Grafting for Root-knot Nematode Control and Yield Improvement in Organic Heirloom Tomato Production

Organic heirloom tomatoes (Solanum lycopersicum L.) are difficult to grow in Florida as a result of root-knot nematodes (Meloidogyne spp.) (RKN) and hot, humid growing conditions. Although grafting with resistant rootstocks has been shown effective for RKN management in tomato production, little research has been conducted on grafted heirloom tomatoes under Florida conditions. In this 2-year study, two susceptible heirloom tomato scions, ‘Brandywine’ and ‘Flamme’, were grafted onto two hybrid rootstocks, i.e., interspecific tomato hybrid rootstock ‘Multifort’ (S. lycopersicum · S. habrochaites) and tomato hybrid rootstock ‘Survivor’ (S. lycopersicum). Non-grafted and self-grafted scions were used as controls. Three field trials were conducted including the 2010 and 2011 organic field trials as well as a transitional organic field trial in 2011. There was a lack of RKN pressure in the organic field in 2010. In 2011, the RKN population was higher in the transitional field than the organic field, whereas grafting with hybrid rootstocks significantly reduced root galling (P # 0.0001) in both fields. In the organic field, the hybrid rootstocks performed similarly and significantly reduced root galling compared with the non-grafted and self-grafted scions by ’80.8%. In the transitional field, compared with non-grafted scions, the root galling reduction by ‘Survivor’ (97.1%) was significantly greater than that by ‘Multifort’ (57.6%). In general, tomato plants grafted onto ‘Multifort’ tended to be more vigorous than all other treatments. There was no clear relationship between root galling and tomato yields. Grafting did not significantly affect the total marketable yield for the scion ‘Flamme’ in both years. Total marketable yield was similar among treatments in 2010 but varied in 2011 for the scion ‘Brandywine’. In 2011, the non-grafted ‘Brandywine’ and ‘Brandywine’ grafted to ‘Survivor’ produced significantly higher (P < 0.05) yields than other treatments in the organic field. However, in the transitional field, ‘Brandywine’ grafted to ‘Multifort’ resulted in significantly higher (P < 0.05) yields than the nonand self-grafted ‘Brandywine’ treatments. Grafting with appropriate rootstocks may play an effective role in RKN management during the transition to organic production when high populations of nematodes are present. Modern tomato breeding has led to improvements in postharvest attributes including shelf life, but this has come with a noticeable decrease in fruit flavor (Klee, 2010). Educated consumers have begun demanding heirloom tomatoes for their superior flavor and unique appeal (Bland, 2005; Jordan, 2007; Klee, 2010). This increased interest has helped expand a niche market for local organic growers (Jordan, 2007). However, heirloom tomatoes can be difficult to grow in Florida as a result of high pest and disease pressure. One of the major pest management challenges is RKNs (Meloidogyne spp.), which thrive in warm weather and moist, sandy soils (Roberts et al., 2005; Sasser, 1980). RKNs cause root galls that damage the root system and result in stunted plant growth and significant yield loss. RKNs persist in the soil for many years and have a broad host range. These characteristics make RKN difficult to control on organic farms. The small size of many organic farms may prevent use of the long rotation times needed to ameliorate soil conditions between planting of susceptible crops. Organic growers often face pest and disease challenges with few effective control methods, making organic heirloom tomato production even more difficult and potentially less profitable than conventional production (Rivard and Louws, 2008; Rivard et al., 2010a). With the use of appropriate rootstocks, grafting may be a useful technique for vegetable producers to overcome soilborne pathogens including RKN. Vegetable grafting began in Japan and Korea in the 1920s to manage fusarium wilt (caused by Fusarium oxysporum Smith) in watermelons and is currently widely used in cucurbitaceous and solanaceous crop production in Asian and Mediterranean countries (Lee, 1994; Lee et al., 2010). Recently, growers and researchers in the United States have begun examining vegetable grafting as an integrated pest management tool for successful vegetable production. Research has focused on grafted seedling production, use, and economics (Kubota et al., 2008; Rivard et al., 2010b); grafting as an alternative to methyl bromide in field production (Freeman et al., 2009); and the use of resistant rootstocks for controlling RKN and soilborne diseases such as bacterial wilt [caused by Ralstonia solanacearum (Smith) Yabuuchi et al.], fusarium wilt, and southern blight (caused by Sclerotium rolfsii Sacc.) (Bausher, 2009; López-Pérez et al., 2006; Rivard and Louws, 2008; Rivard et al., 2010a). With the phase-out of methyl bromide for soil fumigation and the continued rise in demand for organic produce in the United States, the need for alternative disease control methods that do not rely on synthetic biocides has increased (Greene et al., 2009; King et al., 2008; Louws et al., 2010). Tomato hybrids (Solanum lycopersicum L.) and interspecific tomato hybrids (S. lycopersicum · S. habrochaites S. Knapp & D.M. Spooner) have been used worldwide as diseaseresistant rootstocks in grafted tomato production (King et al., 2010). It is unclear how the differences between tomato hybrid rootstocks and interspecific tomato hybrid rootstocks will affect field production of indeterminate heirloom tomatoes. Hence, rootstock evaluations for heirloom tomato production in open-field conditions should involve both types of rootstocks. Field studies conducted in North Carolina demonstrated that southern RKN [M. incognita (Kofoid & White) Chitwood] could be managed by grafting heirloom tomatoes onto interspecific hybrid rootstocks (Rivard et al., 2010a). However, the inconsistent function of the Mi resistance gene in these rootstocks suggested the need for more examinations of rootstock performance in fields infested with RKN to provide recommendations for effective use of rootstocks. Interest in tomato grafting is emerging among small and organic growers in Florida. The results from the North Carolina studies are promising and suggest that grafting may be applicable in Florida heirloom tomato production. However, Received for publication 23 Nov. 2011. Accepted for publication 13 Mar. 2012. This research was funded in part by Southern Region Sustainable Agriculture Research and Education (SARE) Graduate Student Grant awarded to Charles Barrett. We thank Takii Seeds USA for providing ‘Survivor’ rootstock seeds and De Ruiter Seeds Inc. for providing ‘Multifort’ rootstock seeds. We also thank Buck Nelson and crew at the PSREU, Citra, FL, for support and comments concerning the field trials for this article. Graduate Research Assistant. Assistant Professor. Professor. To whom reprint requests should be addressed; e-mail [email protected]. 614 HORTSCIENCE VOL. 47(5) MAY 2012 appropriate rootstocks for Florida conditions need to be determined. The purpose of this study was to assess heirloom tomato grafting for RKN control under organic production in naturally infested Florida sandy soils. It is hypothesized that grafting onto resistant rootstocks can reduce nematode galling incidence. Tomato hybrid and interspecific tomato hybrid rootstocks were compared with respect to their influence on nematode resistance, crop vigor, and fruit yield. Materials and Methods Scion and rootstock cultivars. Grafted tomato seedlings were produced using certified organic heirloom tomato seed and commercially available non-treated rootstock seeds. The heirloom tomato cultivars Brandywine and Flamme were used as nongrafted controls and as scions (Tomato Fest, Little River, CA). ‘Brandywine’ (BW) is a large, red, open-pollinated, indeterminate type valued for its excellent flavor and large size, whereas ‘Flamme’ (FL) is a golf ball-sized, orange, open-pollinated, indeterminate type. ‘Multifort’ (De Ruiter Seeds, Bergschenhoek, The Netherlands) and ‘Survivor’ (Takii Seeds, Salinas, CA) were used as rootstocks. ‘Multifort’ (MU) is an interspecific hybrid (S. lycopersicum · S. habrochaites) and ‘Survivor’ (SU) is a tomato hybrid (S. lycopersicum). Both rootstocks were chosen for their high resistance to soilborne pathogens and RKNs (Meloidogyne spp.) and vigorous growth habit as claimed by the seed companies. Transplant production. Rootstock seeds were sown 2 d before scion seeds on 16 Feb. 2010 and 11 Feb. 2011. Seedlings were grown in Fafard Organic Formula Custom potting mix (Apopka, FL) using 128-cell-count Speedling Flats (Sun City, FL). At the four to five true leaf stage, seedlings were tubegrafted. Grafting procedures were adapted from Rivard and Louws (2006) in which young seedlings are grafted and held together using 2.0-mm or 1.5-mm silicone clips (Hydro-Gardens, Colorado Springs, CO). Grafting took place 34 d and 28 d after scions were sown for 2010 and 2011, respectively. The grafted seedlings were then placed in a temperatureand humidity-controlled walkin cooler at 25 C and 95% RH with no light for 24 h. Thereafter, the grafted seedlings were gradually exposed to light, and humidity was reduced for 6 d until the seedlings healed. Grafted seedlings were then transported to the greenhouse before transplanting into the field. Field trials. Three field trials were conducted at the University of Florida Plant Science Research and Education Unit in Citra, FL. One trial was conducted in the Spring of 2010, whereas two were conducted in the Spring of 2011. In both years, one trial was grown on certified organic land following the rules outlined by the National Organic Program (U.S. Department of Agriculture, Agricultural Marketing Service, 2002). The organic research land was certified by Quality Certification Services, Gainesville, FL. Organic yellow squash (Cucurbita pepo L.), an excellent host for RKN (Wang et al., 2004), was grown during the 2010 Fall growing season to encourage a more uniform natural RKN infestation and increase RKN numbers for the 2011 organic field trial. Additionally, in the Spring of 2011, a trial was conducted on a site with a history of continuous nematode infestation that had been managed conventionally in previous years. Plants used in this trial were produced and grown following organic practices. This trial was designed to reflect growing conditions during a typical 3-year transition period from conventional to organic production. The soil type found in all three field trials is Candler sand, 0 to 5% slopes, hyperthermic, uncoated Typic Quartzipsamments with a pH of 6.0. In all trials, there were eight treatments consisting of: non-grafted and self-grafted scion controls for ‘Brandywine’ (NGBW, Fig. 1. Cumulative marketable yield for non-grafted and grafted heirloom tomato cultivars Flamme (A) and Brandywine (B) from the organic field trial conducted in 2010. Each harvest was analyzed using a oneway analysis of variance and means were separated by Fisher’s least significant difference (LSD) test. LSD bars represent the least significant difference at P# 0.05. NGBW and NGFL = non-grafted ‘Brandywine’ and ‘Flamme’; BW/BW and FL/FL = self-grafted ‘Brandywine’ and ‘Flamme’; BW/MU and FL/MU = ‘Brandywine’ and ‘Flamme’ grafted onto the interspecific tomato hybrid rootstock ‘Multifort’; BW/SU and FL/SU = ‘Brandywine’ and ‘Flamme’ grafted onto the tomato hybrid rootstock ‘Survivor’. Table 1. Effect of grafting treatments on root-knot nematode galling ratings of heirloom tomato cultivars Brandywine and Flamme in 2011. Treatment Organic field Transitional field