Grafting Rootstocks onto Heirloom and Locally Adapted Tomato Selections to Confer Resistance to Root-knot Nematodes and other Soil Borne Diseases and to Increase Nutrient Uptake Efficiency in an Intensive Farming System for Market Gardeners

Grafting Rootstocks onto Heirloom and Locally Adapted Tomato Selections to Confer Resistance to Root-knot Nematodes and other Soil Borne Diseases and to Increase Nutrient Uptake Efficiency in an Intensive Farming System for Market Gardeners

Summary

Yields of grafted organic heirloom tomatoes were higher than non-grafts in both high tunnels and adjacent field plots at the Center for Environmental Farming Systems, Goldsboro, NC. Marketable fruit in the high tunnel system was greater than in the field system and harvests began 25 days earlier. In a controlled environment experiment, shoot and root biomass, height and total tissue nutrient concentration (N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, and B) were greater in grafted treatments. In on-farm trials, soilborne diseases such as bacterial wilt, root-knot nematode, Fusarium and southern stem blight were effectively managed utilizing rootstocks.

Objectives/Performance Targets

1. Improve grafting, acclimation and transplanting techniques.
2. Select appropriate rootstocks for root-knot nematodes and other soilborne diseases.
3. Evaluate appropriate rootstocks for increased nutrient uptake efficiency and other horticulturally valuable traits, such as fruit quality, earliness, vigor, and resistance to pests.
4. Test training and establishment techniques for grafted rootstocks, including single and multiple head systems.
5. Compare performance of scions grafted onto resistant rootstocks, self-grafted and non-grafted controls under realistic conditions of soilborne disease pressure.
6. Compare performance of scions grafted onto resistant rootstocks, self-grafted and non-grafted controls, following organic practices at a research station.
7. Evaluate a grafted rootstock-high tunnel tomato system for feasibility, including a preliminary assessment of the economics. Compare the tunnel system with open-field production.
8. Identify promising avenues for future research and development.

Accomplishments/Milestones

1. Improve grafting, acclimation and transplanting techniques:

In both 2006 and 2007 all grafted plants were produced on the NCSU campus using the Japanese tube-grafting technique. Scion and rootstock seedlings were cut at a 45 degree angle and reattached using a tube-shaped silicon clip. The newly-grafted plants were exposed to specific light and humidity conditions while the scion and rootstock stems fused and the vascular tissue reconnected. As the stem diameter increases, the silicon clips fall off, and the plants are allowed to harden off. Twelve to fourteen days after grafting the plants are ready for field-planting. We also conducted a demonstration in Spanish for workers at a greenhouse currently growing grafted heirloom tomatoes using transplants grown in Canada. In Spring 2008 we held a workshop in Pittsboro, NC with more than 30 growers attending. As growers and extension agents learned about our project from cooperating farms or our outreach, the demand for grafted transplants far exceeded the number we could produce. We are currently trying to recruit a commercial propagator who would specialize in this area.

2&5. Select appropriate rootstocks for root-knot nematodes and other soilborne diseases and compare performance of scions grafted onto resistant rootstocks, self-grafted and non-grafted controls under realistic conditions of soilborne disease pressure:

Five field trials were carried out at on-farm locations and two research stations in order to identify rootstock which could be utilized to manage soilborne diseases. Each trial evaluated two or three rootstocks and one scion, and disease pressure was the result of native soilborne pathogens. The scion was selected based on the grower’s market preferences while the rootstock was chosen based on the anticipated disease pressure. Nutrient management, training and harvest practices followed normal grower practices for each farm.

Root-knot nematodes (Meloidogyne spp.) are a prevalent soilborne plant pathogen in southeastern soils that causes root galling, stunting, and can lead to severe crop losses. A trial in eastern NC was initiated to investigate rootstock that could reduce the symptoms of root-knot nematodes in naturally-infested soils. Many of the commercially-available rootstock contain the Mi gene, which confers complete resistance to RKN, although this particular gene has been shown to break down under high soil temperatures and high populations levels. Based on NCDA nematode assays, the population levels of RKN were extremely high (>10,000 nemas/500cc) in our trial. ‘Maxifort’ and ‘Beaufort’ rootstock had expected results, and galling caused by RKN was not only delayed, but reduced significantly. Interestingly, one rootstock showed no symptoms of galling throughout the season even under these ideal conditions for the pathogen. In this trial, yields were significantly higher among rootstock which carried the Mi gene compared to the non-grafted and self-grafted controls, and similar among themselves.

In previous studies, we have shown that bacterial wilt can be managed using resistant rootstock. However, the rootstock utilized for these previous trials are not commercially-available. In 2007, two commercially-available, rootstock-specific hybrids, ‘DP 105’ (USA) and ‘Asahi’ (Japan) were trialed in an infested field to determine the feasibility of these rootstocks for growers with bacterial wilt. In this on-farm trial, no symptoms of bacterial wilt were observed in plants grafted onto ‘DP 105’. In the self-grafts and those with ‘Asahi’ rootstock, symptoms did not appear until 3 weeks later than in control, and end of season wilt incidence was 40% in the Asahi grafts, 75% in the self-grafts and 100% in controls. Furthermore, yields were significantly higher (P<0.05) on the ‘DP 105’ rootstock than those in the self-grafted and non-grafted treatments. Yields in plants grafted onto ‘Asahi’ were intermediate, but not significantly different from any of the other 3 treatments.

Southern stem blight is a difficult disease for southeastern growers as it is often prevalent under hot conditions, and there is no known genetic resistance to this fungal pathogen in tomato. In 2007, we saw this disease in three different field trials, probably due to the unusually hot summer experienced in central NC. At each location, different combinations of two or three rootstock varieties were tested, and several rootstock varieties were able to reduce the onset of disease while the incidence of southern stem blight among non-grafted and self-grafted controls ranged from 35-100%. In an on-farm trial, yields in plants grafted onto ‘Beaufort’ and ‘Maxifort’ were significantly higher than those for controls and self-grafted plants.

3. Select appropriate rootstocks for increased nutrient uptake efficiency and other horticulturally valuable traits, such as fruit quality, earliness, vigor, and resistance to pests:

At this point only limited numbers of rootstocks are offered for sale in the US, and we were not able to locate any rootstock shown to improve nutrient uptake. For this reason, standard commercial rootstocks were utilized in our trials. In a 2007 study in the North Carolina State University Phytotron, two tomato cultivars, ‘Trust’ (commercial hybrid) and ‘German Johnson’ (heirloom), were grafted onto the ‘Maxifort’ rootstock to determine if growth and nutrient uptake were greater in grafted plants. Total nutrient uptake efficiency was calculated for both macro- and micro- nutrient content in the leaf tissue (leaf biomass x leaf nutrient concentration) and plant growth indicators measured. The experiment was a completely randomized block design with 5 replications, consisting of 6 treatments: ‘Maxifort’-‘Trust’ grafts, self-grafted ‘Trust’, non-grafted ‘Trust’, ‘Maxifort’-‘German Johnson’ grafts, self-grafted ‘German Johnson’, and non-grafted ‘German Johnson’. Five successive weekly destructive harvests were conducted representing the period 4-8 weeks post-grafting. Shoot biomass, root biomass, and height of ‘Maxifort’-‘Trust’ and ‘Maxifort’-‘German Johnson’ grafts were significantly higher, compared to the non-grafted treatments. The shoot biomass and height of the self-grafted treatments were also significantly higher than the non-grafted treatments. The total nutrient content of the leaf tissue of the ‘Maxifort’-‘Trust’ and ‘Maxifort’-‘German Johnson’ grafts were significantly higher for: N, P, K, Ca, Mg, Fe, Mn, Zn, Cu, B compared to the non-grafted treatments (P<0.05). The total nutrient content of the leaf tissue of the self-grafted treatments was significantly higher for: N, P, K, Mg, Zn, Cu, and B than the non-grafted treatments. Higher total concentrations were a result of both higher shoot biomass and higher nutrient content in the leaf tissue of the grafted plants.

In the 2007 CEFS trial (see description under objective 6), three treatment levels of total nitrogen were applied to both the field and tunnel plots at the following rates: 100lbs/A, 150lbs/A, and 200lbs/A. The intermediate (standard) rate of 150lbs/A (168 kg N/ha) produced the highest yields of fruit in both grafted and non-grafted plants under the high-tunnel system but did not show significant effects in the field system. Preliminary data analysis did not show any differences between the grafting treatments in the response to N, but none of the N treatments appeared to cause nutrient stress among the plants during the harvest period. The statistical analysis of the leaf tissue concentration data from the CEFS experiments has not been completed, but no nutrient deficiencies were noted in any treatments.

In a 2007 on-farm trial in Orange County, NC, the heirloom tomato, ‘German Johnson’ was grafted onto ‘Big Power’ and ‘Beaufort’ as well as onto its own rootstock (self-graft). Control (ungrafted) ‘German Johnson’ plants were also included in the trial. German Johnson plants on both commercial rootstocks had significantly higher tissue concentrations of N, K, Ca, Mn and B than self-grafts and controls. Mg levels were significantly lower in grafts onto the commercial rootstocks. S, and Cu tissue concentrations were significantly higher in grafts onto ‘Big Power’ than the other treatments. P concentrations were significantly higher in controls than the grafting treatments, with lowest concentrations in plants grafted on ‘Beaufort’.

4. Test training and establishment techniques for grafted rootstocks, including single and multiple head systems:

Trials in 2005 and 2006 showed inconsistent results of using alternative cultural training methods. The CEFS trial in 2007 evaluated a twin-row stake-and-weave system and also a standard stake-and-weave system with alternative in-row plant spacing. The twin-row stake-and-weave system had similar results to the standard stake-and-weave. However, plants were grown at 24” and 36” in-row plant spacing in the standard stake-and-weave system had similar yields based upon the per acre productivity. This trend shows that in-row spacing may be increased without losing overall productivity, and furthermore per plant productivity was dramatically increased at the higher plant spacing.

An on-farm trial in central NC showed how cultural training methods may be adapted to better suit grafting technology. This grower utilizes European greenhouse-style training methods, in which all suckers are pruned and a single leader is allowed to dominate the plant architecture. It was shown in the on-farm trial, that similar yields could be attained when ‘Maxifort’ rootstock was used and two leaders were trained from a single rootstock. This cultural practice is able to significantly reduce the economic risk associated with using grafted plants, and still provides the protection conferred by the disease resistant rootstock.

5. Compare performance of scions grafted onto resistant rootstocks, self-grafted and non-grafted controls under realistic conditions of soilborne disease pressure: See results reported under Objective 2.

6. Compare performance of scions grafted onto resistant rootstocks, self-grafted and non-grafted controls following organic growing conditions at a research station:
In 2007, a trial comparing the performance of grafted and non-grafted heirloom tomatoes grown in a high-tunnel system compared to an open-field system was conducted at the Center for Environmental Farming Systems (CEFS) located in Goldsboro, NC. Both production systems were managed following the National Organic Program (NOP) guidelines. The tomato cultivar, Cherokee Purple (Solanum lycopersicum) was grafted onto two rootstocks, ‘Maxifort’ and ‘Beaufort’ (Solanum lycopersicum x Solanum habrochaites). ‘Maxifort’ is the most important rootstock utilized commercially in the US. It confers vigor in addition to disease resistances and is ‘standard’ in the US and Europe for greenhouse tomato grafting. However, it is not clear whether more vigorous plants are desirable in all situations. Plants that are too big can be difficult to handle in a tunnel or field system. Therefore we also included in our Orange County NC trial the rootstock ‘Beaufort’ which the distributor (DeRuiter Seeds) claims confers less vigor to the scion, and ‘Big Power’, which the distributor (Rijk Zwaan) claims will make the plant less vegetative. At CEFS, grafting significantly increased the cumulative total yield of fruit (lbs/A) in both the field and high-tunnel systems (P<0.05). Highest yields were seen in the ‘Maxifort’ grafts, although differences between ‘Maxifort’ and ‘Beaufort’ were not always significant. Subplots in the main CEFS experiment included 3 levels of N fertility as described under Objective (3).

7. Evaluate a grafted rootstock-high tunnel tomato system for feasibility, including a preliminary assessment of the economics. This system will be compared with open-field production:
Trial was conducted as above (Objective 6). An IPM program was employed to manage insect pests using beneficial organism releases and OMRI-approved insecticide applications, mainly Bacillus thuringiensis. Environmental monitoring equipment was utilized to compare the microclimates between the two systems. The tunnels provided frost protection for early planting dates as well as a 25-day earlier harvest. The first planting in the tunnel occurred March 20. A second planting occurred in the tunnels on April 3. The final planting date corresponded with the field planting date, April 19. Temperatures were significantly (10-20F) warmer in the tunnels through mid-April, but after than date, tunnel temperatures were only a few degrees higher than field temperatures. Effects of both planting date (tunnel plants only) and grafting treatment (control, self-graft, ‘Beaufort’ or ‘Maxifort’) on stem biomass at the end of the season were significant (P<0.05). Stem biomass was highest in ‘Maxifort’ grafts and lowest in controls. Predictably in the tunnels, highest stem biomass was seen in the first planting date, with lowest in the final planting date. Yields in the tunnels followed the same pattern, with the later plantings never ‘catching up’. Although differences in yield between the March 20 and later tunnel plantings were greatest in the first 6 weeks, the cumulative total yield of marketable fruit in the early plantings was still higher at the end of the season. Similarly cumulative yields in the high-tunnel system were greater than that of the field system (P<0.05) so field plants also never ‘caught up’ in terms of total yield. Compared to the open field system, plants grown in tunnels were more susceptible to fruit cracking, blossom end rot, and catfacing, but less susceptible to insect damage and tomato spotted wilt virus (P<0.05). The economic analysis will be completed in 2008.

8. Identify promising avenues for future research and development:

We continue to see more vigor in grafted plants. This is a desirable characteristic in general, especially for the heirloom tomatoes, which typically lack vigor. Furthermore, several rootstocks were highly effective at reducing diseases caused by soilborne plant pathogens. In the case of root-knot nematodes and southern stem blight, the unexpected resistance that was seen indicates the importance of grafting technology for deploying major resistance genes. Besides adding greatly to the success of heirlooms, these rootstock genotypes may provide new alternatives to the traditional array of resistance genes that are common in hybrids.

Additional studies are needed to verify 2007 studies would be help determine how to match specific rootstocks with specific scions, and in particular, whether heirlooms or other low-yielding cultivars are more responsive to grafting. Some evidence of greater response to grafting in the heirloom cultivar than the commercial hybrid was seen in the Phytotron experiment, but this requires additional documentation to see if it is general. It would also be useful to implement additional studies on training and spacing of grafted plants. Wider spacing and double heading could reduce the number transplants required, reducing costs. Another area in which more research would be useful is the optimal training systems for heirloom tomatoes in the tunnel. In fact, we had difficulty in finding consistent training recommendations for heirloom tomatoes in general. We have followed greenhouse rather than field cultural practices, which include more severe pruning of suckers. This particular cultivar, Cherokee Purple, which is know to be cracking susceptible, had high levels of fruit cracking in both the field and tunnel, but especially in the tunnel. Allowing more leaves to develop, particularly in the top of the canopy, and potentially also installing shading in or over the tunnels, may reduce cracking. Alternatively a less crack-prone cultivar could be utilized. This cultivar also experienced higher levels of catfacing in the tunnel than in the field. Presumably this was because of low temperatures experienced by flowers in the weeks before the field plots were planted. Spring 2007 was unusually stressful; with a severe Easter freeze followed by temperatures in the 90’s. It would be interesting to compare microclimatic data in the tunnels and field with catfacing, cracking, and blossom-end rot levels. We will be able to do this in 2008, but experienced some problems with the temperature sensors in early 2007. Another interesting area for future research would be to evaluate drought tolerance of grafted tomatoes. Some rootstocks display a larger root systems, as shown by larger root biomass. Perhaps grafted plants are more drought-tolerant; there are some reports in the literature of greater drought tolerance.

Impacts and Contributions/Outcomes

The interest in both grafting and high tunnels has increased greatly in the past year. Group members have been requested to give many talks and demonstrations on grafting and high tunnels. We have also received many more requests for grafted transplants than we could satisfy with available personnel and supplies. Events and activities included:

• Grafting demonstrations for students in Vegetable Crop Production (HS431) in Fall 2007

• Grafting demonstration and lecture for students in Greenhouse and High Tunnel Food Production (HS543) in Spring 2007. Three of the four extension agents enrolled, one graduate and one undergraduate student incorporated grafting into their required class project. We provided additional grafting assistance to one of these extension agents, and he was able to utilize these plants in a July 2007 demonstration at NC A&T.

• On-farm grafting training was provided to two NC growers, including a demonstration conducted in Spanish.

• This project directly supported two graduate student grafting research projects, (MS and Ph.D.) and a CEFS intern. In addition training was provided to CEFS interns and apprentices.

• Grafted tomato seedlings were provided to 3 NC growers and one in Pennsylvania for on-farm research in replicated field trials. Growers were selected on the basis of their willingness to take data and on being leaders in the sustainable agriculture area. The Pennsylvania grower was able to obtain a NE-SARE producer grant to support further development of the grafting project and a 2008 workshop.

• Invited presentations by one or more project personnel: Southeastern Veg. Expo 2007 Dec. 13, Myrtle Beach, SC; Grafting workshop, March 2007, Pittsboro, NC March 2007; Sustainable Agriculture Conference, Durham, NC, November 2007; Organic Growers School Flat Rock, NC, March 2007; CEFS Workshop, Goldsboro, NC, Oct. 11, 2007; CEFS Fall Festival Presentation, September 2007; Tomato Field Day, Fletcher, NC, August 2007. Over 30 growers attended the Pittsboro workshop in March 2007.

• Research reports and Presentations at professional meetings: Rivard, C.L., Peet , M.M. and Louws, F.J. 2007. Disease management and crop productivity utilizing grafted tomatoes. Proc. of the Int. Res. Conf. on Methyl Bromide Alternatives and Emissions Reduction, San Diego, CA. Page 61/1-61/3; Louws, F., Rivard, C. and M. Peet. 2007. Grafting for High Tunnel production of Organic Heirloom Tomato. Proceedings Southeast Vegetable & Fruit Expo 2007 Yearbook-Volume IX. P. 49-50. Myrtle Beach, SC Dec. 12-13 2007.

• CEFS tours have been held of the research tunnels constructed in 2007. A handout has been prepared for distribution to tour groups, and information on the project appears on the CEFS website: http://www.cefs.ncsu.edu/tomato.htm and in CEFS publications about on-going research.

• High tunnels and grafting have been added as topics to the Greenhouse Food Production Website http://www.ces.ncsu.edu/depts/hort/greenhouse_veg/. We will be posting additional information here as resources are developed.

• A handout on grafting techniques was prepared and 1000 copies printed. Almost all paper copies have been distributed, but we have made the document available in .pdf format online under the grafting topic, and it is now available at numerous other websites as well. A grower in Missouri accessed this information on-line, experimented in his greenhouse, and successfully produced grafted transplants in 2007. His farm was included on the SARE tour in 2008, and he is expanding his production of grafted transplants in 2008. At least 10 other individuals on the tour, including 5 growers, requested the URL for the grafting information. In addition, the handout was utilized in a Mississippi training session on grafting led by an extension worker from Alabama and a university professor in Utah is planning on developing a lab on herbaceous grafting for his plant propagation courses.

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