Management strategies for Tomato spotted wilt virus and curtoviruses in Utah

Final report for OW19-343

Project Type: Professional + Producer
Funds awarded in 2019: $31,149.00
Projected End Date: 02/28/2022
Host Institution Award ID: G219-19-W7502
Grant Recipient: Utah State University
Region: Western
State: Utah
Principal Investigator:
Claudia Nischwitz
Utah State University
Co-Investigators:
Dr. Diane Alston
Utah State University
Richard Heflebower
Utah State University Extension - Washington County
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Project Information

Abstract:

Vegetables are a $19 million industry in Utah and production acreage is expanding to accommodate grocery stores that want to offer more local produce. Tomato spotted wilt virus (TSWV), curtoviruses and their vectors are threatening the production of vegetables in Utah and neighboring states. Incidence of curtoviruses have always occurred in tomato and pepper production but has increased in recent years. In addition, the host range for curtoviruses has expanded to pumpkins and gourds with up to 25% losses in individual fields. TSWV incidence and yield losses quadrupled since 2012. Indications are the virus is established in the farmscape and transplant greenhouses and not introduced from out-of-state yearly. Annual losses for both viruses range from $1,300 to $6,600 per acre in fresh market production and $2,000-3,000 per 1,000 transplants for transplant producers. Management of these viruses is difficult. The objectives of this project are (1) to identify virus reservoirs near vegetable fields and in transplant greenhouses, (2) determine suitability of the identified virus reservoirs as reproductive hosts for the insect vectors and (3) evaluate floating row cover and reflective mulch as management options in on-farm trials, (4) develop outreach materials including field days with presentations by participating growers and Extension faculty and fact sheets. The expected outcome is that the control of identified virus reservoirs in the farmscape and in transplant greenhouses will reduce the incidence of TSWV and curtoviruses and minimize the distribution of virus infected transplants across the state. The use of floating row covers and reflective mulch will repel the insects and prevent them from feeding on vegetable crops and transmitting viruses especially early in the season when plants are most susceptible. These management strategies can be used by both organic and conventional producers.

Project Objectives:
  1. Identify TSWV and curtovirus reservoirs in the farmscape (ten fields) and greenhouse environment (five transplant operations) and determine their suitability as reproductive hosts for insect vectors (2019).

Weeds have been identified as reservoirs for many viruses in many states but have not been evaluated in Utah. The survey will show which weeds are hosts to TSWV and curtoviruses. The insect vectors acquire the viruses as larvae. It is therefore necessary to determine the suitability of identified weeds as reproductive hosts for the insects. Good reproductive hosts are more likely a good source for virus dispersal and would be a priority for growers to remove from their fields and greenhouse operations. Controlled greenhouse studies will determine reproduction rates. Transplant producers will be less likely to sell infected plants and growers will see a reduction in TSWV and curly top in their fields.

  1. Evaluate the use of floating row covers as a tool to reduce curly top incidence. Participating growers will conduct field trials using floating row covers for tomatoes and peppers. Insect presence under row cover is monitored with sticky cards (2020).

It is expected that floating row covers will reduce insect vector presence on covered plants and reduce curly top incidence.

  1. Evaluate the use of reflective plastic as a tool to reduce disease incidence. Growers will conduct field trials using reflective mulch for tomatoes, peppers and cucurbits. Insect presence in beds with reflective mulch vs black plastic mulch or no mulch is monitored with sticky cards (2020).

It is expected that reflective mulch will reduce insect vector presence and reduce curly top and TSWV incidence.

  1. Deliver results of objectives 1-3 in on-farm field days (one each in Northern Utah and Southern Utah), fact sheets and presentations at other grower meetings (2020 and 2021).
Timeline:

Year 1: Weed field surveys and weeds/ornamental plants survey in transplant greenhouses will be conducted monthly from April to August 2019. Samples will be analyzed within four weeks of arrival to the lab and growers immediately notified of the results. In year 2, participating producer will conduct field trials on the use of reflective mulch and floating row covers to minimize virus incidences. The reflective mulch and floating row cover trials will be set-up April 2020 in Southern Utah and end of May 2020 in Northern Utah. Sticky cards will be evaluated monthly and insects on the cards identified. Vegetable plants in trials will be inspected for symptomatic plants once a month and symptomatic plants tested for TSWV and curly top viruses. Reproductive studies will be conducted in Year 2 by Dr. Alston’s lab from May to October 2020.  Two on-farm producer meetings will be held in Year 2 (September 2020). Project results will be presented in Years 1 and 2 in January and February 2020 and 2021 at the Utah Farm and USU Urban and Small Farms Conferences. The project evaluations will take place during the field days and the conferences (Timeline tables for each year attached).

Cooperators

Click linked name(s) to expand/collapse or show everyone's info
  • Bryce Frei - Producer
  • Mark Fryer - Producer
  • Lyle Holmgren - Producer
  • Kelby Johnson - Producer
  • Benjamin Scow - Technical Advisor - Producer
  • Benjamin Scow - Technical Advisor - Producer

Research

Materials and methods:

The survey for virus reservoirs was conducted by four participating producers on their farms once a month in 2019 starting in April in southern Utah and June in northern Utah through August. Participating Extension faculty collected samples from three additional commercial fields and three transplant producers. The samples were tested for presence of TSWV and curly top using antibody-based ELISA (enzyme-linked immunosorbent assay) test specific to TSWV and molecular tests for curly top. For TSWV testing, 90 plant samples were collected per field per month. The ID of each sample was recorded. Plant sap was extracted from each sample and stored at -20C until ELISA test was conducted. For curly top testing, the total DNA from each sample was extracted and polymerase chain reaction (PCR) performed using curtovirus specific primers to detect the virus. If a new host was identified, the resulting PCR products were sequenced for additional confirmation. Due to more expensive testing for curly top ten samples were collected per site per month. The testing was done in Dr. Nischwitz’ lab at Utah State University in Logan, UT.

Dr. Alston’s lab at Utah State University in Logan conducted trials to determine reproductive suitability of identified virus reservoirs. Six plants of each identified species were placed in insect-proof cages and ten western flower thrips were placed on each plant. After four weeks, the number of larvae and nymphs per plant was recorded and leaves stained to count eggs laid.

 

One participating grower conducted trials in 2019, 2020 and 2021 using floating row cover over tomatoes. Four 25-ft. sections each with and without floating row cover will be set up. Sticky cards were be placed under the row covers and in uncovered plots. Insect numbers and IDs were recorded once a month. Extension faculty helped with the ID of insects.

Three participating growers conducted trial using reflective mulch in tomatoes. Four 25ft sections were set up with reflective mulch and four 25 ft. sections with either black mulch or no mulch depending on farm operations. Sticky cards were placed in each plot and insect numbers and ID recorded once a month. Extension faculty helped with the ID of insects.

 

In lieu of on-farm field days, two in-person winter webinars in Southern Utah and a virtual webinar in North Utah were organized by Extension faculty on participating farms to present results of the trials and incorporating management recommendations in farm operations.

Research results and discussion:

In all three years of the project, thrips were significantly reduced using silver mulch or floating row covers early in the season (see table Thrips tables).  There were some difference during the years with regard to incidence of TSWV and curly top as well as the presence of leaf hoppers. Later in the summer due to high temperatures, the number of thrips dropped and no data was collected after July. Plants infected early in the season are more affected by TSWV than plants infected later in the season when they are bigger. Beet leaf hoppers were found in very low numbers overall. No statistics were done on leaf hopper.  Below are the results by year.

In 2019, we sampled tomato seedlings from eight transplant producers. We did not detect TSWV or curly top in the transplants. Collecting weeds monthly around two tomato fields and tomato leaves from five fields, we detected TSWV in two weeds but not in tomatoes. Including transplants and weeds, we tested 1,920 samples for TSWV in 2019. Curly top was detected in tomatoes in three fields. One field had about 1% infection rate and the rest less than 1%. No weeds were found positive for curly top. One interesting observation was the higher incidence of curly top near table beets. This may be because the vector of curly top viruses is the beet leafhopper. It may be attracted to the beets and in the process also feeds on other nearby plants including tomatoes. The table beets were also infected with curly top.

Due to Covid-19 the second year (2020), we could only collect a few some tomato transplant samples from both Southern Utah and Northern Utah. None had TSWV or curly top. Despite the pandemic three growers implemented trials. We identified Powell's amaranth (Amaranthus powellii) as a host for curly top in Utah. One tomato trial had no curly top despite beet leafhopper being observed. In a second trial, curly top was seen and confirmed by my lab in tomatoes on bare ground (the grower delivered the symptomatic plants).  According to the grower no tomato plants grown on silver mulch had curly top.  No TSWV was found in any trial in 2020. There were significant differences in thrips numbers in the Southern Utah trial in May and June (p-value 0.0314 and <0.0001, respectively). Black plastic mulch had significantly more thrips on sticky cards compared to silver mulch and row cover treatments. In the May sampling the sticky cards in black plastic had more than double the amount of thrips compared to silver much and row cover. In June, the differences were even more pronounced with black plastic having 2.5 times more thrips compared to silver mulch and 6.5 times more thrips compared to floating row cover.

In 2021, three grower trials were implemented. Data were collected from two trials. The tomatoes in the third trial died prematurely after being infested with russet mites. As was seen in 2020 in the Southern Utah trial, sticky cards in the black plastic treatment had significantly more thrips than silver mulch and floating row cover in May and June (p-value 0.0024 and 0.0005, respectively). After floating row covers were removed on June 11, 2021 high numbers of thrips were recorded from that treatment as well. In July, there was no significant difference between all three treatments. In the northern Utah trial, we found lower numbers of thrips on sticky cards in silver mulch compared to black mulch but even the lower number was still over 1,000 thrips per card compared to over 3,000 thrips on cards in no plastic treatments. No statistics were done of the thrips numbers in Northern Utah because of loss of many sticky cards due to high wind. Thrips get easily blown around by wind and may have ended up on the sticky cards because of wind gusts. With the high wind in the Northern Utah trial location, floating row cover treatments were not feasible. In 2021, no leaf hoppers were detected and no curly top was found in tomatoes or any other host plants in the state. This may have been due to very high temperatures early in the season. Utah had day time high temperatures of close to 100F starting in early June. Thrips reproduction studies were conducted in the greenhouse on Palmer's amaranth as well as dahlias. Dahlias were identified as a host for TSWV. Many small farm growers are looking to diversify their crops and are going into cut flower production putting vegetables like tomatoes next to dahlias and other flowers.  Thrips were able to reproduce on both amaranth as well as dahlias but only in very low numbers. A total of four nymphs were found on dahlias and two nymphs on amaranth after four weeks. Only one live adult was recovered from one of the dahlias. Both plants seem to be poor hosts for reproduction considering that thrips can lay up to 300 eggs in their lifetime. But as long as any reproduction takes place on dahlias and amaranth thrips larvae can acquire the TSWV from infected plants.

There was concern about sunburn of the fruit grown on reflective mulch but no sunburn was observed in any trial over the three year period.

Participation Summary
3 Farmers participating in research

Educational & Outreach Activities

10 Consultations
3 Webinars / talks / presentations

Participation Summary:

90 Farmers
90 Ag professionals participated
Education/outreach description:

I gave a presentation at the USU Urban and Small Farms conference on March 4, 2020 discussing management of TSWV and curly top in tomatoes as well as cut flowers. During the spring and summer 2019, farmers were individually advised on TSWV and curly top in tomatoes.

Due to Covid-19, all meetings went virtual. I gave a presentation in the Utah State University's Vegetable IPM Twilight series on tomato diseases (https://www.youtube.com/watch?v=vuHgAXYxtmQ) that included TSWV and curly top. To date (February 18, 2022)  we reached over 1,900 people on Facebook and had 663 people watched it on Youtube. In January 2022, I gave two presentations in Southern Utah as part of the Vegetable IPM Winter webinar series. The webinars are geared towards small farms. We were not able to hold field days last fall. We could not find dates in Northern and Southern Utah that would have worked for large number of growers. We therefore used the webinars to present the information as well as talk to producers directly. A research manuscript is in preparation.

Learning Outcomes

18 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key changes:
  • Learned about symptoms of TSWV and curly top in tomatoes

  • TSWV resistant tomato varieties and other management options

Project Outcomes

3 Farmers changed or adopted a practice
24 Farmers intend/plan to change their practice(s)
1 New working collaboration
Project outcomes:

Both silver mulch and floating row covers reduced thrips populations on tomatoes early in the season. The use of floating row covers or silver mulch on tomato beds can be very useful to reduce thrips, leaf hopper and other insect pressures. Even though the cost for reflective mulch is about $25 for a 50 ft long piece, the cost for insecticides has significantly increased last year and there is a shortage on some products making silver mulch and floating row covers more economically viable. They are also a good alternative in urban farming where applications of insecticides may not be feasible due to the close proximity of residential houses as well as for certified organic growers or growers who don't want to use chemical pesticides. An additional benefit of the silver mulch is that it suppresses weeds thus reducing labor costs and time for weed management. Floating row covers are effective in areas that are not prone to strong winds that would blow them away. The row cover used in the trial had 85% light transmission and very small mesh size to keep thrips and other insects out.

Due to weather condition we saw very few beet leaf hopper in 2020 and no beet leaf hopper in 2021. Based on observation in 2019, we recommend not to plant tomatoes next to table beets. We saw a higher incidence of curly top in tomatoes that were growing next to table beets.

 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.