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.
- 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.
- 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.
- 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.
- 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).
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The survey for virus reservoirs will be conducted by the five 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 will collect samples from five additional commercial fields and five transplant producers. The samples will be 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 will be collected per field per month. The ID of each sample will be recorded. Plant sap will be extracted from each sample and stored at -20C until ELISA test is conducted. For curly top testing, the total DNA from each sample will be extracted and polymerase chain reaction (PCR) performed using curtovirus specific primers to detect the virus. If a new host is identified, the resulting PCR products will be sequenced for additional confirmation. Due to more expensive testing for curly top ten samples will be collected per site per month. The testing is done in Dr. Nischwitz’ lab at Utah State University in Logan, UT.
Dr. Alston’s lab at Utah State University in Logan will conduct the trials to determine reproductive suitability of identified virus reservoirs. Ten plants of each identified species will be placed in insect-proof cages and ten western flower thrips or five beet leafhopper will be placed on each plant. After ten days, the number of larvae and nymphs per plant will be recorded and leaves stained to count eggs laid.
Four participating growers will conduct a trial using floating row cover over tomatoes and peppers. Four 20-ft. sections each with and without floating row cover will be set up. Sticky cards will be placed under the row covers and in uncovered plots. Insect numbers and IDs will be recorded once a month. Extension faculty will help with the ID of insects.
Four participating growers will conduct a trial using reflective mulch in tomato, pepper and cucurbits. Four 20ft sections will be set up with reflective mulch and four ten ft. sections with either black mulch or no mulch depending on farm operations. Sticky cards will be placed in each plot and insect numbers and ID recorded once a month. Extension faculty will help with the ID of insects.
Two on-farm field days will be organized by Extension faculty on participating farms to demonstrate the trials and provide information on virus reservoirs. Participating growers will talk about their experience with weed control, floating row covers and reflective mulch.
Educational & Outreach Activities
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.facebook.com/utahpests/videos/363086517989534/?__so__=channel_tab&__rv__=all_videos_card) that included TSWV and curly top. To date (March 17, 2021) we reached over 1,900 people on facebook and had 293 people watch it on Youtube.
Learned about symptoms of TSWV and curly top in tomatoes
TSWV resistant tomato varieties and other management options
This is just the first year of the project. At this point growers have become aware of the viruses and how they can be spread. 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 last year. Including transplants and weeds, we tested 1,920 samples last year for TSWV. Curly top was detected in tomatoes in three fields. One field had about 1% infection rate and the 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 feed on other nearby plants including tomatoes. The table beets were also infected with curly top.
Due to Covid-19 the second year of the project was very challenging. We were able to collect some tomato transplant samples from both Southern Utah and Northern Utah. None had TSWV or curly top. Despite the pandemic three growers implemented trials. A fourth grower could not farm this past year due to cancer. We’ll have to see if he can have a trial this year. We collected weed samples in Southern Utah and identified Powell’s amaranth (Amaranthus powellii) as a host for curly top in Utah. The tomato plants were inspected for symptoms of curly top and TSWV once per month. 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 but not the silver mulch according to the grower (the grower delivered the symptomatic plants). No TSWV was found in any trial in 2020. Statistics run on thrips numbers in the different treatments in the Southern Utah trial in 2020 showed significant differences. Black plastic mulch had significantly more thrips on sticky cards compared to silver mulch and row cover treatment. In the April sampling the sticky cards in black plastic had double the amount of thrips compared to silver much and row cover. In May 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. Later in the summer due to heat number of thrips drop and no data was collected later in the summer. In addition, plants infected early in the season are more affected by TSWV than plants infected later in the season when plants are bigger. Beet leaf hoppers were found in very low numbers overall. No statistics were done on leaf hopper. Sticky cards from the other trials are still being analyzed. Due to the pandemic and restriction to operations in some labs, the greenhouse studies for insect reproduction on identified weeds could not be conducted last year as planned. It will be conducted this year.