This two-year project demonstrated the effectiveness of using grafting with appropriate rootstocks for root-knot nematode control in organic heirloom tomato production. The two commercial rootstocks tested significantly reduced root galling. Under severe root-knot nematode pressure, use of the interspecific rootstock resulted in significantly higher yields than the non- and self-grafted treatments. Grafting did not exhibit any consistent effects on fruit quality attributes. Grafting could be an economically feasible pest control measure to help maintain a profitable production given that the risk of economic crop losses due to root-knot nematodes outweighed the higher cost of grafted transplants.
The purpose of this project was to study the benefits of grafting to improve disease resistance and to determine the effects of grafting on productivity and fruit quality of heirloom tomatoes. In addition, we aimed to investigate the feasibility of implementing grafting as an economically-viable practice on organic farms. Heirloom tomato varieties with superior flavor and outstanding eating quality have been grown increasingly by small growers as a niche product with a high price premium. However, the lack of a good disease resistance package in many varieties has been a great challenge for successful production of heirloom tomatoes. Pest management under organic production may be even more challenging as cultural control measures like crop rotation and cover cropping tend to be less effective due to the long-term survival and wide host range of certain soilborne pathogens such as Fusarium oxysporum and root-knot nematodes.
Grafting has been used in many cropping systems in a number of countries to obtain crop resistance or tolerance to soilborne diseases (Leonardi and Romano 2004). Grafting can create a new “hybrid” plant through a physical union of a rootstock plant and a scion plant. This technique is more rapid than breeding in uniting positive genetic and physiological traits that confer elevated vigor and productivity to the resulting plant (Edelstein 2004; Pogonyi et. al. 2005). Interest in tomato grafting is increasing with growers in Florida. Our recent interview with some heirloom tomato growers revealed their strong interest in innovative and integrated alternative practices for soilborne pest management. In addition to disease management, increased efficiency of nutrient and water absorption and improved tolerance to abiotic stresses have been observed on grafted vegetables (Davis et al., 2008; Lee, 1994). It is expected that using appropriate rootstocks may promote marketable fruit yield by reducing the incidence of fruit cracking and misshapen fruits in heirloom tomato production.
Despite the numerous benefits of employing vegetable grafting technology, the cost of using grafted plants in commercial production is often perceived as a major concern of growers. Compared with traditional heirloom tomato production, additional costs associated with grafted tomato production are reflected mainly in producing grafted transplants, i.e., seeds, space, water, nutrients and labor for growing rootstock seedlings, tools, materials, space and labor for making grafts, and care of grafts during the healing process. Hence, high marketable yield and quality is the key to profitability in production of grafted heirloom tomatoes. In this project, an updated, comprehensive, and objective cost-return analysis will be performed to provide recommendations for innovative and integrated use of grafting to grow heirloom tomatoes in an organic production system.
Objective 1: Examine the effectiveness of using rootstocks in organic production of grafted heirloom tomatoes to provide resistance or tolerance to root-knot nematodes.
Objective 2: Assess growth promotion, yield increase, and fruit quality in grafted heirloom tomato production under organic growing conditions.
Objective 3: Analyze the costs and returns of producing and using grafted heirloom tomato transplants in organic farming systems and provide updated information on the economic feasibility of adopting tomato grafting technology for these systems.
Objective 4: Outreach; Teach growers and extension agents how to graft and about the benefits of grafting and work with growers and extension agents to increase and promote sustainability in agriculture.
SCION AND ROOTSTOCK CULTIVARS. Grafted tomato seedlings were produced using certified organic heirloom tomato seeds and commercially available non-treated rootstock seeds. The heirloom tomato cultivars Brandywine and Flamme were used as non-grafted 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, while ‘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 x S. habrochaites) and ‘Survivor’ (SU) is a tomato hybrid (S. lycopersicum). Both rootstocks were chosen for their high resistance to soilborne pathogens and root-knot nematodes (Meloidogyne spp.) (RKN) and vigorous growth habit as claimed by the seed companies.
TRANSPLANT PRODUCTION. Rootstock seeds were sown two days 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 4-5 true leaf stage, seedlings were tube grafted. 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 temperature and humidity controlled walk-in cooler at 25°C and ~95% RH with no light for 24 hr. 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 while 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. Dept. Agr., 2002). The organic research land was certified by Quality Certification Services (Gainesville, FL). Organic yellow squash (Cucurbita pepo L.) 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. The plants used in this trial were produced and grown following organic practices. This trial was designed to reflect growing conditions during a typical three-year transition period from conventional to organic production. The soil type found in all three field trials is Candler sand, 0 to 5 percent 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, BW/BW) and ‘Flamme’ (NGFL, FL/FL), and the scion-rootstock combinations including ‘Brandywine’ and ‘Flamme’ grafted onto the rootstocks ‘Multifort’ (BW/MU, FL/MU) and ‘Survivor’ (BW/SU, FL/SU). The seedlings were transplanted on 10 Apr. 2010 and 2 Apr. 2011. A randomized complete block design was used with five blocks (replications). In the 2010 trial there were 12 plants per treatment in each block. In 2011, there were 15 plants per treatment in the organic field and 8 plants per treatment in the transitional field. In all three trials the in-row plant spacing was 0.46 m with 1.83 m between row centers. The plants were grown in raised beds with black plastic mulch and drip irrigation. A preplant application of Nature Safe organic fertilizer 10N-0.9P-6.6K (Cold Spring, KY) was applied and supplemental liquid fertilizer applications were injected into the drip system weekly throughout the season using Neptune’s Harvest 2N-1.3P-0.8K (Gloucester, MA). Supplemental calcium was also supplied through injection with Calplex (Botanicare, Chandler, AZ). All the nutrient inputs were approved by the Organic Materials Review Institute (OMRI, Eugene, OR) for use in certified organic production. The plants were staked and trellised as needed throughout the season following the stake and weave system common to Florida tomato production (Olson et al., 2011).
NEMATODE GALLING. Assessments of nematode infestation on plant roots were conducted after the final harvest. On 13 July 2010 and 30 June 2011, roots of five plants per treatment in each block in the organic fields and three plants per treatment in each block in the transitional field were assessed for nematode galls. The rating scheme proposed by Zeck (1971) that estimates nematode infestation levels on a plant was used. This scheme is a scale from 0-10 (0 = no galling, 10 = plant and roots are dead). Three researchers assessed each plant individually and then the ratings were averaged. In addition, two nematode samples from each field were submitted to the University of Florida Nematode Assay Laboratory for identification of species.
FRUIT YIELD. Tomato harvests began 58 days after transplanting (DAT) in 2010 and 63 DAT in 2011. In 2010, there were four harvests occurring on 7, 13, 17, and 25 June. In 2011, there were six harvests in the organic field and five harvests in the transitional field occurring on 4, 8, 13, 16, 22 (organic only), 23 (transitional only), and 28 (organic only) June. Fruit were harvested at the breaker stage and were graded and weighed at each harvest. Non-characteristic fruit and those exhibiting blossom end rot, catfacing, splitting, and insect/disease damage were also counted and weighed for calculation of non-marketable yield.
CROP VIGOR. The aboveground portion of one representative plant per treatment was destructively harvested in each block after the final harvest on 25 June 2010 and 30 June 2011 in the organic fields. Leaf area was measured using a LI-COR area meter (LI-3100, Lincoln, NE). After recording leaf area, each plant was dried in a forced air drying room at 75°C for 5 days and weighed for aboveground biomass.
FRUIT COMPOSITION ANALYSES. Vitamin C, soluble solids content (SSC), pH, and total titratable acidity (TTA) were determined for tomato fruit from four treatments including NGBW, BW/BW, BW/MU, and BW/SU, harvested on 13 June 2010 and 8 June 2011. Five representative fruit were picked at the breaker stage, which ensured that the fruit samples were at a uniform developmental stage. The harvested fruit were allowed to ripen at ambient storage temperature (approximately 24°C) for 3-4 d before being cut into pieces and blended for 30 s to form a homogenate for each treatment. Three replicates of the composite sample of each treatment were analyzed in both years. The homogenate was centrifuged for 20 min at 17,600 gn and 5°C. Then the resulting supernatant was filtered through eight-layer cheese cloth to obtain a clarified extract. Vitamin C was measured using a microplate spectrophotometer (PowerWave XS2; BioTek, Winooski, VT) with absorbance at 540 nm (Terada et al., 1978). Percent SSC was measured with an Abbe Mark II, digital refractometer (Reichert Technologies, Depew, NY). Initial pH was recorded and TTA in terms of percent citric acid milliequivalents was determined by potentiometric titration of 6 mL of tomato extract to an end point of pH = 8.2 with 0.1 N NaOH using a 719 S Titrino automatic titrator (Metrohm, Herisau, Switzerland).
CONSUMER SENSORY ANALYSES. Consumer sensory tests were conducted in 2010 and 2011 at the University of Florida Sensory Analysis Laboratory in Gainesville. Fruit were harvested at the breaker stage of development and allowed to ripen to full color before analysis. In the 2010 study, tomato fruit from NGBW, BW/BW, BW/MU, and BW/SU were harvested on 13 June and stored at ambient temperature for 3 d prior to the sensory evaluation. In 2011, tomatoes were harvested on 13 June and assessed after 4 d at the same ambient temperature. Fruit from the five field blocks were pooled for each treatment to provide enough ripe fruit for >100 sensory analysis samples. Tomatoes were cut from stem end to blossom end into wedges about 1-inch thick. Each serving sample consisted of two wedges of tomato fruit. There were 75 and 69 consumer panelists for the 2010 and 2011 sensory tests, respectively. Sensory test data were collected and analyzed using Compusense Five (Compusense, Guelph, ON, Canada) software. All procedures used were approved by the University of Florida Institutional Review Board.
The sensory tests began with three demographic questions: gender, age, and frequency of fresh tomato consumption. Each panelist was then asked to “indicate how much you like or dislike the following attributes” using a hedonic scale. The attributes assessed were overall appearance, overall acceptability, firmness, tomato flavor, and sweetness. The hedonic scale ranged from 1-9 (1 = dislike extremely, 2 = dislike very much, 3 = dislike moderately, 4 = dislike slightly, 5 = neither like nor dislike, 6 = like slightly, 7 = like moderately, 8 = like very much, 9 = like extremely). Panelists cleansed their palates between samples using cracker and water. After completing the taste test, each panelist was compensated for their participation. The order in which the tomato samples were presented to each panelist was randomized to reduce bias caused by the order in which the samples were presented. All possible orders were presented about an equal number of times. Following each sensory panel, the data were collected from all computers and compiled.
STATISTICAL ANALYSES. Data analyses were performed using the GLIMMIX procedure of SAS Version 9.2 (SAS Institute, Cary, NC). Yield, crop vigor, and nematode galling data were analyzed using a one-way analysis of variance with multiple comparisons conducted using Fisher’s least significant difference test at P ? 0.05. Fruit composition data and the consumer sensory test data were analyzed using a one-way analysis of variance with Tukey’s test (P ? 0.05) for multiple comparisons among treatments.
NEMATODE GALLING. In the 2010 field trial no RKN galls were observed on the tomato plants regardless of the treatment. Yellow squash was grown during the fall of 2010 in the organic field to build a natural RKN population for the spring 2011 season. RKN trials had been conducted in the transitional field for >10 years and there was a well established RKN population in that site. In the 2011 trials, almost all treatments showed RKN galling despite the use of rootstocks. However, RKN galling index ratings were significantly lower (P<0.0001) in both fields for tomatoes grafted onto ‘Survivor’ and ‘Multifort’ compared to the non- and self-grafted ‘Brandywine’ and ‘Flamme’ treatments. The nematode species found in both fields was identified as M. javanica (Treub) Chitwood. In the organic field, the two rootstocks performed similarly and significantly reduced root galling compared to the nongrafted and self-grafted scions by approximately 80.8%. In the transitional field, both rootstocks significantly reduced root galling for both scions in comparison with the non- and self-grafted scion treatments. However, the rootstock ‘Survivor’ led to the lowest galling ratings for both scion cultivars. Compared with nongrafted scions, the root galling reduction by ‘Survivor’ (97.1%) was significantly greater than that by ‘Multifort’ (57.6%). The self-grafted ‘Brandywine’ treatment had significantly lower galling index ratings than the nongrafted ‘Brandywine’ treatment. This reduced galling in the transitional field was unexpected and the cause is unclear. It may be that for the scion ‘Brandywine’, the act of grafting promoted a defense response which resulted in reduced galling ratings, but further investigation will be required to elucidate a cause.
The RKN galling ratings were generally higher in the transitional field than the organic field, suggesting a more severe infestation. This trend was observed with ‘Multifort’ but not with ‘Survivor’. Under different field infestation levels, the high resistance to RKN was consistent when the two heirloom tomato cultivars were grafted onto ‘Survivor’. In contrast, the resistance conferred by ‘Multifort’ appeared to break when the soil RKN infestation increased. In this study, ‘Multifort’ performed similarly to ‘Beaufort’ and ‘Maxifort’ which were assessed by Rivard et al. (2010). These three interspecific tomato hybrid rootstocks are released by the same seed company and tend to exhibit tolerance to RKN rather than resistance. RKN resistance is conferred by the Mi-1 gene that was introduced into commercial tomato rootstocks and cultivars from the wild tomato relative Solanum peruvianum L. (Lopez-Perez et al., 2006; Medina-Filho and Stevens, 1980). ‘Beaufort’ carries the Mi gene but exhibited tolerance to RKN (M. incognita) rather than resistance since RKN populations reproduced and increased on the ‘Beaufort’ rootstock while the fruit yield of a susceptible tomato cultivar was improved when it was grafted onto ‘Beaufort’ (Lopez-Perez et al., 2006). Our relatively high galling ratings for the interspecific rootstock ‘Multifort’ and lower galling ratings for the tomato hybrid rootstock ‘Survivor’ were consistent with the study by Lopez-Perez et al. (2006) in which ‘Hypeel45’, a processing tomato cultivar with Mi-gene resistance was found to have lower galling ratings than ‘Beaufort’.
According to a pot study by Devran et al. (2010), the root galling index for ‘Beaufort’ plants in response to M. incognita at 32°C soil temperature was significantly higher than that at 24°C soil temperature. In contrast, some other studies did not reveal a clear relationship between soil temperature and RKN susceptibility of ‘Beaufort’ and ‘Maxifort’ rootstocks (Lopez-Perez et al., 2006; Rivard et al., 2010). In our study the average soil temperatures at 10 cm depth were above 28°C during May and June 2011 with the maximum reaching 36-37°C. However, the high temperature sensitivity of Mi-mediated resistance to RKN may not have been seen in our study, as grafting with ‘Multifort’ resulted in differential responses to M. javanica infestion in organic and transitional fields during the same production season.
FRUIT YIELD. The two heirloom tomato scions ‘Brandywine’ and ‘Flamme’ exhibited differential responses to the two rootstocks used in terms of yield performance. For the cultivar Flamme in 2010, the grafted plants produced significantly lower marketable yields than the nongrafted control for the first two harvests. However, there were no significant differences in total marketable yield for the ‘Flamme’ treatments in 2010 and 2011. Reduced early yields may be an effect of the grafting process. In our study the effect of grafting on early yields was inconsistent and varied with scion cultivars and growing seasons. Surprisingly, although ‘Flamme’ was susceptible to RKN and the infestation was significantly decreased in plants grafted onto resistant rootstocks, total marketable fruit yields did not differ significantly between nongrafted and grafted treatments. There were no significant differences in marketable yield at any harvest dates for the scion ‘Brandywine’ in 2010. In 2011, there was variability in marketable yields for the ‘Brandywine’ treatments between the organic and transitional fields. In the organic field, the NGBW and BW/SU treatments produced significantly higher total marketable yields than the BW/BW and BW/MU treatments. However in the transitional field, BW/MU demonstrated significantly higher total marketable yields than the BW/BW and NGBW treatments. The BW/SU treatment resulted in statistically similar yields to all other ‘Brandywine’ treatments in the transitional field.
Some of these yield differences in 2011 may be attributed to the presence of RKN. With no RKN pressure in the 2010 trial, there were no differences in total marketable yield with either scion cultivar. However, with high RKN pressure in the 2011 transitional field, the highest marketable yield for the scion ‘Brandywine’ was achieved when grafted onto ‘Multifort’. Our results were consistent with the study by Lopez-Perez et al. (2006), in which significantly higher tomato fruit yield was observed with resistant rootstocks at high RKN (M. incognita) densities. According to Rivard et al. (2010), total and marketable tomato fruit yields were higher on interspecific hybrid rootstocks under severe RKN and southern blight disease pressure. In contrast, grafting did not exhibit any significant effect on heirloom tomato yield under low disease pressure and it was unclear if grafting onto interspecific hybrid rootstocks would be beneficial in such circumstances (Rivard and Louws, 2008).
In our study, NGBW and BW/SU preformed similarly at intermediate levels of RKN infestation in the 2011 organic field trial, whereas BW/BW and BW/MU yielded significantly less marketable fruit. It is unclear why tomato yield was reduced for BW/MU since it showed significantly lower root galling ratings compared to NGBW and BW/BW, and did not differ significantly from BW/SU in terms of RKN resistance. This response could be related to the genetic background of ‘Multifort’. This rootstock was developed from a breeding line of greenhouse rootstocks aimed at enhanced crop vigor and extended growing seasons in addition to disease resistance. It could be that the increased vigor and vegetative growth that is useful in greenhouse conditions had a deleterious effect on ‘Brandywine’ fruit yield in the shorter field- growing season. More studies are warranted to examine the influence of vigorous interspecific hybrid rootstocks on the yield of indeterminate cultivars grown in field production systems.
CROP VIGOR. Rootstock effects were also observed in leaf area and aboveground biomass evaluations. When grafted to the rootstock ‘Multifort’, the scion ‘Brandywine’ produced significantly greater leaf area and aboveground biomass than NGBW, BW/BW, and BW/SU in 2010. In the 2011 trial, the leaf area and aboveground biomass of BW/MU were significantly greater than that of BW/BW and BW/SU, but there was no significant difference between BW/MU and NGBW. For both years, FL/MU produced significantly greater leaf area than NGFL, FL/FL, and FL/SU. In 2011, aboveground biomass was significantly greater for FL/MU than all other ‘Flamme’ treatments.
The interspecific rootstocks ‘Beaufort’ and ‘Maxifort’ have been shown to increase leaf area for the heirloom tomato ‘Cuore di Bue’ when grown in a greenhouse (Di Gioia et al., 2010). The effect of the rootstock should be carefully examined when grafting is used for a specific growing condition. King et al. (2010) pointed out that disease management might be considered the most important purpose for growing grafted heirloom tomatoes by organic growers, and using more vigorous rootstocks might adversely affect crop yields. The interspecific hybrid rootstock ‘Multifort’ used in this experiment is similar to ‘Maxifort’ in terms of vigorous growth. It may be advantageous to use these vigorous rootstocks for greenhouse tomato production where season extension is strongly emphasized; however, when used in the open field, this increase in vegetative growth may not be beneficial. This is due to the shorter field production cycle in Florida where frost in early spring and late fall and hot, humid conditions in summer can limit tomato production seasons. On the other hand, the increased vigor provided by ‘Multifort’ might be related to the tolerance exhibited by this rootstock to high populations of RKN.
FRUIT COMPOSITION MEASUREMENTS. No significant differences in vitamin C, SSC, pH, TTA, and SSC:TTA ratio between treatments were found in either year for ‘Brandywine’ fruit. Few consistent effects of grafting or rootstock on quality attributes of tomatoes have been reported from previous studies. Khah et al. (2006) demonstrated that pH, Brix, lycopene content, and firmness were unaffected when the commercial tomato cultivar Big Red was grafted onto ‘Primavera’ or ‘Heman’ tomato rootstocks. Similarly, Di Gioia et al. (2010) showed that SSC, TTA, and SSC:TTA ratio were not significantly influenced by grafting heirloom tomatoes onto interspecific hybrid rootstocks. While vitamin C concentrations did not differ significantly between grafted and nongrafted tomato fruit in this study, decrease of fruit vitamin C levels as a result of grafting with selected tomato rootstocks has been observed by others (Di Gioia et al., 2010; Turhan et al., 2011; Vinkovic Vrcek et al., 2011). Other studies have also indicated that grafting could negatively affect nutritional quality of tomato fruit including the antioxidant properties (Davis et al., 2008; Edelstein, 2004; Vinkovic Vrcek et al., 2011). In contrast, some previous research has shown that grafting with certain rootstocks may have a positive impact on tomato fruit quality by increasing carotenoid content (Fernandez-Garcia et al., 2004).
Rouphael et al. (2010) pointed out that grafted fruit quality attributes may be dependent on the selection of scions and rootstocks as well as growing environment. There are many possible scion-rootstock combinations and growing environments. Clearly, more of these scenarios will need to be examined before researchers fully understand the impact of grafting with different rootstocks on fruit quality attributes. These studies will need to assess specific rootstocks for specific growing conditions in a variety of regions.
Environmental conditions may influence fruit quality more than the impact of grafting with rootstocks. It was demonstrated that total soluble solids content and titratable acidity of tomato fruit varied significantly with harvest date despite the similar levels between grafted and nongrafted tomatoes (Di Gioia et al., 2010). Our results showed that SSC values in 2010 were lower than that measured in 2011 by approximately 36.6% on average, whereas the average SSC:TTA ratio in 2010 was about 43.2% lower than that in 2011. The vitamin C level in the 2010 fruit samples was also lower than the 2011 study by 17.6%. Given the considerable seasonal variability and even the changes of fruit composition during the harvest season, multiple samplings are suggested when examining the influence of grafting on fruit composition and quality. Moreover, fruit ripening needs to be well considered in a grafted tomato study as SSC and carotenoid level often increase in tomato fruit with advancement of ripening stages (Raffo et al., 2002).
CONSUMER SENSORY ANALYSES. In the 2010 study, the consumers perceived some differences between ‘Brandywine’ fruit harvested from the nongrafted and grafted treatments in the sensory attributes evaluated. There were significant differences regarding overall appearance, overall acceptability, and tomato flavor. For the overall appearance, fruit from nongrafted ‘Brandywine’ were rated significantly higher than fruit from ‘Brandywine’ grafted onto ‘Survivor’ rootstock. Grafting onto either ‘Multifort’ or ‘Survivor’ rootstocks resulted in a significant decrease in the overall acceptability rating compared with nongrafted tomatoes. Interestingly, fruit from self-grafted ‘Brandywine’ also showed significantly lower ratings of over acceptability in contrast to the nongrafted tomatoes. Regardless of these differences detected, consumer ratings of tomato firmness and sweetness did not differ significantly between nongrafted and grafted treatments. Although grafting with the two rootstocks had a consistent negative effect on heirloom tomato flavor in the 2010 study, this trend did not persist in 2011. No significant differences were observed between nongrafted and grafted treatments for the measured sensory analysis attributes in the 2011 consumer sensory test. The 2011 season was generally drier and warmer with lower foliar disease incidence than the 2010 season. Crop vigor and tomato yields were higher in 2011 than in 2010. The grafting effect on tomato sensory attributes in response to variations in seasonal environmental conditions may deserve further studies. According to Di Gioia et al. (2010), grafting did not influence the sensory attributes “sweetness”, “sourness”, and “tomato-like taste” for the heirloom tomato cultivar Cuore di Bue.
Tomato flavor is a complex balance of sugar and acid contents with aroma volatiles (Krumbein and Auerswald, 1998). Cultural practices and environmental conditions during fruit development can affect the ratios of flavor compounds in the fruit. Harvest maturity also has a major influence on flavor in climacteric fruits like tomato (Mattheis and Fellman, 1999). When tomato fruit are picked before reaching full ripeness and ripened off the plant, they tend to have less “tomato-like” flavor intensity, are less sweet, and have more off-flavor than tomatoes ripened on the plant (Kader et al., 1977). In the present study, environmental conditions were more favorable and yields were much greater in 2011 than in 2010. As a result, more fruit were available at similar stages of development for the fruit composition measurements and consumer sensory tests in 2011. The sensory differences among ‘Brandywine’ fruit in 2010 between nongrafted and grafted plants may reflect the more variable SSC and ripeness stages of fruit in 2010.
Educational & Outreach Activities
Barrett, C.E. 2011. Organic production of grafted heirloom tomatoes: nematode management, fruit quality, and economics. Master’s thesis, University of Florida, Gainesville, FL.
Barrett, C.E., X. Zhao, C.A. Sims, J.K. Brecht, E.Q. Dreyer, and Z. Gao. 2012. Fruit composition and sensory attributes of organic heirloom tomatoes as affected by grafting. HortTechnology 22:804-809.
Barrett, C.E., X. Zhao, and R. McSorley. 2012. Grafting for root-knot nematode control and yield improvement in organic heirloom tomato production. HortScience 47:614-620.
Barrett, C.E., X. Zhao, and A.W. Hodges. 2012. Cost benefit analysis of using grafted transplants for root-knot nematode management in organic heirloom tomato production. HortTechnology 22:252-257.
Barrett, C., X. Zhao, and J.K. Brecht. 2011. Grafting does not influence the nutritional content of organic heirloom tomatoes. American Society for Horticultural Science Annual Conference, Waikoloa, HI.
Barrett, C., X. Zhao, R. McSorley, and C.A. Sims. 2011. Grafting heirloom tomatoes for improved crop vigor, yield, and fruit quality. Southern Region American Society for Horticultural Science Annual Conference, Corpus Christi, TX.
Zhao, X. 2011. Use of grafted plants to control soilborne diseases. Florida Small Farms and
Alternative Enterprises Conference, Kissimmee, FL.
Vegetable grafting exhibit at the Florida Small Farms and Alternative Enterprises Conference, Kissimmee, FL. July 2012.
This project was conducted in response to the growing interest in grafting as an innovative environmentally-friendly approach to managing devastating pest problems in vegetable production. The results of this study will help develop and promote research and education programs that investigate the potential of integrated use of grafting for tomato production especially in organic cultivation systems. Our findings provided useful, up-to-date information to both researchers and farmers with respect to the influence of grafting with selected tomato rootstocks on plant growth, fruit yield, RKN resistance, fruit quality, and economic feasibility.
This study focused on organic heirloom tomato production because of the unique opportunity for vegetable grafting to be adopted by these typically smaller growers. Heirloom tomatoes often command a higher market price than regular fresh market tomatoes. Organic produce also commands a higher price at market. Therefore, organic heirloom tomatoes offer a niche market with price premiums that afford more opportunity for growers to experiment with grafted transplants. This is especially true when using grafted transplants with high resistance/tolerance to soilborne diseases for yield improvement in fields with a history of severe infestation.
When assessing whether or not to use grafted tomato plants for RKN management, growers need to consider the severity of the RKN infestation, the growing system, and the scion and rootstock cultivars to be used. Growers interested in using grafted plants need to be aware that scion-rootstock interactions are still not fully understood. It is suggested that different grafting combinations be evaluated under site-specific conditions before incorporating this technique on a large production scale.
Nongrafted ‘Brandywine’ and grafted ‘Brandywine’ with ‘Multifort’ were considered in the cost benefit analysis. Sources and prices for materials and labor used to perform the partial budget analysis were identified for estimating the cost of producing grafted heirloom tomato transplants. A partial budget analysis was conducted using data acquired during this grafted heirloom tomato study. All phases of grafted and nongrafted transplant production were recorded to provide accurate estimates for labor, materials, and total transplant production costs. Production costs were based on a target of 1000 grafted and nongrafted transplants. Costs for constructing a simple healing chamber were based on experience gained at the University of Florida, and reflected a practical option for small growers and grafters. Although a walk-in cooler was used in this study for healing the grafted transplants, a simpler structure may be conveniently substituted.
Sensitivity analyses were conducted to compare partial net returns for grafted and nongrafted plants grown under organic and transitional to organic growing conditions. These partial net returns per plant were calculated by subtracting the cost of the transplant from the estimated return and do not account for other production costs (e.g., transplanting labor, field preparation, mulch, fertilizer, etc.). The tomato yield data used to construct the grafted and nongrafted analyses for the organic field were pooled from 2010 and 2011 to form a more representative data set. The range of tomato fruit prices used for the sensitivity analyses were derived from the published monthly average price of a 10-lb carton of organic heirloom tomato fruit (U.S. Dept. Agr., 2009).
Transplant cost analyses showed that grafted transplants required more materials, seeds, and labor and were more expensive to produce than nongrafted plants. In this study, grafted transplants cost $0.78 per plant while nongrafted transplants cost $0.17 per plant. The bulk of this price difference was associated with the price of the rootstock seeds, which accounted for 36% ($0.28/plant) of the total cost of the grafted transplants and 46% of the cost difference between grafted and nongrafted transplants. The cost of building an inexpensive but effective healing chamber was considered in the cost estimation of grafted transplants instead of the walk-in cooler that was actually used since a walk-in cooler may not be available to some growers. However, it should be noted that using a walk-in cooler as a healing chamber might help reduce the cost of producing grafted transplants by achieving a high graft survival rate and avoiding the cost of building a healing chamber.
Sensitivity analyses demonstrated that grafting may not be economically feasible when applied to fields with low RKN pressure and insignificant yield improvement as a result of grafting.
For example, nongrafted ‘Brandywine’ plants produced a mean marketable yield of 1.8 lb per plant and at that yield for the lowest tomato price per pound ($1.80/lb) the estimated partial net return was $3.07/plant. This was $1.49 more than the estimated partial net return for the mean marketable yield of grafted ‘Brandywine’ at 1.31 lb per plant at the same price of $1.80/lb. However, when high levels of RKN infestations occurred, the grafted plants demonstrated great potential for maintaining fruit yield and reducing economic crop losses. For the transitional organic field trial with high RKN pressure, the nongrafted ‘Brandywine’ had a mean marketable yield of 0.35 lb per plant with an expected partial net return of $0.46/plant at $1.80/lb of heirloom tomato fruit. The mean marketable yield of grafted ‘Brandywine’ was 1.44 lb per plant which was 311.4% higher than that of nongrafted plants. The expected partial net return per plant for grafted ‘Brandywine’ was $1.81 at $1.80/lb. This represents a $1.35/plant difference between the grafted and nongrafted estimated partial net return in the transitional field. These findings suggest that grafting could be an economically feasible approach to controlling RKN in heirloom tomato production in organic and transitional organic systems with severe RKN pressure.
In this project, we provided some grafted tomato transplants to two organic growers in Florida for evaluations of their performance including RKN resistance and fruit yields in high tunnels. A small demonstration trial was also conducted at a local small farm. Benefits and challenges of using grafting in vegetable production have been introduced to farmers through the presentation and vegetable grafting exhibit at the Florida Small Farms and Alternative Enterprises Conferences as well as the Florida Organic Growers Organic Production and Certification Workshop. Over 130 participants attended these outreach events.
Grafting could be a useful tool for growers in the transition process from conventional to organic farming systems with high populations of soilborne pathogens and nematodes. Many of the pest control strategies employed in organic or alternative cropping systems can take multiple seasons to have a beneficial effect, whereas grafting effects are immediate. Resistant rootstocks can also reduce field infestation levels for subsequent crops and provide a non-host root system in a crop rotation. Grafting may also reduce the need for expensive fumigants in conventional farming systems thereby reducing input costs. While this study focused on the use of grafting to control tomato RKN, grafting has been used successfully in the U.S. for managing bacterial wilt, fusarium wilt, and southern blight in tomato production.
The high cost associated with using grafted plants still remains as the main barrier for wide adoption of vegetable grafting in the U.S. In the case of organic production of grafted tomatoes, lowering the cost of grafted transplants and rootstock seeds could increase adoption of tomato grafting by growers. The price reduction of grafted transplants may be more important for commercial producers who rely on propagators to supply large quantities of high quality transplants. Economic analysis conducted in this study indicates that decreasing the rootstock seed cost is the most important factor for decreasing the cost of grafted seedlings. In addition, improving the graft survival rate and efficiency of grafted transplant production will also help reduce the cost of grafted plants. Simple and efficient grafting systems that are easy to operate will be of great interest to growers who are willing to graft their own plants to meet their site-specific needs.
Areas needing additional study
It is expected that more growers in the U.S. may consider using grafted transplants for soilborne disease control in the near future. Researchers, extension agents, and growers will need to work together to ensure that rootstocks suited for open-field conditions and the appropriate scion-rootstock combinations are used to optimize the benefits of this technique. Future research should be conducted in major production regions with multiple rootstocks, including different types of disease-resistant hybrid rootstocks, in order to fully elucidate the scion-rootstock interactions. This will help growers select the most suitable rootstock for their production systems and reduce the risk of economic losses.
The economic viability of grafted vegetable production will ultimately depend on farmers to identify the soilborne disease problem on their production site and choose the suitable rootstock and scion to meet their needs. Further work should examine local production methods and costs to provide accurate information on costs and returns of grafted vegetable production for growers in diverse environments. More rootstocks should be developed for open-field production as the vigorous rootstocks that are adapted to the greenhouse conditions may not be suitable for field production. Early harvests may be more important to open-field tomato growers to capture highest market prices.
The impact of rootstocks on fruit quality at harvest and during postharvest also deserves further studies. Grafting may affect the maturation of tomato fruit. Grafted plants can have delayed early harvests compared to nongrafted plants because the grafting and healing processes may delay fruit development. Harvest time has been shown to markedly affect the consumer perceived appearance of tomato fruit. It would be advisable to conduct consumer sensory tests using fruit from different harvests during the production season to fully elucidate the influence of grafting on tomato sensory attributes. More in-depth studies are warranted to assess the effects of grafting per se and the use of rootstocks on fruit ripening and shelf life.