Incorporating Natural, Non-toxic Arthropod Resistant Tomato Varieties into Southern Production Systems

Progress report for LS19-305

Project Type: Research and Education
Funds awarded in 2019: $299,963.00
Projected End Date: 03/31/2022
Grant Recipient: Clemson University
Region: Southern
State: South Carolina
Principal Investigator:
Juang-Horng Chong
Clemson University
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Project Information

Abstract:

Arthropod pests critically limit yields and quality of tomatoes in the Southeast. Whiteflies and thrips transmit viruses that can result in 100 percent crop loss. Additionally, whiteflies, thrips and spider mites reduce yield through foliar feeding, and spider mites and thrips cause fruit scarring. The management of these major pests and the viral diseases they vector to tomatoes relies heavily on pesticides. But tomato producers are caught in a classic “pesticide treadmill” — using frequent pesticide applications that increase the risks of resistance development (due to pests’ rapid life cycles) and cause secondary pest outbreaks (due to the pesticides’ non-target effects on natural enemies), which necessitate further pesticide applications. Therefore, the introduction of new, pest-repellent tomato lines and hybrids can break this cycle at its roots by reducing pest abundance (through repellency to pests) and solving the shortcomings of existing TSWV-resistant lines (which are not arthropod resistant and can still suffer feeding damage).

Such new pest-repellent tomato lines and hybrids, which incorporate natural non-toxic insect resistance from wild tomato, are available from Cornell University tomato breeding program. The leaves and stems of the new lines have trichomes that produce acylsugars, which are highly repellent to many pest species (including thrips and whiteflies) and reduce pest feeding, oviposition, and virus prevalence. But new lines require field testing to integrate into current production practices. Therefore, the proposed project will assess a selection of four experimental thrips-and whitefly-resistant tomato lines and hybrids (varying in acylsugar type and level) for resistance to spider mites, and compatibility with natural enemies of these major pests in the tomato production systems of the Southeast.

We will determine in laboratory studies if select experimental acylsugar lines/hybrids known to control whiteflies and thrips also control twospotted spider mites, which have not yet been directly verified. We will also evaluate non-target effects of these lines on selected natural enemies to confirm that acylsugar-mediated insect resistance can be integrated with biological control. We will measure the effects of acylsugars on pest and natural enemy abundance and virus prevalence in commercial fields in GA and SC. We will consult with our collaborating growers and field test the tomatoes lines in their farms, scale and small, and spring and fall production. We have planned for a variety of outreach activities to engage our stakeholders directly.

A summary of project results prepared for seed companies will encourage mass production of seeds, and will be posted on the Cooperative Extension websites of Clemson University and the University of Georgia. In addition, project updates will occur at vegetable grower meetings, and demonstrations are planned to coincide with the annual Vegetable Field Days in GA and SC. Finally, we will organize workshops on natural plant traits for agriculture for improved pest management at the Southeast Vegetable and Fruit Expo and Southeast Regional Fruit and Vegetable Conference. Therefore, our coordinated research and outreach efforts will help break the “pesticide treadmill” currently challenging sustainable production of tomatoes.

Project Objectives:
  • Determine if acylsugar producing tomato lines provide sufficient resistance to twospotted spider mites and select the most spider mite-resistant line.
  • Determine the compatibility of acylsugar producing lines with biological control of thrips and whiteflies by four known biological control agents.
  • Determine the compatibility of acylsugar producing lines with biological control of spider mites by three predatory mites.
  • Examine impacts of acylsugar producing lines on pest and natural enemy abundance, and virus incidence in southern commercial tomato fields.

Cooperators

Click linked name(s) to expand
  • Zachary Snipes (Educator)
  • Dr. David Riley (Researcher)

Research

Materials and methods:

Objective 1: Determine if acylsugar producing tomato lines provide sufficient resistance to twospotted spider mites and select the most spider mite-resistant line.

Acylsugar tomato lines were provided by the tomato breeding program at Cornell University led by Mutschler. The program is producing seeds from acylsugar lines that produce different types and/or levels of acylsugars, and providing the seeds for use by co-PIs and cooperators in lab and field trials of their impact on insects, mites and virus transmission. Since the initial lines with CU071026 background suffered from reduced fruit set, seed set and germination, a new set of lines with greatly improved seed production and germination was created with CU17NBL background. These improved seeds are now being provided to co-PIs. The new acylsugar lines in CU17NBL background were developed by backcross breeding transferring acylsugar QTL from the lines in CU071026 background to CU17NBL using marker assisted selection. Seed production is all done in greenhouses, for maximum control of plant conditions and to protect against possible seed borne pathogens.

Research team at Clemson University, under Chong and Gill, received the seeds of four experiment lines (CU071026, CU17NBL, QTL6/CU17 and QTL6/AS) from co-PI at Cornell University. Team at Clemson University grew the hybrid tomato lines as well as a commercial variety (Amelia) from seeds in a greenhouse at Clemson University’s Pee Dee Research and Education Center (PDREC), Florence, SC, for evaluation of susceptibility of acylsugar tomato lines for twospotted spider mite.

Spider mite adults used in all experiments were collected from the laboratory colony maintained on “Amelia” tomato plants at PDREC. Two-choice tests on detached leaves in the laboratory and all-choice tests on the whole tomato plants in the greenhouse had been conducted. In the two-choice tests, the five tomato lines were paired and exposed to twospotted spider mites. Each tomato line was paired with itself or another line, resulting in 10 comparisons. Ten adult female spider mites were introduced to the plastic platform connecting both leaves. Each pair was replicated eight times. The numbers of spider mite motiles (i.e., adults and nymphs) and eggs on each leaf were recorded after 72 hours. In all-choice test, the acceptance of the five tomato lines to twospotted spider mites was evaluated in the greenhouse. Potted plants from each experimental line were randomly placed together in a block, resulting in a total eight blocks. Spider mites dispersing from infested plants maintained in the same greenhouse were allowed to infest the experimental plants for 12 weeks. After that, three fully expanded leaves (one each from top, middle and bottom thirds) were randomly collected from each plant weekly for three weeks, and the numbers of spider mites (eggs, nymphs and adults) were counted under microscopes.

Research team at Clemson University also conducted studies to evaluate the susceptibility of the five selected tomato lines to sweetpotato whitefly. Whitefly adults used in all experiments were collected from a greenhouse colony maintained on collard plants at PDREC. No-choice tests on detached tomato leaves in the laboratory and no-choice tests on whole tomato plants in the greenhouse were performed. No-choice test on detached tomato leaves was performed in plastic cups. In each cup, a scintillation vial filled with water was glued and a hole was drilled in its lid to insert the petiole of fully expanded leaf (third from the top) from each line. A total of 12 leaves from each line were used in this experiment. Acylsugar droplets were removed from six leaves (half of total number of leaves) with a strong stream of water. Five pairs of whiteflies were introduced into each arena to lay eggs, and after 72 hours the whiteflies (adults, nymphs and eggs) were counted under microscopes. For the no-choice test on full plant, one potted plant of each acylsugar and commercial line was isolated in each cage covered with fine mesh. A total of 40 plants (eight from each line) were used. Similar to the no-choice test on detached leaf, five pairs of whiteflies were introduced to the plant from each line. Three fully expanded leaves (one each from top, middle and bottom thirds) were randomly collected from each plant biweekly for 8 weeks, and the numbers of whiteflies (eggs, nymphs and adults) were counted under the microscope.

Leaf samples of all acylsugar and commercial lines used this these experiments were collected and dried at PDREC, and sent to Cornell University for acylsugar analysis using standard assay developed in the Mutschler lab. The analysis is on-going. The analysis will enable co-PIs to correlated different levels ad types of acylsugars with the observed experimental data on the susceptibility of these selected lines.

Objective 2: Determine the compatibility of acylsugar producing lines with biological control of thrips and whiteflies by four known biological control agents.

Research team at the University of Georgia (UGA), under Schmidt, conducted a greenhouse study to evaluate the efficacy of suppressing whitefly abundance on selected acylsugar tomato lines with the predatory mite, Amblyseius swirskii. (Amblyseius swirskii is also a predator of thrips.) The tomato varieties used in this experiment were three acylsugar lines (CU071026, FA7/AS and QTL6/AS) and two commercial varieties (Amelia and Florida 47). Tomato plants were grown inside the bug dorms (W60 x D60 x H60 cm; mesh size = 150 x 150 | 160 µm aperture). Tomato seedlings at five-leaf stage were infested with whiteflies that were obtained from a clean non-virus colony maintained on the cotton seedlings at Riley’s lab at UGA. Whiteflies were allowed to colonize the plants in a choice test, all varieties in same enclosed area, for a period of three weeks. Each of the six bug dorms contained five plants, one plant of each variety (total of 30 plants). After release of whitefly adults into the dorms, the team sampled whitefly populations weekly by removing one leaflet and counting all eggs, nymphs and adults for three weeks. Each plant received a treatment of one mite sachet, which hung on each plant. After predatory mite introduction, weekly samples of whitefly populations were taken by removing one leaflet per plant variety from each of the cages, for a total of 30 leaves per time period over four weeks. The leaflet samples were transported to the lab for the count of whitefly eggs and nymphs, and count of A. swirskii eggs and adults under stereomicroscopes.

Objective 4: Examine impacts of acylsugar producing lines on pest and natural enemy abundance, and virus incidence in southern commercial tomato fields.

The abundances of whiteflies and natural enemies on selected acylsugar and commercial lines had been evaluated in an organic field at UGA’s Tifton Campus. In addition to the target predatory mite, A. swirskii, the research team at UGA (led by Schmidt and Dutta) also monitored for other endemic natural enemies colonizing the tomato plants. The tomato varieties used for this experiment were two acylsugar lines (CU071026 and FA7/AS) and two commercial lines (SV7631TD, which is susceptible to Tomato Yellow Leaf Curl Virus, and Skyway 687, which is resistant to TYLCV). The seeds were sown in the greenhouse and planted to the fields at four- to five-leaf stage. Whiteflies were allowed to infest the plants naturally and the mite treatments were applied to the plants right after transplant. In this field experiment the research team tried three different mite application types, i.e. dusting the mites on the top of the plants (top), dusting the mites on the base of the plants (basal) and hanging the mite sachets on the plants (sachet), and prepared a treatment without biological control. The team documented the abundances of whiteflies and A. swirskii over time by removing leaflets from plants then assessing the numbers of arthropods colonizing the plants. The presence of TYLCV was quantified using an immunostrip test.

Research results and discussion:

Objective 1: Determine if acylsugar producing tomato lines provide sufficient resistance to twospotted spider mites and select the most spider mite-resistant line.

In two-choice laboratory experiments conducted by research team at Clemson University, the abundance of twospotted spider mite founded on all acylsugar lines was significantly lower compared to the commercial line (Amelia). Among the acylsugar lines, QTL/CU17 harbored fewer spider mites compared to CU07, whereas equal numbers of mites were found on CU17, QTL6/CU17 and QTL6/AS. In all-choice test, more spider mites were found on commercial line than all acylsugar lines throughout the experiment.

In the first no-choice test to assess the susceptibility of the acylsugar lines to sweetpotato whitefly, the per capita numbers of whitefly eggs at 72 hours were not significantly different among the acylsugar and commercial line (Amelia). However, in the repeat of the no-choice test, the per capita numbers of eggs after 72 hours were significantly lower on QTL6/CU17 and CU07 compared to the commercial line. Throughout the duration of experiment (72 hours), the presence of acylsugars (i.e. washed vs. unwashed leaves) did not significantly affect whitefly egg numbers in both laboratory no-choice tests. In the greenhouse no-choice test, the team found that the numbers of eggs were significantly lower on all acylsugar lines compared to the commercial line. An interaction between line × time was significant for the numbers of nymphs, which could be explained by higher number of nymphs on commercial line compared to the acylsugar lines at four weeks after whitefly introduction. The numbers of adults were also significantly higher on the commercial line than all the acylsugar lines at all sampling times.

These studies showed that when assessed in a small isolated space during the first 72 hours of introduction, sweetpotato whiteflies did not exhibit marked preference for commercial lines over acylsugar lines. However, when assessed over a longer duration and provided with full plants, all acylsugar lines appeared to have significantly lower numbers of whitefly eggs, nymphs and adults. All acylsugar lines were highly resistant to spider mites. These results demonstrate that the acylsugar lines, when planted in the field, will likely be able to harbor lower abundance of whiteflies and spider mites.

Objective 2: Determine the compatibility of acylsugar producing lines with biological control of thrips and whiteflies by four known biological control agents.

Consistent with field results generated under Objective 4, the populations of whiteflies were lower in acylsugar lines (lowest in FA7/AS) compared to the commercial lines. The whitefly egg and nymph populations were particularly low in the later sampling dates. Predatory mite recovery was very low during the entire experiment. As a result, the decrease of whitefly population likely could not be attributed to control by the predatory mites, but more likely related to acylsugar lines. This study suggested that A. swirskii established equally poorly on all tomato varieties (similar among the acylsugar lines). This result indicates that while acylsugar lines are good at reducing whitefly numbers, they did not improve the establishment of a known and highly effective whitefly predator, putting in doubt the practicality of combining a highly resistant host plant with biological control.

Objective 4: Examine impacts of acylsugar producing lines on pest and natural enemy abundance, and virus incidence in southern commercial tomato fields.

In the organic field experiment, whitefly abundance was significantly affected by tomato variety, with lower numbers observed on acylsugar lines than on the commercial lines. The UGA team observed low recovery of A. swirskii from all tomato varieties, and few endemic natural enemies. Due to poor establishment of predatory mites on tomato plants, A. swirskii application methods had no significant effect on reducing whiteflies on all tomato lines. Although acylsugar lines significantly reduced whitefly abundance compared to commercial lines, they were not effective in reducing disease incidence.

Participation Summary

Education

Educational approach:

The project team planned to disseminate research findings and recommendations to the vegetable growers and allied industry groups in GA, FL, SC and NC via several routes and outlets. These routes and outlets include county, state and regional grower meetings, existing newsletters and online information delivery tools, and demonstration plots for the annual Vegetable Field Days at university campuses and commercial fields. Data will also be provided to seed companies to encourage the mass production of acylsugar-producing tomato varieties. Additional opportunities for scientific outreach include the annual meetings of the American Phytopathological Society (APS), Entomological Society of America (ESA) and American Society of Horticulture Science (ASHS). Results from this project will also be published in peer-reviewed journals.

While some outreach activities have been conducted virtually, travel and meeting restrictions imposed by universities and organization during COVID-19 pandemic have severely limited the project team’s ability to deliver training opportunities, field demonstrations and on-farm research in 2020. During a project team meeting in February 2021, the team took stock of the project progress and decided to request a one-year no-cost extension of this project to allow for more time to deliver outreach and education activities in 2021 and 2022.

This project also provided training opportunity to graduate students, post-doctoral research associate and technicians on plant breeding, host plant resistance screening, and entomological research.

Educational & Outreach Activities

4 Consultations
2 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

173 Farmers
112 Ag professionals participated
Education/outreach description:

The initiation of this project was announced and introduced to the attendees of the South Carolina Organic and Sustainable Grower Meeting, held virtually 8 December 2020.

The 2020 VBI Field Day (held virtually due to COVID-19 restriction) was held 24 to 26 August 2020. This is an educational event to provide information to seed companies and breeders. Mutschler presented a report on the progress in creating and characterizing new acylsugar lines. Despite being a virtual event, attendance to this field day more than tripled the previous attendance, with significant increase from both national and international seed companies.

The experience of VBI Field Day suggested that the most effective route for this project to disseminate its project results might be through virtual meetings, conferences and field days.

Paper presentations have been provided at the virtual annual meetings of the Entomological Society of America (ESA) and at the Southeastern Branch of ESA. Additional presentations will be given at the annual meetings of the ESA and Acarological Society of America. Efforts have been expanded in proposing education session on resistant tomato varieties at the Southeast Fruits and Vegetable Conference.

 

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.