Evaluating Sustainable Alternatives of Rootstock Selections in Grafted Tomatoes to Enhance Yield Potential in High Tunnel and Field Production

Final report for GNC15-215

Project Type: Graduate Student
Funds awarded in 2015: $9,900.00
Projected End Date: 03/31/2018
Grant Recipient: University of Minnesota
Region: North Central
State: Minnesota
Graduate Student:
Faculty Advisor:
Dr. Emily Hoover
University of Minnesota
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Project Information

Summary:

Introduction/Summary:

Tomatoes, especially heirloom varieties that often are susceptible to many soil-borne diseases, are a popular and profitable crop for market farmers. After continuous production, disease pressures within soil reduce yield without either chemical treatment or alternative practices.

Grafting heirloom scions to disease resistant root stock has been shown to be an effective method of mitigating the problem with disease load in soils. Grafting to commercial hybrid root stock seed is the current practice. Many open pollinated and hybrid tomato seeds are bred to suppress several of the same diseases controlled by the many commercially available root stock cultivars. Sustainable production practices discourage the purchasing of off-site inputs. The expensive commercially available root stock seed is typically 100x the cost of alternative hybrid and open pollinated seeds.

This study was designed to evaluate a more sustainable production alternative by determining the relative advantage of using “saved seeds” from open-pollinated tomato varieties shown to suppress diseases. The primary objective was to increase yields. It was demonstrated that saved seeds from open pollinated varieties provide an effective root stock alternative for grafting onto heirloom varieties (scions) that cannot be produced without treatment in soil containing high disease loads.

Saving seeds from parent plants exhibiting improved production qualities and inherent natural disease resistance had been a practice traditionally used by farmers to provide site-specific improvements in yield. Although saving seeds has not been practiced by most contemporary farmers, revisiting this practice may offer an additional advantage as a source of disease suppressing root stock when grafting to scion varieties that cannot survive without treatment.

This study was designed to first determine whether the apparent disease suppression qualities of tomato plants found to survive in soils containing disease pressure could be inherited by the subsequent generations within three open-pollinated varieties: Large Red Cherry, Roma, and Rutgers. Even as the disease load in the test soil plots increased each year, saving seeds from viable parent plants resulted in an increasing percentage of surviving plants that produced salable tomatoes. This would indicate that the disease suppression is a genetic trait that can be positively influenced by seed saving.

After three seasons of saving seeds from the plants that demonstrated superior disease resistance that were raised in soil contaminated with Fusarium species, the seeds from resistant offspring were used in the 2017 growing season as root stock for a grafting study. A commercially popular and well-studied root stock, RST-04-105T, was compared to the open-pollinated root stocks, traditional hybrid tomato varieties (Celebrity and Early Girl), and controls. Several tomato cultivars, including many heirloom varieties known to lack inherent disease suppression, were compared in many growing conditions, under different grafting techniques, and with different root stocks to determine whether grafting provides farmers an advantage over non-grafted tomatoes.

The primary objective was to increase yields using production techniques that increase sustainability. This study found that this objective could be met through two primary practices. First, by encouraging farmers to returning the practice of saving seeds from open pollinated varieties as a method of naturally reducing losses from soil-borne diseases in both traditional (non-grafted) production and with tomato production utilizing grafting. Second, encourage farmers to include grafting of tomatoes when crop rotation cannot mitigate disease pressure (typically in high-tunnels) or when highly susceptible heirloom varieties are grown. Although requiring “off-site” purchases, under some circumstances, grafting to commercially available root stock offerings may be the best choice.

The resulting outcome did not fully meet the desired result. The first objective, showing the advantages of saving seeds, had very positive results. Farmers can provide ongoing site-specific advantages in both traditional (non-grafting) and grafting production by saving seeds from open pollinated tomatoes demonstrating superior performance. The second objective also demonstrated that grafting can provide farmers an advantage in increasing yields, especially with heirloom varieties of tomatoes. However, the commercial variety RST-04-105T, outperformed all other root stocks with all scions tested.

Although the root stock alternatives from saved seeds used in this study demonstrated great advantages over non-grafted and self-grafting (same variety as both root stock and scion) controls (63-92% increase in yield compared to controls with the "Large Red Cherry" variety), they failed to meet the effectiveness of the RST-04-105T (58-117%), especially with high tunnel production.

The broader range of diseases controlled by the RST-04-105T compared to others tested may likely explained why the commercially breed root stock consistently produced the greatest yield in each of the scions tested. The greater root mass of the RST-04-105T compared to other root stocks tested may also suggest that other factors, such as greater Mycorrhizal colonization may have been a factor.  Growers need to consider all the potential soil borne diseases and select root stock that can suppress the greatest range of possibilities. We focused on selecting root stock varieties for Fusarium species, the primary disease issue we experience within our soils. It is likely that multiple soil-borne diseases were present and had an influence on yield.

The additional cost (an additional $.73/grafted unit increase) of using the RST-04-105T seed may not justify the production increases that were achieved because production increases did not occur with the RST-04-105T grafted plants until the end of the season (October) when production often exceeds potential sales. The advantage of the RST-04-105T was not apparent until the days shortened, and the air/soil temperature dropped. This was especially apparent when comparing the field production yield to the warmer tunnel production in October. Growers who maintain production beyond the four months of harvest recorded in this study (July-October) may determine that the commercially sold root stock such as RST-04-105T may be the best alternative.

 

Project Objectives:

The objective of this study was to increase yield in tomatoes with more sustainable production techniques. Two primary ideas were evaluated. Potential benefits of saving seeds to reduce disease issues and increase yield, and evaluating the potential of using open pollinated tomato varieties as a source of root stock for grafting.

These objectives were evaluated by trials. No changes in the original experimental design was made. More extensive experimentation was performed on alternative grafting methods (age of plants at time of graft and side compared to tip graft). Additional varieties for both root stock and scions were added.

Research

Materials and methods:

This project was divided into two parts. The first phase was simply raising three varieties of open pollinated tomato seeds (Large Red Cherry, Roma, and Rutgers) in soil that previously had a complete tomato crop failure due to Fusarium species. As expected, the next planting (2014 season) of open pollinated tomatoes had nearly a total loss. Seeds from surviving plants were saved for three growing seasons. The survival rate each generation increased until the seeds saved from 2016, the third year, were used as root stock for the next phase of the project. Newly purchased open pollinated Large Red Cherry seeds (Seed Saver's Exchange) planted in this test plot had a 100% crop loss before September 1st due to Fusarium.

The second phase of this study was to evaluate if these open pollinated seeds, that seem to have a natural resistance to soil-borne diseases, including Fusarium, may replace the expensive commercially sold root stock varieties typically used in tomato grafting. Many farms strive to become more sustainable, reducing purchased inputs is a part of this sustainable production paradigm. The study wanted to compare the open pollinated seeds (Large Red Cherry, Roma, and Rutgers), two inexpensive hybrid seeds (Celebrity and Early Girl), the RST-04-105T (a well researched and popular commercially available root stock offering), and controls to determine if grafting to less expensive seeds may be effective. Both determinate and indeterminate selections were used. The scions included many popular open pollinated, hybrid, and heirloom varieties (32 in total). We selected scion varieties from the most popular tomato varieties in our area.

Standard testing methods were followed under organic production practices on our farm in Foley, MN. This farm is close to USDA Hardiness Zone 3. Transplants we planted out May 23-26, 2017. Production was in loam soil that had not been in crop production for five years. Both open field and high tunnel production was used. Tomatoes were raised in 200' rows, planted 24" O.C. (50 plants per row) with 52" cattle panels used to trellis. Rows were planted E-W with 60" spacing between rows for air flow. Organic mulch was used to suppress weeds and irrigation was added as needed. Randomized planting was used but colored tags helped readily identify the root stock of each scion variety.

The harvest and recording was done once weekly July-October during the 2017 season. First blush of color to fully ripe was harvested on this 7 day harvest schedule (would recommend twice weekly harvest).  Yields were weighed by salable and cull tomatoes each week. The grading standard is somewhat subjective, some minor defects and spots from disease may be seen and still considered salable. The standard was based on what our customers would find acceptable. However, by August, there was a significant difference in the disease exhibited on the plants and tomatoes. Many non-grafted plants died before October 1st.

Research results and discussion:

Comparisons between the yields of salable tomatoes raised either as grafted plants (different varieties as scion and root stocks) or controls, harvested between July-October 2017 under both open field and high-tunnel growing conditions, were evaluated to determine the relative advantage of using saved seed as an alternative root stock. Results show that grafting a separate root stock variety to the scion provided a benefit in all scion varieties tested. Two primary characteristics of the root stock varieties considered should include the ability to suppress disease and the ability to created a large plant and root structure. The RST-04-105T achieved both these objectives best, but the open pollinated variety, "Large Red Cherry" also provided a close alternative that can be raised from saved seed.

Other varieties tested as root stock (Celebrity, Early Girl, Roma, and Rutgers) would not be recommended. The hypothesis that a close match between the mature root stock stem size with the scion did not seem to affect graft integrity. Even when the scion at maturity was about half the diameter of the larger stem size of the root stock, the grafts did not fail. The reverse, small diameter root stock compared to scion diameter, will cause graft failure. Also, "Roma" and "Celebrity", two determinate/semi-determinate type tomatoes used as root stock, did not seem to influence the scion. The roots continued to remain vigorous and there was no noticeable effect on yield until the final harvest date 10/23/2017.

The open pollinated variety, "Large Red Cherry" was the root stock that performed best out of the three open pollinated varieties when "Roma", "Rutgers", and "Large Red Cherry" were compared. The "Large Red Cherry" root stock developed large plants with good disease control. Grafted plants using "Roma", "Rutgers", Early Girl" (hybrid seed), and "Celebrity" (hybrid seed) showed disease resistance and developed few cull tomatoes but the smaller plant size influenced yield.

This research study had hypothesized that an open pollinated tomato may effectively replace a commercially available root stock selection, RST-04-105T, but the results show that under season extension production with high-tunnels (where grafting is most often practiced), the RST-04-105T would be a preferred choice. The broader range of disease resistance bred into RST-04-105T may be a good insurance for producers using tunnels.

Over the 105 days of harvest, a 63-92% increase in yield of salable tomatoes from tomatoes grafted to seeds collected from (open pollinated) "Large Red Cherry" compared to controls were recorded. RST-04-105T performed better, with 58-117% increase in yield of salable tomatoes compared to controls. However, the RST-04-105T did not begin to out perform the "Large Red Cherry" grafted plants until 10/9/2017.

The most significant improvements in yield were seen in grafted heirloom varieties during the 6 weeks beginning 8/14/2017 when high temperatures resulted in high numbers of culls (up to 95% in "Brandywine"). An average of 5 pounds of tomatoes were not salable on heirloom varieties on non-grafted plants. This translates to over $10 ($2/pound) in lost potential sales per plant as a result of raising non-grafted plants. Using RST-04-105T as root stock, between 10/9/2017-10/28/2017, 2.1-3.4 additional pounds of salable tomatoes per plant were harvested compared to plants grafted with the "Large Red Cherry" root stock. However, in our market, at this time of the season, these tomatoes do not have a strong demand with with customers and would not likely be sold.

Selecting hybrid cultivars resistant to diseases as scions showed the least benefit of disease control (based on percentage of culls) when used as a grafted plant.

When "Large Red Cherry" and RST-04-105T were used as the root stock, the vigor and size of the grafted plant were nearly double the size of same non-grafted scion varieties and a greater yield was reported. These two root stocks also produced earlier harvest (3-11 days sooner) compared to other root stock varieties. By September, the overall plants were nearly double the size of similar scion varieties grafted to "Roma" or "Rutgers" root stock. Upon harvest, the roots of "Large Red Cherry" and RST-04-105T could not be pulled by hand from the soil. The network of roots were fibrous and extensive compared to the roots of "Roma" and "Rutgers". As a result, "Roma" and "Rutgers" would not be recommended as root stock selections.

Participation Summary

Educational & Outreach Activities

4 Consultations
3 On-farm demonstrations
11 Tours
3 Workshop field days

Participation Summary:

73 Farmers
1 Ag professionals participated
Education/outreach description:

We were fortunate to have many farmers, gardeners, and youth interested in visiting the farm and learning about saving seeds and grafting potential. Several youth and homestead/hobby farmers who tried grafting and took home their grafted tomatoes followed up at the end of the the season to report positive increases in yield when compared to the non-grafted plants. Many participants did not want to discard the scion transplants and chose to remove tips above the first set of true leaves and planted the scions transplants after tips were removed for the grafts. This seemed to be a good practice if suitable soil is available for these disease susceptible varieties. The only issue observed with graft unions was in selection of matches between scion/root stock alternatives. Where the size of the scion exceeded the diameter of the root stock as the plants matured, grafts failed. Several heirlooms varieties have very large diameter stems, "Large Red Cherry" is a good large root stock variety for these scions.

Many visitors had never raised heirloom varieties and after tasting and smelling these tomatoes were eager to raise these "new" varieties again.

Younger participants seemed to have an easier time creating successful graft unions than older participants. We later tried grafting demonstrations with more established plants (six true leaves showing) with a mature group of gardeners. Although easier to make grafts, the grafts did not remain as well on the more established plants and many graft unions separated as fruit set in summer. The recommendation with grafting demonstrations may be to use side-grafting rather than tip grafts, using 72-cell size plants at the three true-leaf stage, and use tape rather than clips. Several humidity chambers were tried (individual and creative designs were invented by youth) and regardless how elaborate, each design seemed to work and successful graft unions formed. A plastic tent with a pan of water will suffice.

One youth chose not remove the roots from the scion after the graft union formed and the yield from five of his plants with two roots outperformed the four "normal" grafted plants with only the single (root stock) root attached, even when three of the scion roots eventually rotted away (likely from disease). This small test seemed to show that retaining both sets of roots may be an option because the yield on these plant were greater than the plants with a single root. 

Three local vegetable farmers implemented grafting into their production with mixed results. A large percentage of the grafts did not hold with one of the farmers. The other two said they will continue to use grafted plants in high-tunnels but will adopt better rotation in fields rather than use grafted plants.

The discussion about seed saving did not seem to resonate with visitors who seem to prefer the annual rotation of new varieties, at least with tomatoes. Farmers prefer hybrids that are not good candidates for seed saving. Although not the focus of the primary presentation, many visitors were curious about mycorrhizal fungi addition to transplants (the roots of treated and untreated plants were a remarkable proof of the effect of colonization), crop rotation to reduce disease pressure, poly culture (lettuce sown between tomatoes in May, then basil sown between plants in July), and cover crop ideas to reduce weeds and increase soil organic matter/fertility.

Salsa, made with heirloom peppers and tomatoes, provided to guests was the "thing most enjoyed" about the presentations. Free plants were a close second.

There have been no articles accepted for publication. Future field days for other research projects on the farm will continue to include discussions about seed saving and grafting potential.

Project Outcomes

14 Farmers reporting change in knowledge, attitudes, skills and/or awareness
3 Farmers changed or adopted a practice
1 Grant received that built upon this project
Project outcomes:

Accomplishments

A significant improvement was observed from the 2015 season to the 2016 season from seeds saved. The variation of plants in open pollinated varieties such as Red Cherry, Roma, and Rutgers seems to transfer the improvements on disease resistance to their offspring. If farmers can retain the seeds from plants showing the greatest disease resistance to pressures found in their particular environment, these plants may provide a sustainable root stock option for grafting scion tomato varieties with little genetic disease resistance. If successful, farmers can save seeds from both root stock varieties and scion varieties to produce healthy tomatoes through grafting rather than relying on chemical-based treatments for diseases.

Knowledge Gained:

This project increased our farm's resolve to find new ways to further bring our production system into a more sustainable paradigm. Sharing the information learned in this project with others was rewarding. We continue to experiment and perform research and use our farm as an example to area youth, gardeners, and farmers as an example of sustainable production practices. 

Some benchmarks of our objective to become more sustainable include:

  1.  In the 2018 season, over 80% of our production area contained either perennials or vegetable raised from saved seeds. Our goal is to be at 90% by the 2020 season.
  2. We produced enough compost in 2017 from changes in livestock production practices to eliminate "off-site" fertilizer purchases for the current 2018 season. Animal bedding was locally sourced at no material cost.
  3. All of our production area now uses Red Clover in walking aisles and on driving paths. This clover is cut and collected to supplement livestock feed.
  4. Cover cropping, mulches (organic and geotextile fabric), deep bedding for animals to provide material for compost, "vertical gardening", inter-cropping, companion planting, biological control measures, and poly culture is extensively used as a result of ideas generated from other SARE projects.
  5. Our farm's marketing has implemented a more sustainable approach by including CSA and PYO (fruit and vegetables). This reduces wasted resources (time and spoiled/unsold product) from our farm's former reliance on farmer's markets and brings many guests to our farm. Guests seem interested in sustainable farming ideas and integrating livestock into our production model, this provides our farm with a niche that will likely increase sales.
  6. Unsold harvest is used in value-added offerings. Over 1,500 quarts were canned in 2017, we intend to double this in the 2018 season. Cull production and many weeds can be used as supplemental animal feed.
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.