fall 2010 (samples #10 and 15). Other soil samples from Fresno and Kings Counties were collected from fallow fields and weedy orchards (samples #12, 14 and 16), which were previously confirmed to harbor TSWV-infected weeds and were nearby tomato fields had TSWV outbreaks in 2010. Soils were placed in large plastic tubs, covered with a mesh top and kept in a greenhouse. Thrips emergence was monitored with yellow sticky cards and indicator plants used to detect TSWV.

We captured only a few thrips from some of the soil samples (Table 3). Most of

the emerging thrips were captured in first or second week of the experiment. Thrips populations on these yellow sticky cards were low (1-27 thrips/card), and these came mainly from soil samples collected from fall crops rather than soils from tomato fields that had been plowed. None of the thrips from these soils (or control thrips captured on yellow sticky cards that were placed outside of the cages in the greenhouse) tested positive for TSWV by the RT-PCR technique. Interestingly, in most cages many weeds and volunteer crops germinated and started to grow during the experiment. These plants, as well as the fava bean indicators, did not develop symptom of TSWV infection, consistent with the results indicating that the thrips were not carrying TSWV. For example, soil samples from tomato fields from Fresno and Kings Counties had high populations of volunteer tomato seedlings as well as variety of common weeds. The fact that these plants did not develop TSWV symptoms supports the idea that viruliferous thrips had not emerged from this soil. Furthermore, these results indicated that our experimental conditions mimicked the natural conditions that promote emergence of adult thrips. To further test whether these volunteer tomatoes or weeds were infected with the TSWV, leaf samples from these plants as well as the fava beans indicator plants that were placed in each cage were analyzed with RT-PCR. All of these plants were tested negative for TSWV. However, even though no thrips were captured from soil sample #7 from Merced, two months later, one of the indicator fava bean indicator plants in this cage developed some leaf necrosis. This fava bean plant tested negative for TSWV with immunostrip, but was weakly positive with RT-PCR.

Together, these results indicated that thrips can stay dormant in the soil for some

period of time, and that some may well retain TSWV based on the single confirmed case of an infected indicator plant, which was likely infected by viruliferous thrips emerging from soil. However, this must be reviewed as a preliminary result, and these experiments should be repeated to conclude whether soil emerging thrips can be an inoculum source

for tomatoes. Because, thus far, our results indicated that most of the soil samples had no or very low levels of overwintering thrips, it appears that viruliferous adult thrips emerging from soil may not be an important inoculum source for early-planted tomatoes. However, it is important not to exclude the possibility that such viruliferous thrips may be sources of TSWV for susceptible weeds or bridge crops, in which the virus can be amplified.

Table 3. Summary of the assessment of the potential role of the soil-emerging thrips

Sample #

Source of the soil samples

Collection Date

Previous/Current Crop Type

Number of

captured

RT-PCR tests of thrips

RT-PCR tests of plants

Soils Discarded

Yolo & Colusa Counties1 Yolo 1 Rd 14 11-Feb Fall Radicchio 2 Negative Negative 5-May

2 Yolo/Colusa 2 County Line 11-Feb Olive Orchard 1 Negative Negative 5-May

3 Yolo/Colusa 3 County Line 11-Feb Proc. Tomato 3 Negative Negative 5-May4 Yolo 4 Rd 29 11-Feb Fava Beans 17 Negative Negative 5-May

Merced County5 CD Chileds Ave. - Merced 24-Feb Fall Radicchio 27 Negative Negative 9-May

6 LG La Grand Rd. -Merced 24-Feb Fall Radicchio 7 Negative Negative 9-May7 BU Burchell Rd. -Merced 24-Feb Late Fresh Mark. To 0 N/A Positive 9-May8 PL Plainsburg Rd.-Merced 24-Feb Weedy Almond 0 N/A Negative 9-May

Fresno County9 Russel -Fairbaugh 23-Feb Proc. Tomato 0 N/A Negative 9-May

10 Five Point 23-Feb Fall Lettuce 0 N/A Negative 9-May11 North TP -Fairbaugh 23-Feb Proc. Tomato 0 N/A Negative 9-May

12 Five Star -Five Point 23-Feb Fallow field/Vineyard 0 N/A Negative 9-MayKings County

13 Jones 23-Feb Late Fresh Mark. To 0 N/A Negative 9-May14 30th & Oniele 23-Feb Fallow field/Vineyard 0 N/A Negative 9-May15 Westside 23-Feb Fall Lettuce 0 N/A Negative 9-May16 Plymouth 23-Feb Weedy Almond 0 N/A Negative 9-May

17 UC Davis Greenhouse 23-Feb Sterile soil; (-) control 0 N/A Negative 9-May

Genetic diversity of TSWV Isolates from the Central Valley

Tomatoes as well as other crops such as pepper, radicchio and lettuce, and weeds with or without obvious virus-like symptoms were collected and tested for TSWV in 2011 by RT-PCR. From some isolates, the N gene DNA fragment was amplified, cloned and sequenced to determine the genetic diversity of the TSWV in the Central Valley of California. Sequence analysis of TSWV N genes revealed a very closely related group of isolates. No major differences were found, irrespective of the host, location and year of isolation (data not shown). Similar results were obtained with TSWV sequences amplified from thrips. These results indicate that in Central California, the TSWV

population is still fairly homogeneous, and likely represents a geographically isolated population.

Detection of TSWV in thrips and studies of thrips biology

Thrips from flower samples that were collected from each monitored fields were pooled in a way to represent the biweekly population from each field, and tested by RT-PCR throughout the season in 2011. RT-PCR tests performed with thrips in early season revealed that most of the insects were negative for TSWV, whereas lab-reared viruliferous thrips controls and some of the thrips collected from infected tomatoes were positive (data not shown). These results indicate that most of the adult thrips present in processing tomato fields early in the season are not carrying the virus. In contrast, most of the thrips samples that were collected after mid-June and early July, when incidences of TSWV were increasing in fields, tested positive for the TSWV. These results are consistent with our hypothesis that levels of TSWV inoculum early in the season are low, and that most of the thrips migrating to processing tomatoes were not carrying the virus. However, as the season progressed, TSWV was amplified in susceptible crops (e.g., tomato), and then later generations of thrips readily acquire the virus (recall that, larvae, which must acquire the virus for adults to be viruliferous, are common in tomato flowers) and can transmit the virus within or between tomato fields. This is also consistent with the relatively low levels of TSWV in tomatoes in early growing season in 2011, despite high thrips populations in many of these fields (Fig. 13). If more of these thrips were carrying the virus early in the season, it is likely that the incidence of TSWV would have been much higher. Thus, there was no correlation between thrips populations and TSW disease incidence.

Comparison of thrips transmission efficiency for the TSWV-California isolates between two thrips colonies from Fresno and Yolo

In these experiments, laboratory-reared male and female thrips colonies were used to conduct TSWV transmission assays. To assess transmission efficiency of male and female thrips and to compare their transmission efficiency with each other (i.e., gender and geographic origin), progeny thrips reared from Fresno and Yolo colonies were separately tested for the ability to transmit TSWV over five independent experiments. The results are summarized in Table 4. Based on our results, regardless of the geographic origin, male adult thrips transmitted TSWV more efficiently than female adult thrips. Interestingly, the overall transmission efficiencies of these two colonies were different, with thrips from the Fresno colony having a higher transmission rate (35.6%) than thrips

from the Yolo colony (19.4%). This is an exciting finding and indicated that these two western flower thrips colonies, at least under our experimental conditions, had significant differences in their transmission efficiency. This is also consistent with the finding that thrips populations are often higher in tomato fields in Yolo County than Fresno County, but the incidences of TSWV are higher in Fresno County than Yolo. However, because other factors that may affect the efficiency of thrips to transmit TSWV, further experimentation must be done to conclude whether it is the different transmission rates of the thrips that are responsible for different levels of TSWV in the field in these locations.

Table 4. Summary of assessment and comparison studies in transmission efficiency of

male and female thrips collected from Fresno and Yolo Counties. Fresno Yolo

Trial Male Female Male Female I 5/9 3/11 - - II 3/10 4/10 5/10 4/10 III 9/10 7/10 0/10 1/10 IV 1/10 1/11 1/7 2/11 V 3/11 0/9 1/8 0/6

Subtotal 21/50 (42%) 15/51 (29.4%) 7/35 (20%) 7/37 (18.9%) Total 36/101 (35.6%) 14/72 (19.4%)

Insecticide Trial

The 2011 thrips insecticide trial to assess the performance of insecticides against Western flower thrips was conducted at the UC WSREC. In 2011, several materials reduced the levels of thrips when compared with the untreated control (Table 5). These materials included Radiant (either with or without Pre-Am), Athena with Beleaf and Venom. Results of the 2011 trial generally supported results of earlier trials in which Radiant was consistently among the top-performing materials and Beleaf, either alone or tank mixed with a pyrethroid, also has provided a level of control better than the untreated control. However, unlike 2007 and 2008 results, but similar to 2010 results, Venom also reduced thrips population densities in 2011.

In 2011, there were clearly differences among the foliar treatments in terms of

yield and percentage of fruit with TSWV symptoms (Table 6), as well as symptoms in plants (Table 7). While there also were differences in terms of symptom incidence on plants in 2009, we did not see other differences that season. Furthermore, no differences in treatments vs. untreated controls were observed in 2010. We believe that because the

level of TSWV around the field station was lower in 2011, that the incidence and spread of TSWV was from mostly from in-field viruliferous thrips, which could be controlled by application the chemical. In 2010, we believe that high amounts of virus brought in from thrips immigrating from surrounding fields may have obscured the effect of the chemicals, whereas this additional virus pressure was not present in 2009 or 20111. Regardless of the year we have never seen a reduction in disease associated with the drip-applied materials. Together, these results indicate that some materials can reduce thrips populations to some extent and reduce TSWV incidences but that grower can not rely on insecticide application for efficient control.

Table 5. Insecticide efficacy results against Western flower thrips at WSREC in 2011.

Treatment rate fp/acrez Nymphs/25 flowersy

Radiant 6.0 fl oz + Prev-Am 1qt 6.0 cx

Athena 17 fl oz + Beleaf 50SG 2.8 oz 7.0 c Radiant 6.0 fl oz 7.3 c Venom 70SG 0.895 lb 8.0 c HGW86 20.5 fl oz 10.5 bc Athena 17 fl oz 12.8 abc Requiem 2 qts 20.3 a Untreated 17.0 ab

z Treated 6 Aug with Co2-pressurized back pack sprayer at 40 gal/acre. y Collected 25 samples per plot in 70% EtOH and counted nymph and adults under dissecting scope. x Means followed by the same letter are not different (LSD0.05.)

Table 6. Influence of insecticide programs for control of thrips on incidence of fruit expression of Tomato spotted wilt virus and yield and other quality parameters in Fresno Co., 2011.

Treatmentz Yield (tons/ acre)y

Fruit quality (% by weight)x PTABw

Injections into drip irrigation system buried to 10 in red grn rot sun

brn TSWV color solids pH

Platinum75SG 3.7 oz (22 Jun), Venom 6.0 oz (12 Jul) 29.5 55.6 6.3 12.6 5.4 19.6 23.42 5.833 4.56 Platinum75SG 3.7 oz (22 Jun), Venom 6.0 oz (22 Jul) 29.2 61.4 8.1 9.3 3.3 17.9 24.17 5.508 4.45 Untreated 33.9 62.7 7.0 9.1 3.1 18.2 24.17 5.667 4.54 Drip injection, probabilityv NS NS NS NS NS NS NS NS NS Foliar applications Yield

(tons/ acre)

Fruit quality (% by weight) PTAB Trans-plant drnch 17 May

24 Jun 6 Jul 14 Jul 21 Jul red grn rot Sun

brn TSWV color solids pH

HGW Radiant 10.0 fl oz

Dimeth 4EL 1pt.

Radiant 10.0 fl oz

Dimeth 4EL 1pt.

38.0 64.3 7.6 9.5 2.8 15.9 24.11 5.522 4.53

Radiant 10.0 fl oz

Dimeth 4EL 1pt.

Radiant 10.0 fl oz

Dimeth 4EL 1pt.

30.4 59.6 9.0 11.0 4.7 15.7 23.44 5.622 4.55

Radiant 10.0 fl oz

Dimeth 4EL 1pt.

30.2 61.1 7.4 9.5 3.1 18.9 24.44 5.667 4.53

Untreated 25.0 54.6 5.2 11.3 5.1 23.8 23.66 5.862 4.58

LSD p=0.05 4.72 NS NS NS NS 6.193 NS NS NS AB NS NS NS NS NS NS NS NS NS CV (%) 15.4 13.8 49.2 55.5 71.8 33.66 4.20 4.61 1.10

z Experimental area was transplanted on 17 May with cv. H8004 processing tomato plants at UC West Side Research and Extension Center. All drip applied materials were applied over 45 mins and water was run for1 hour after the injections. Foliar applications were made with a backpack sprayer at 30 gpa.

y Twenty to 25 lbs samples were taken from a mechanical harvester, and hand sorted for red, green, sun burn, rot and blossom end rot. Fruit in each category were weighted and percentage by weight was calculated. This sort was performed on the day of the harvest.

x A sample of 50 red fruit from each plot were tested for color solids and pH by Processing Tomato Advisory Board laboratory in Helm, CA. w Yields per acre were calculated based on machine-harvested 100 ft-long plots, which were harvested on 21 Sep, that were positioned in 4

locations within each center bed of three beds receiving the irrigation treatments. v Means within columns above a probability value greater than 0.05 are considered the same as determined by Analysis of Variance. If probability

was greater than 0.10, a NS appears below the means.

Table 7. Influence of insecticide programs for control of thrips on incidence of Tomato spotted wilt virus symptomatic plants, Fresno Co., 2011.

Treatmentz TSWV %

Injections into drip irrigation system buried to 10 in 22 Jun 12 Jul 12 Aug 25 Aug

Platinum75SG 3.7 oz (22 Jun), Venom 6.0 oz (12 Jul) 2.0 14.4 51.2 50.0 Platinum75SG 3.67 oz (22 Jun), Venom 6.0 oz (22 Jul) 1.6 12.8 52.9 41.7 Untreated 2.3 12.1 56.8 43.5 LSD, P=0.05 NS NS NS NS Foliar applications

TSWV % Trans. drench 24 Jun 6 Jul 14 Jul 21 Jul HGW Radiant 10 fl oz Dimeth 4EL 1pt. Radiant 10 fl oz Dimeth 4EL 1pt. 1.5 9.3 40.2 32.4 Radiant 10 fl oz Dimeth4EL 1pt. Radiant 10 fl oz Dimeth 4EL 1pt. 2.2 15.0 48.4 40.3 Radiant 10 fl oz Dimeth 4EL 1pt. 2.0 14.4 54.9 40.6 Untreated 2.1 13.8 71.0 66.8 LSD, P=0.05 NS 5.2 8.5 7.6 AB NS NS NS 0.0334 CV (%) 91.27 39.93 16.04 16.98

z Experimental area was transplanted on 17 May with cv. H8004 processing tomato plants at UC West Side Research and Extension Center. All

drip applied materials were applied over 45 mins and water was run for1 hour after the injections. Foliar applications were made with a

backpack sprayer at 30 gpa. v Means within columns above a probability value greater than 0.05 are considered the same as determined by Analysis of Variance. If probability

was greater than 0.10, a NS appears below the means.

Development a phenology model and a risk assessment system for thrips and tomato spotted wilt disease in processing tomato fields in Central Valley of California

In 2011, we developed the TSWV risk index (TRI) for California processing tomatoes as a tool to help growers calculate the relative level of risk for TSWV development in a given field based upon a variety of factors known to influence disease development. We assigned points (a relative numeric weight) to each factor so that an overall level of risk can be estimated. In the TRI the higher total points a field receives indicates higher levels of risk.

Based on our extensive investigations of TSWV in California, we now know a

number of factors that are important for TSWV development and management. These factors are listed in Table 8. We are initially evaluating the TRI with monitored fields in 2011. We are currently gathering information about each monitored field from growers, and we will calculate the risk index for each field and compare this with the already known TSWV incidence in the field. This will allow us to modify or change the factors and/or their proposed point values. For example, a monitored organic processing tomato field in Yolo County in 2011 was transplanted on recommended planting date and was adjacent to other tomato fields. This field was planted with double rows (about 9000 transplants/acre), and was in close proximity to some weedy areas. Based on this information, the TRI was 110 points, which meant that the risk of losses due to TSWV was moderate (data not shown). Indeed, TSWV incidence in this field was sporadic (<1%), and there was no economic loss due to TSWV. Another example is a late transplanted (after most tomato fields were harvested) fresh market tomato field in Gustine, Merced County in 2011. Based on the available information about this field, we calculated a risk index of 170 for this field. This point value indicated that this field is in a high risk situation. Indeed, based on our initial surveys of this field, the TSWV incidence in some parts of the field was up to 10% which plants were still early development. Thus, we believe that TRI may be a tool that will help growers determine if TSWV poses a risk in a given field.

In 2011, we developed a second predictive tool to assist growers to determine

when thrips populations begin to increase. This phenology model is based on a “degree day’ model that utilizes the understanding of the effects of weather (temperatures) on thrips biology. Currently, this model generates the degree-day accumulations for each year from January 1 to October 31 in a map format (heat availability). We are testing this model with monitored fields in Fresno, Kings, Merced, Yolo and Colusa Counties from

2009-2011. To do this, we selected weather stations from these Counties and used weather parameters to map climate conditions for each county. We then used this data and the phenology model to predict appearance of thrips generations in a particular area and year. The predicted data were then compared with actual thrips populations that were documented during our surveys in 2009-2011. This data is shown in Figs. 9, 10, 11 and 12.

Based on our initial attempts, the model predicted at least five thrips generations

during the tomato growing season (March-August). We reasoned that after emergence of each thrips generation there should be some degree of population increase in the fields. Thus, we looked at the generation times predicted by the model, and compared with actual populations dynamics determined from yellow sticky card data to assess whether there was an increase in thrips populations after each generation and whether predicted and actual values were in agreement. The model predicted the timing of the generations with overall accuracy of 78% and 67% for Merced and the other counties, respectively (Figs. 9, 10, 11 and 12). In some instances, the model was up to 80% accurate in predicting timing of thrips generations. For example, in Fresno County in 2009 and 2010, after 4 of the 5 predicted generations, levels of the average thrips populations per yellow sticky cards increased; thus, accuracy of this prediction was calculated as 80% (Fig. 9). However, in some cases, thrips generations predicted by the model were early or late, compared with actual thrips population counts. In most of these cases, these difference were in small or acceptable windows of time (± 2 days off), but there were cases that the level of accuracy was very low (40%). This is an unacceptable level of accuracy, and could lead to severe consequences if relied on for decision-making in thrips chemical control. It is possible that these discrepancies are due, at least in part, to inaccurate weather station data used and/or the unavailability of weather stations close enough to some of the field locations. Thus, evaluations are now ongoing for some individual fields by selecting weather stations closer to the field to vigorously assess the model’s accuracy. However, these initial results are very promising and suggest predicting thrips build-up in the fields and making thrips management decisions.

Table 8. Tomato spotted wilt virus Risk Index for Processing Tomatoes in the Central Valley of California (2011 index)

Tomato Variety1 Examples Risk Index Pointsa,b,c stunted plt w less fruit, very severe, dead like 50d,e,f Res. size plt w less fruit, severe symptoms 40g,h,i Nor. size plt w many fruits severe symptoms 30j,k,l Nor. plt w many fruits some symptoms 20m,n,o Vigor.Plt w many fruits almost no symptom 10p,q,r with SW5 0Planting Date2

Prior to February 1 First planted fields in any given region 10February 1-29 week or two later than first planted fields 15March 1-15 week earlier than recommended period 10March 16- April 31 Recommended period (Majority of fields) 5May 1-20 week or two later than majority of fields 15May 21- June 5 tree week or more later planted from major 25After June 5 latest planted fields in a given region 35Plant Population3

Less than 1 plant per foot single row (7000 per acre) 252 to 3 plants per foot double row (9000 per acre) 15More than 3 plants per foot double row but more dens (>9000 per acre) 5Planting MethodDirect seeded 10Transplanted 5Proximity to Known Bridge Cropsadjacent radicchio, lettuce, fava, fallow field, pepper or tomato 25less than 1 mile radius distance (if TSWV confirmed add 10 more points) 151-2 mile radius distance 10greater than 2 mile or None 5Proximity to Thrips Sourceadjacent wheat, pea, alfalfa or weedy patches etc. 20less than 1 mile radius distance 151-2 mile radius distance 10None 5At-Plant InsecticideNone 15for other pests (+ thrips) 10specifically for thrips 5Weed situation/Herbicide usew/out herbicide but weedy In-field ONLY weed population 15w/out herbicide but not so weedy 10w/out pre emergence herbicide or NO weed 5Total Points (35-195) Risk of Losses Due to TSWVLess than or equal to 75 LowGreater than 80 or equal to 150 ModerateGreater than 150 High Note that point values in the index are not final and shown here as an example.

PredictedActual Activity

Prediction Results

Translated Acuracy

2-Apr drop 2 days late11-May increase Match4-Jun increase Match29-Jun increase Match18-Jul increase Match

80%

Fresno Average 2009

Prediction Results5th Generation Adults

1st Generation Adults2nd Generation Adults3th Generation Adults4th Generation Adults


Prediction Results

Translated Acuracy

6-Apr increase Match20-May increase Match16-Jun increase Match8-Jul increase Match26-Jul drop 8 days early

80%

Fresno Average 20101st Generation Adults2nd Generation Adults3th Generation Adults4th Generation Adults5th Generation Adults

Prediction Results


Prediction Results

Translated Acuracy

18-Apr increase Match27-May drop 4 days late24-Jun drop 3 days late14-Jul increase Match3-Aug drop week early

40%

67%

Fresno Average 2011

5th Generation AdultsPrediction Results

Combined Prediction Accuracy (2009-2011)


Thrips Populations/Dynamics Overwintering Adults Egg Hatch Adult Thrips Generations

Fresno Average Thrips Populations per Card 2009

1

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18-Dec 6-Feb 28-Mar 17-May 6-Jul 25-Aug0

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6


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0

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4500

3-Dec 22-Jan 13-Mar 2-May 21-Jun 10-Aug 29-Sep 18-Nov0

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1

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3

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7-Jan 26-Feb 17-Apr 6-Jun 26-Jul 14-Sep 3-Nov0

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5

6

Fig 9. The phenology model predictions for thrips generations in Fresno County and their comparison with actual thrips dynamics

recorded in 2009-2011.


Prediction Results

Translated Acuracy

2-Apr increase Match11-May increase Match3-Jun drop 6 days early28-Jun increase Match16-Jul drop 4 days early

60%

4th Generation Adults5th Generation Adults

Prediction Results

Kings Average 20091st Generation Adults2nd Generation Adults3th Generation Adults


Prediction Results

Translated Acuracy

8-Apr increase Match23-May drop 3 days late19-Jun increase Match10-Jul increase Match29-Jul drop 6 days early

60%5th Generation Adults

Prediction Results


Kings Average 2010


Prediction Results

Translated Acuracy

9-Apr increase Match20-May increase Match19-Jun drop 4 days early9-Jul increase Match30-Jul increase Match

80%

67%

Prediction Results


2nd Generation Adults3th Generation Adults4th Generation Adults5th Generation Adults

Kings Average 20111st Generation Adults


Kings Co. Average Thrips Populations per Card 2009

1

2

3

4

5

0

500

1000

1500

2000

2500

3000

3500

18-Dec 6-Feb 28-Mar 17-May 6-Jul 25-Aug 14-Oct0

1

2

3

4

5

6


1

2

3

4

5

0

500

1000

1500

2000

2500

3000

3500

3-Dec 22-Jan 13-Mar 2-May 21-Jun 10-Aug 29-Sep 18-Nov 7-Jan0

1

2

3

4

5

6


1

2

3

4

5

0

1000

2000

3000

4000

5000

6000

18-Nov 7-Jan 26-Feb 17-Apr 6-Jun 26-Jul 14-Sep0

1

2

3

4

5

6

Fig 10. The phenology model predictions for thrips generations in Kings County and their comparison with actual thrips dynamics



Prediction Results

Translated Acuracy

4-Apr increase Match14-May increase Match7-Jun increase Match1-Jul increase Match21-Jul drop 3 days early

80%


Prediction Results

Merced Average 20091st Generation Adults2nd Generation Adults3th Generation Adults


Prediction Results

Translated Acuracy

16-Apr increase Match30-May drop 2 days late24-Jun increase Match15-Jul increase Match5-Aug increase Match


Prediction Results


Merced Average 2010


Prediction Results

Translated Acuracy

14-Apr N/A23-May increase Match21-Jun increase Match10-Jul increase Match31-Jul steady 4 days early

75%

78%

Prediction Results


2nd Generation Adults3th Generation Adults4th Generation Adults5th Generation Adults

Merced Average 20111st Generation Adults


Merced Co. Average Thrips Populations per Card 2009

1

2

3

4

5

0500

1000150020002500

30003500

18-Dec 6-Feb 28-Mar 17-May 6-Jul 25-Aug 14-Oct 3-Dec 22-Jan0

1

2

3

4

5

6


1

2

3

4

5

0

500

1000

1500

2000

2500

3-Dec 22-Jan 13-Mar 2-May 21-Jun 10-Aug 29-Sep 18-Nov0

1

2

3

4

5

6


1

2

3

4

5

0500

10001500200025003000350040004500

18-Nov 7-Jan 26-Feb 17-Apr 6-Jun 26-Jul 14-Sep 3-Nov0

1

2

3

4

5

6

Fig 11. The phenology model predictions for thrips generations in Merced County and their comparison with actual thrips dynamics



Prediction Results

Translated Acuracy

28-Mar increase Match9-May increase Match1-Jun drop 5 days early26-Jun increase Match17-Jul drop 1 day late7-Aug increase Match


Prediction Results

3th Generation Adults4th Generation Adults5th Generation Adults

Yolo&Colusa Average 2009

1st Generation Adults2nd Generation Adults


Prediction Results

Translated Acuracy

27-Mar increase Match12-May increase Match12-Jun increase Match4-Jul drop 2 days late24-Jul drop 6 days early

60%Prediction Results



1st Generation Adults2nd Generation Adults3th Generation Adults


Prediction Results

Translated Acuracy

5-Apr N/A14-May increase Match17-Jun increase Match10-Jul increase Match3-Aug drop 12 days early

75%

67%Combined Prediction Accuracy (2009-2011)

5th Generation AdultsPrediction Results




Yolo&Colusa Co. Average Thrips Populations per Card 2009

1

2

3

4

5

6

0

2000

4000

6000

8000

10000

12000

28-Dec 27-Jan 26-Feb 28-Mar 27-Apr 27-May 26-Jun 26-Jul 25-Aug 24-Sep0

1

2

3

4

5

6

7


1

2

3

4

5

6

0500

100015002000250030003500400045005000

13-Dec 22-Jan 3-Mar 12-Apr 22-May 1-Jul 10-Aug 19-Sep 29-Oct 8-Dec0

1

2

3

4

5

6

7


1

2

3

4

5

6

0500

10001500200025003000350040004500

7-Jan 6-Feb 8-Mar 7-Apr 7-May 6-Jun 6-Jul 5-Aug 4-Sep 4-Oct0

1

2

3

4

5

6

7

Fig 12. The phenology model predictions for thrips generations in Yolo and Colusa Counties and their comparison with actual thrips

dynamics recorded in 2009-2011.

Integrated pest management for TSWV in Central California Our accumulated research findings on thrips population densities and TSWV

development on processing tomatoes in Central Valley of California are presented in Fig. 13. Based on this understanding, the following IPM approach for managing TSWV in processing tomatoes has been developed. This approach has been presented to growers through many presentations, reports and UCIPM flyer. The flyer is available for interested parties upon request. The IPM program is outlined below and continues to be modified and validated. A) Preplant

i) planting location/time of planting-avoid hot spots known to have had high TSWV the previous year and avoid planting near winter fields of potential bridge crops (e.g., radicchio and lettuce).

ii) resistant cultivars-these are available, but may not be necessary if other practices are followed. Resistant cultivars should be used in hot-spot areas or in late planted fields, especially in near those fields in which TSWV infections have already been identified.

iii) weed management-maintain weed control in and around tomato fields and especially in fallow fields, as weeds are potential TSWV hosts. Indeed, our results here indicated that if weeds are allowed to grow in fallow fields, they can amplify thrips and TSWV and serve as inoculum sources for processing tomatoes.

B) Production i) monitoring for thrips/TSWV-monitoring thrips populations and TSWV incidence can indicate when to apply insecticides for thrips control, thereby reducing TSWV spread. All evidence indicates that thrips management should be initiated early (e.g., April/May) to reduce the development of virus-carrying adult thrips that can spread the virus within and between fields. This may even need to be done before disease symptoms are observed. More accurate timing of such treatments may be come from using the TRI and phenology model.

ii) weed management-maintain effective weed control in and around tomato fields.

C) After harvest i) sanitation-immediately plow under crop residue following harvest. ii) bridge crops-minimize the planting of bridge crops that will maintain thrips/TSWV in the absence of tomato and or provide inoculum by overlapping with

spring-planted tomatoes. If this is not possible (e.g., lettuce in Fresno and radicchio in Merced), thrips management should be practiced in these field (note that our results indicate that thrips populations are low in the winter) and prompt sanitation (plowing under old crops and residues) should be practiced.

Current situation of thrips and TSWV in CA

• Western flower thrips and tospoviruses have emerged as major pests in California crops and are likely to continue to be a problem in crops such as lettuce, pepper, radicchio and tomato.

• It is difficult to predict when and where TSWV outbreaks will occur. • Not all aspects of thrips and TSWV biology fully understood. • No single approach is adequate for management of thrips or TSWV. • Evidence that the IPM approach is effective:

-Losses due to TSWV in monitored fields have been minimal -Growers using some or all of the IPM practices have not experienced significant losses due to TSWV

• Progress in managing thrips and TSWV in radicchio has been associated with substantial reductions in TSWV in Merced

• Economic losses due to TSWV in 2011 were minimal to none, although some late-planted fields did have higher TSWV incidences (~20%)

• A phenology (degree day) model has been developed for predicting when thrips populations will began to increase.

• A risk index for tomato fields has been developed that can help growers predict the chances of developing TSWV in their field.

Development of TSWV in Processing Tomato Fields

Persistence in weeds, reservoir hosts, bridge crops (i.e., radicchio

and lettuce)

Potential for higher incidence/epidemics

and economic losses in late-planted crops. Late

infections may be limited to some shoots.

Infections with TSWV –low incidences, depending on

populations of virus carrying thrips

TSWV overwinters at low levels in weeds,

bridge crops and thrips

Off SeasonLate SeasonEarly-Mid SeasonWinter

Fall: Populations decrease

Summer: Peak populations

Spring: Thrips populations increase

Winter: Thrips overwinter at very low

levels

Western Flower Thrips Population Dynamics in the Central Valley of California

High

Low

High

Low

Target: 2nd and 3th

Adult thrips Generations

December January February March April May June July August September October November

Increased Viruliferous Increased Viruliferous thrips populationsthrips populations

Amplification in susceptible crops (dependent on initial inoculum,

thrips populations)

Fig. 13. Schematic presentation of thrips population dynamics and development of tomato spotted wilt diseases in processing tomatoes

in the Central Valley of California. The recommended thrips management period is highlighted with dashed loop.

Documents

Project title: Assessment of thrips populations and Tomato