Implementation of Novel Grafting Methods and Rootstocks for Organic and Heirloom Tomato Growers in the Midwest

View the project final report

Commodities

• Vegetables: tomatoes

Practices

• Education and Training: on-farm/ranch research
• Pest Management: integrated pest management
• Production Systems: organic agriculture, transitioning to organic
• Sustainable Communities: sustainability measures

Grafting with inter-specific hybrid rootstock is effective for tomato (Solanum lycopersicum) growers looking to reduce soilborne disease organically and increase fruit yield in the Southeastern US. However, production with grafted tomatoes has not been tested in the Great Plains region of the US.  Small-acreage growers would like to produce grafted plants themselves, but many have difficulty with propagation due to water stress in the scion post-grafting and/or high temperatures within healing chambers. Growers may be able to reduce water stress post-grafting by removing the upper portion of the shoot to reduce leaf surface area, but no data exist on the potential effects of this practice on mature plant yield. Five high tunnel and one open-field study were conducted in 2011 and 2012 to investigate yield effects related to the use of two rootstocks and shoot removal during the grafting procedure. Grafting significantly increased fruit yield in five of the six trials (P<0.05). The average yield increases by Maxifort and ‘Trooper Lite’ rootstocks were 53% and 51%, respectively, across all trials. In some trials shoot removal during the grafting process reduced yield and could depend upon rootstock vigor. Another series of experiments were performed testing the efficacy of shoot removal for graft survival during the healing period prior to field planting.  We also determined environmental conditions by monitoring temperature and relative humidity (RH) in all healing chambers. Five healing chambers designs were evaluated, and no significant effects of treatment design were observed upon grafted seedling survival. Plants grafted with no chamber had success rates of 81% to 91%. Additionally, three grafting leaf removal techniques were studied, and a partial leaf removal method had significantly higher success rates as compared to fully foliated and defoliated plants (P<0.05). Partial leaf removal may be recommended as a way to reduce water stress in the plant, and could potentially be a way to simplify the grafting process for small-scale producers.  Environmental monitoring within the healing chambers showed the various microclimates of the chamber designs. Interestingly shade cloth increased RH humidity slightly while plastic chambers with no humidifier held very high RH (>85%).

Introduction:

The domestication and cultivation of food sources gave rise to human settlement and ultimately modern civilization. Humans have sought methods to improve crop success for several millennia.  Grafting, an ancient technique of unknown specific historical origin, was developed to improve production in woody plants by means of growing the vascular systems of two related species.  Ultimately, grafting improves yield quality and volume via the union of desired qualities from two initially separate plant bodies.   Despite the application of this technique originating with fruit trees, grafting may be employed with vegetable – specifically solanaceous and cucurbitaceous – crops in order to improve yield and combat soilborne pathogens.  Furthermore, the resurgence in growers’ interest in organic cultivation practices and mandated phaseout of some soil fumigants have made vegetable grafting a major topic of interest in the horticultural community throughout the past decade (Davis et al, 2009; Kubota et al., 2008, Louws et al., 2010). 

Although grafting is useful for managing soilborne diseases, many of them are not common at high frequencies for Midwestern growers, as they are not as established in the region compared to other areas of the United States due to cropping history.  Therefore, a major point of interest in the case for herbaceous grafting is that of increasing crop productivity.  Specifically, yields are increased in grafted watermelon and cucumber (Pavlou, Vakalounaki, and Ligoxigakis, 2002; Upstone, 1968), Tomato grafting usually leads to increased fruit yield via larger fruit size (Pogonyi at al., 2005; Augustin, Graf, and Laun, 2002).  This increase in output vigor is linked to heightened rootstock growth, thereby affecting water uptake and nutrient content (Leonardi and Giuffrida, 2006; Ruiz, Belkbir, and Romero, 1996; Fernandez-Garcia et al., 2002).  

         Many organic growers are implementing the use of high tunnels as a way to reduce foliar disease and extend the growing season (Carey et al., 2009). However, managing soilborne diseases can be very difficult in these systems as crop rotation intervals are often reduced as a way to sustain profitability. Research suggests that grafting is effective at reducing the incidence and/or severity of  soilborne diseases such as root-knot nematodes (Rivard et al., 2010b), southern blight (Rivard et al., 2010b), fusarium wilt (Rivard and Louws, 2008), and verticillum wilt (Groff, 2009).  In the Midwest, organic tomato growers encounter verticillium wilt, fusarium wilt, and root-knot nematodes regularly, and southern blight incidence can vary, depending on summer growing conditions and soil type. Furthermore, many organic growers in Kansas and throughout the US are utilizing specialty cultivars such as heirlooms in order to cater to local niche markets.  These varieties are highly susceptible to soilborne as well as foliar diseases and the implementation of high tunnels and grafting together can be particularly useful as an Integrated Pest Management (IPM) approach to disease management for heirloom tomato production.  Available research suggests that grafting will be beneficial for tomato growers in the US (Kubota et al., 2008) and particularly for organic growers who are utilizing heirloom cultivars to capture specialty markets (Rivard et al., 2010a).  

         Although grafting may be very useful for tomato growers in the Midwest, there is virtually no market availability of grafted plants propagated in the US, and more than 30 million grafted plants are imported from specialty nurseries in Canada for the US (Kubota et al., 2008). Even though a current and future market for grafted tomato plants exists, very few (if any) propagators in the United States have started grafting at a commercial scale for fruit production. This market gap has resulted in a large increase in interest recently in regards to tomato grafting for the US. However, still little information is available in the US in regards to grafted tomato propagation.  

         This project seeks to conduct a research and extension program that will provide support and answer relevant questions for this clientele as the US propagation industry continues to progress in this area.  Grafted propagation techniques have not been explored in a comprehensive way and there is a strong need to determine healing chamber management and post-grafting environment manipulation in a systematic way.  Furthermore, there is a significant need to demonstrate propagation methods in regards to economic optimization.  A recent publication showed that grafting can add $0.46 to $0.74 per plant depending on heating costs and other factors (Rivard et al., 2010c). In this case, the growers were utilizing the tube-grafting technique (Lee, 1994; Rivard and Louws, 2006), and this technique is commonly used in commercial production systems worldwide (Lee, 1994; Lee, 2003).  Once the plants are grafted, they are placed into a “healing chamber” which is a small tent that allow for the proper environmental manipulation to promote graft union (Rivard and Louws, 2006). 

         Although growers can perform their own grafting, managing the grafted plants can be difficult (Groff, 2009; O’Connell et al., 2009). Grafted plants are usually placed inside “healing chambers” to maintain high humidity and reduce light intensity (Rivard et al., 2010c). Plastic healing chambers built inside greenhouses can overheat, leading to plant wilting and death. Healing chambers also add to the cost of producing a grafted transplant, as they require additional materials and labor (Rivard et al., 2010c). Reducing leaf area could reduce water stress and reduce the need for humidity management. This method is commonly used with ornamental and woody plants (Christopher, 1954; Harris, 2003). Reducing leaf area via elimination of scion shoots, reliance on the healing chamber can be greatly reduced or eliminated altogether. However, there is little information available as to whether this process scion shoot removal will affect tomato yield and fruit quality in a production setting in addition to healing chamber design/modifications. Therefore, there were five primary overall objectives for this research: (i) to determine the efficacy of two rootstock cultivars at increasing tomato fruit yield in high tunnels; (ii) to test the effect of scion shoot removal upon mature plant yield and biomass; (iii) to determine how healing chamber design (supplemental humidity and covering) affects graft survival; (iv) to determine how healing chamber design affects the healing chamber environment; (v) and to determine how scion shoot removal affects graft survival in different healing chambers.

This work elaborates upon two experiments designed to analyze different facets and effects of tomato grafting.  The first experiment aimed to determine the efficacy of two rootstock cultivars at increasing tomato fruit yield in high tunnels as well as test the effect of scion shoot removal upon mature plant yield and growth.  The second experiment focused upon the propagation of grafted plants by determining how healing chamber design (supplemental humidity and covering) affects graft survival; determining how healing chamber design affects the healing chamber environment; and determining how scion shoot removal affects graft survival in different healing chambers. Overall, the goal of this research is to investigate and streamline the tomato grafting in order to enhance the effectiveness and utilization of this technology. We were successful at reaching all of our major performance targets by completing the proposed field (high tunnel) studies in addition to the greenhouse work proposed. We were also successful at completing the objectives outlined focused on outreach and dissemination.  We coordinated and delivered a half-day workshop focused on tomato grafting at the Olathe Horticulture Research and Extension Center.  The workshop was well-attended by approximately 30 growers. Evaluations are provided below.  We were also successful at producing several short informational videos discussing the propagation of grafted tomatoes as well as the research supported by this grant.