Inducing Disease Resistance and Increased Production in Organic Heirloom Tomato Production Through Grafting

Inducing Disease Resistance and Increased Production in Organic Heirloom Tomato Production Through Grafting

Summary

The use of grafted tomato for commercial production has been implemented worldwide, where soilborne disease pressure is high. Grafting with resistant rootstock is used to manage Fusarium, Verticillium, Root-knot nematodes, and bacterial wilt in several Asian, Mediterranean, and northern European countries. However, this technique is relatively unknown in the United States. Recently, direct-marketing avenues for sustainable farmers with small acreages have increased, and consumer-based demand for vine-ripened organic heirloom varieties has made this specialty crop especially important. These cultivars are open-pollinated, and are typically very susceptible to an array of soilborne and foliar diseases. A research program was initiated to investigate the potential of grafting as an integrated pest management strategy to reduce soilborne disease and increase crop productivity for organic heirloom tomato production.
Because this research relies heavily on well-developed international techniques and practices, an extension objective was implemented to disseminate information regarding grafting benefits and techniques, and to facilitate local adoption of this technology. An eight-page extension article focused on grafting technique was published through the North Carolina Cooperative Extension Service, and has been distributed widely at both the regional and national level. It is also available on Cary Rivard’s website, which is dedicated to tomato grafting research. Cary has been invited to speak at a number of grower conferences throughout the southern region and nationally as well.
During 2005 and 2006, field trials were initiated to determine the capability of grafting to reduce soilborne disease incidence in heirloom tomato. Bacterial wilt (caused by Ralstonia solanacearum) is a devastating soilborne disease in eastern North Carolina. CRA 66 and Hawaii 7996 genotypes were highly effective at reducing bacterial wilt in naturally-infested soils when utilized as a resistant rootstock for heirloom fruit production. No evidence of wilt was seen among resistant rootstock treatments when terminal disease incidence among non-grafted treatments was 75%, and 79% in 2005 and 2006, respectively. Heirloom scion grafted onto rootstock-specific cultivar, ‘Maxifort’, showed no symptoms of fusarium wilt (caused by Fusarium oxysporum f.sp. lycopersici), and non- and self-grafted controls had 45-50% disease incidence. In the mountain region of NC, verticillium wilt is an especially severe problem for tomato growers as crop rotation is not typically employed. Grafting with ‘Maxifort’ showed high potential as a management tool for this disease based upon increased vigor under continuous and rotational treatments.
Several field trials in 2005 and 2006 investigated the ability of rootstock-specific hybrids to increase crop productivity under organic management practices in a growing environment with little soilborne disease. Grafting with ‘Maxifort’ and ‘Robusta’ did not enhance yields when implemented into a typical on-farm organic production setting. Evaluation of alternative training systems indicated the importance of added vigor by ‘Maxifort’ through enhanced yields under “twin-headed” management in 2005. In 2006, yields were not increased under alternative training methods as compared to standard training system, but grafting with ‘Maxifort’ rootstock showed enhanced crop productivity among both training systems (P=0.005).

Objectives/Performance Targets

The objectives of this project were three-fold:
1) An evaluation of rootstock/scion combinations was carried out in North Carolina through field trials. Grafted heirloom tomatoes were planted in fields where bacterial wilt incidence is historically high. Data was collected pertaining to disease incidence as well as yield and fruit quality. Furthermore, production techniques were analyzed in an effort to increase yield on a per-plant basis, thereby offsetting the added cost of using grafted transplants. The expected outcomes of this aspect are increased disease resistance in areas where bacterial wilt pressure is high and increased productivity even in the absence of disease. The practical application of this technique and its success was evaluated and manipulated in order to cater to the success of North Carolina farmers.
2) To determine the dynamics of induced resistance mechanisms when heirloom scion are grafted onto rootstocks. The grafting process imposes a severe wounding event upon the plant, and wounding has been associated with an induced defense response in tomato. In order to understand how grafting affects this phenomenon, and the expression of defense genes associated with wounding was investigated. This work will lead to a knowledge base that may impact breeding and other cultural techniques.
3) To communicate the results and ideas behind this research through extension and education. An active role was taken in an effort to introduce farmers as well as extension agents to this technology. Demonstration plots and field days were used to illustrate the benefits of grafting directly. Grower workshops and training seminars were conducted to show farmers how to graft their own transplants. Finally, we constructed an extension factsheet that describes both the grafting technique and identifies resistant lines suitable for rootstock.

Accomplishments/Milestones

Bacterial Wilt Trials – Field trials were implemented in 2005 and 2006 to investigate the efficacy of grafting susceptible heirloom cultivars onto resistant rootstock under naturally-infested field conditions. Heavy bacterial wilt disease incidence was observed among non-grafted controls (75% and 79% incidence in 2005 and 2006, respectively), and grafted treatments with resistant genotypes CRA 66 and Hawaii 7996 showed no symptoms of wilt in both years. Yield in 2005 was significantly higher in Hawaii 7996 rootstock treatments compared to non-grafted control (P=0.04). In 2006, yields were not collected due to hurricane events that occurred early in the fruiting season. CRA 66 and Hawaii 7996 genotypes used as resistant rootstock for heirloom tomato production were highly effective at preventing bacterial wilt from endemic populations of R. solanacearum in eastern NC. These lines may be extremely important for future rootstock breeding programs.

Organic Crop Productivity Trials – In 2006, field trials were initiated to determine the efficacy of using commercial rootstock, ‘Maxifort’ and ‘Robusta’, to increase crop productivity for organic heirloom production. In Alamance County, Fusarium wilt (FW), caused by Fusarium oxysporum f.sp. lycopersici, occurred. Self-grafted and non-grafted controls were susceptible and generated similar terminal incidence values of 50% and 46%, respectively. ‘Maxifort’ rootstock completely controlled the incidence of FW and ‘Robusta’ offered moderate control. Cumulative marketable and total yields were not impacted by FW incidence or rootstock treatment. Earlier yields were seen with ‘Robusta’ rootstock and ‘Maxifort’ delayed peak harvest in Alamance County. A third trial was located at CEFS in Goldsboro, NC. This trial was located at the Small Farm Unit, which has been managed organically since 1994. ‘Maxifort’ showed 50% higher yield than the non-grafted and self-grafted controls and was statistically significant (P=0.01). The results of these studies are inconclusive as to whether or not grafting with vigorous rootstock will be able to increase yield where low disease pressure from soilborne plant pathogens is evident.

Crop Rotation Trial – A crop rotation trial at the Mountain Horticultural Crops Research Station (Buncombe Co.) was conducted on land with a history of Verticillium wilt (race 2). Continuous production (12 years) reduced biomass compared to the crop rotation (3 year) (P=0.003). ‘Maxifort’ rootstock improved growth compared to self-grafted and non-grafted controls within each rotational treatment (P=0.0003). Under continuous management, ‘Maxifort’ had similar or better plant vigor than non- and self-grafted treatments under rotation. Grafting with vigorous rootstock may be an alternative way of managing Verticillium wilt by giving a growth advantage over non-grafted plants.

Induced Resistance Experiment – An experiment was carried out to determine the gene expression dynamics of proteinase inhibitor II (PIN II) as a result of grafting. PIN II expression was elevated as a result of grafting alone, indicated by the elevated expression of PIN II in the self-grafted treatments. Furthermore, grafting with various rootstock showed that the quantitative expression of this gene was affected by rootstock genotype. Gene expression of PIN II was highest at 4 days, and had returned back to normal levels at 16 days.

Impacts and Contributions/Outcomes

Our findings illustrate the efficacy of rootstock to manage soilborne disease. We have identified two lines (CRA 66 and Hawaii 7996) with excellent resistance to bacterial wilt. These lines will be very important for future rootstock breeding programs in the US. Currently, rootstock-specific hybrids are available in the US, but very few of these lines have bacterial wilt resistance. This disease is especially important throughout the southeast, and the adoption of grafting for tomato production relies on the ability to manage this disease. Furthermore, this data represents the best management tool for bacterial wilt to-date. Traditional breeding methods have not been able to produce cultivars that contain good fruit quality characteristics and resistance to this disease, and it is hypothesized that these traits are linked. Even chemical fumigation shows poor efficacy against bacterial wilt as the bacterium can quickly re-colonize fumigated land from below the fumigation zone. Clearly, grafting is a viable management tool for bacterial wilt.
We have also identified rootstock than can be utilized to manage fusarium wilt in naturally-infested conditions. This pathogen is particularly difficult to manage for heirloom growers due to lack of genetic resistance and the pathogen’s ability to survive in the soil for a long period time (5-10 years). Although our evidence is preliminary, it appears that grafting may be a viable management tool for verticillium wilt (race 2) as well. This pathogen is extremely difficult to manage for organic and conventional growers as the only effective management tool is soil fumigation. There is no genetic resistance known in tomato to this pathogen, and crop rotations must be greater than 5 years to reduce pathogen populations. ‘Maxifort’ provides an alternative management strategy as the added vigor of this rootstock appears to compensate for the functional effect of the pathogen.
The results of the PIN II study highlight the importance of rootstock genotype for induced resistance. In our findings, it was shown that rootstock genotype quantitatively affected the expression of PIN II, a gene known to be associated with reduced herbivory by insects. However, the functional affect is still unknown. Rootstock breeders could utilize this information and the techniques developed in this study to select for induced resistance as a character trait in future breeding programs.

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