Vegetable Grafting: History, Use, and Current Technology Status in North America Chieri Kubota and Michael A. McClure Department of Plant Sciences, The University ofArizona, 303 Forbes Building, Tucson, AZ 85721-0036 Nancy Kokalis-Burelle, Michael G. Bausher, and Erin N. Rosskopf U.S. Horticultural Research Laborator y, U.S. Department of Agriculture, Agriculture Research Service, Fort Pierce, FL 34945 .4J/ihunal i,u1v n-or(Is. automation, controlled environment, cucurbit, methyl bromide, root-knot nematode rootstock, scion, Solanaceae Abstract. Grafting of vegetable seedlings is a unique horticultural technology practiced for many years in East Asia to overcome issues associated with intensive cultivation using limited arable land. This technology was introduced to Europe and other countries in the late 20th century along with improved grafting methods suitable for commercial production of grafted vegetable seedlings. Later, grafting was introduced to North America from Europe and it is now attracting growing interest, both from greenhouse growers and organic producers. Grafting onto specific rootstocks generally provides resistance to soilborne diseases and nematodes and increases yield. Grafting is an effective technology for use in combination with more sustainable crop production practices, including reduced rates and overall use of soil fumigants in many other countries. Currently, over 40 million grafted tomato seedlings are estimated to be used annually in North American greenhouses, and several commercial trials have been conducted for promoting use of grafted melon seedlings in open fields. Nevertheless, there are issues identified that currently limit adoption of grafted seedlings in North America. One issue unique to North America is the large number of seedlings needed in a single shipment for large-scale, open-field production systems. Semi- or fully-automated grafting robots were invented by several agricultural machine industries in the 1990s, yet the available models are limited. The lack of flexibility of the existing robots also limits their wider use. Strategies to resolve these issues are discussed, including the use of a highly controlled environment to promote the standardized seedlings suitable for automation and better storage techniques. To use this technology widely in North American fresh vegetable production, more information and locall y collected scientific and technical data are needed.
Grafting of herbaceous seedlings is a For members of the Solanaceae, the first tomatoes to jimson weed was not introduced unique horticultural technology practiced record was of eggplant (Solanum inc/on gena commercially, and apparently disappeared, for many years in East Asia to overcome L.) grafted on scarlet eggplant (Solanum presumably as a result of the potential trans- issues associated with intensive cultivation integri/bliurn Poir.) in the 1950s (Oda, port of small amounts of alkaloids to the using limited arable land for vegetable pro- 1999). Grafting tomato (Lvcopersicon escu- fruits, as experimentally proved by Lowman duction. According to Lee and Oda (2003), lenturn Mill.) was introduced commercially in and Kelly (1946), and also because of the a self-grafting technique to produce a large the 1960s (Lee and Oda, 2003). Along with the labor-intensive propagation process. gourd fruit by increasing root-to-shoot ratio rapid development of intensive protected cul- Intensive labor input and resulting high through multiple graftings was described in tivation technologies using high tunnels and costs of grafted seedling production have an ancient book written in China in the 5th greenhouses, which presumably prevented been issues preventing this technology from century and in Korea in the 17th century. The farmers from continuing traditional crop rota- being widely adopted outside of Asia. How- first record of interspecific, herbaceous graft- tion, vegetable grafting became a crucial tool ever, along with the development of efficient ing as a yield increase and pest/disease con- to overcome soilborne diseases and other commercial production techniques for trol strategy was for watermelon [Citrullus pests. In the 1990s, nearly 60% of open fields grafted seedlings and the introduction of lanatus (Thunb.) Matsum.& Nakai], using a and greenhouses in Japan producing musk- new rootstocks with desirable traits compat- squash rootstock (Cucurhita rnoschata melon (Cucumis melo L.), watermelon, ible with locally selected scions, grafting Duch.), reportedly developed by a water- cucumber, tomato, and eggplant were report- technology was introduced to European melon farmer in Japan (Tateishi, 1927). This edly planted with grafted seedlings (National countries in the early 1990s (Oda, 2002) watermelon grafting technique was quickly Research Institute of Vegetables, Ornamental mainly through marketing efforts of interna- disseminated to farmers through extension Plants and Tea, 2001) and 81 % in Korea (1990 tional seed companies and through informa- research programs of regional agricultural data reported by Lee, 1994). Today, over 500 tion exchanges among research communities. experimental stations in Japan, and then later million grafted seedlings are produced annu- The major objectives of using grafted seed- into Korea, during the late 1920s and early ally in Japan (Kobayashi, 2005). One of the lings are: I) to achieve resistances to soil- 1930s. Use of grafted seedlings in commer- large-scale grafting operations in Japan is borne diseases and nematodes; 2) to increase cial vegetable production occurred as early shown in Figure 1. yield and quality; and 3) to improve the as the 1930s in Japan for watermelon grafted Although limited information is available, physiology of plants making them more on Lagenaria siceraria (Mol.) Standl. (Oda, it seems that grafting of tomato was practiced adaptable to harsh environments. Conse- 2002). Research on grafting cucumber at a limited scale in the United States more quently, many countries in Europe, the Mid- (Cucumis sativus L.) also started in the late than 60 years ago. According to Lowman and dle East, Northern Africa, Central America, 1920s, but wider commercial applications did Kelly (1946), grafting tomatoes using jimson and other parts of Asia (other than Japan and not happen until 1960 (Sakata et al., 2008). weed (Datura strarnoniuni L.) as rootstock Korea) adopted the technology and the areas was practiced for many years in the southern introducing grafted plants increased rapidly United States to overcome root-knot nemat- during the past two decades. odes. Isbell (1944) recommended this Until recently, grafted seedling produc- CEAC paper number D-302430-10-07. We thank the USDA CSREES (Project # 2007- method to home gardeners based on their tion and its use were not common in North 51102-03822) for partial support. research conducted from 1935 to 1943 on America. The exception was the home garden To whom reprint requests should he addressed; grafting tomato, eggplant, and sweet pepper practice in the southern United States as e-mail [email protected] on selected weeds. Nevertheless, grafting described previously, and there have been I HORTSC1ENCE VOL. 43(6) OCTOBER 2008 Fig. 1 Graftin g operations iii Canada (A) and in Japan (B). Each operation has the capacity to produce more than 10 million grafted seedlings annually. small numbers of organic growers who prac- lings were used for trials in open fields (less American propagators use a humidification ticed this technique by themselves to over- than 100,000 plants). chamber or a simple covered structure inside come soilborne diseases and pests in their Many of the propagators supplying a shaded greenhouse, whereas more sophis- small operations (M. Peet, personal commu- grafted seedlings to greenhouse growers are ticated indoor healing units with better air nication). Recently, along with the success located in Ontario or British Columbia, temperature, relative humidity, and light of European-based, large-scale greenhouse Canada (Fig. 1). These propagators expanded intensity control capability are commercially operations in North America, the improved their business successfully along with the available in other countries. This is probably yield and fruit quality by using grafted seed- success of greenhouse tomato growers. because tomato grafting and healing methods lings became known to more growers. The Because there are only small experimental are relatively easy and tomato is the major majority of users of grafted seedlings is propagation capabilities in the United States, crop currently grafted in North America. currently greenhouse hydroponic tomato large-scale propagators, each with 10 After healing, grafted tomato seedlings growers, whereas it is still a relatively million seedling production capacity in are acclimatized inside the greenhouse and unknown technique for open-field producers. Canada, are still the major sources of the then often pinched to induce two lateral Recently, several trials in North America grafted seedlings used in the United States shoots. This "two-headed" grafted seedling have been initiated using grafted seedlings and northern Mexico. Many organic farmers is widely accepted in greenhouse production for open-field vegetable production. Strong and growers who produce in small green- systems where it reduces the production cost marketing efforts by seed companies and house operations graft their own tomato while maintaining the yield per head (and genuine interest from practitioners in inte- seedlings because there are no local propa- therefore overall yields) (Kubota, 2008: grated pest management have driven collab- gators available. P. Rorabaugh, unpublished data). oration with producers, universities, and Gm/ted tomato seedling production and Among the North and Central American other research institutions. The authors cur- use. In most propagation operations in North countries, use of grafted tomato seedlings in rently work as an interdisciplinary team to America, grafting of tomato is accomplished open fields seems to be most advanced in develop necessary technologies, collect local by using an elastic plastic tube that holds the Mexico. After 2 years of successful trials, information, and conduct trials in different graft union cut at an angle (so-called "tube tomato producers in Mexico have used climatic zones in the United States consider- grafting method"). Each plastic tube has a slit grafted seedlings on 1250 acres (500 ha) to ing grafted plants as a means to mitigate yield so it falls off as the stem expands in diameter. overcome Fusarium oxysporum f. sp. lyco- losses to pathogens and to partially replace Grafting speed using this tube grafting persici race 3, thereby attracting the interest methyl bromide soil fumigation. In Mexico, method varies from 300 to 500 grafts per of other producers (F. Knol, personal com- Guatemala, and other countries, similar but hour depending on the worker s skill munication). Nevertheless, a recent outbreak larger-scaled projects led by the United (J. Marie, personal communication). Grafting of seedborne bacterial disease in a grafting Nations Industrial Development Organiza- is usually performed 2 to 3 weeks after trial in Mexico alerted propagators and pro- tion contributed to disseminating information seeding. Some rootstock seeds exhibit slower ducers to the seeds and seedlings from unreli- on the efficacy of vegetable grafting germination and require seeding 1 week able sources. Especially when the available (UNIDO, 2007). earlier than the scion to develop compatible sources of grafted seedlings and rootstock stem sizes at the time of grafting. Propagators seeds are limited and, therefore, grafted seed- CURRENT USE OF GRAFTED need to optimize the seeding schedule for lings travel a long distance, both propagators SEEDLINGS each selected combination of scion and root- and producers must be cautious to prevent stock so as to maximize the grafting success. accidental introduction of diseases and Although estimating the growing number Grafting young seedlings is considered viruses through transportation of seeds and of grafted plants used in North America is advantageous as a result of the smaller size seedlings. challenging, surveys conducted by faculty at of seedlings, which allows greater seedling Although there are many tomato root- the University of Arizona in 2002 and 2006 density and reduces the cost. After grafting, stock varieties available worldwide, cur- showed that the total number of grafted seed- seedlings are placed under high humidity rently it seems that only one or two hybrid lings used in North America was over 40 (greater than 95%), ambient temperature tomato rootstock varieties dominate the million with the majority of these used in (27 to 28 °C), and low light intensity (cclOO North American grafting rootstock market. hydroponic tomato greenhouses. A relatively .tmolm 2 . I photosynthetic photon flux) for This is probably the result of their early small number of grafted watermelon seed- 4 to 7 d to heal the grafted union. Many North introduction and success in greenhouse
HORTSCiENCE VOL. 43(6) OCTOBER 2008 1665 tomato hydroponic production using the spe- drawback of the lvii gene, however, is its Limitations of propagators e.\ /)erieflce cific vigorous rootstocks. Making other alter- sensitivity to temperature, and failure of with grafting. The majority of propagators native rootstocks available would help resistance can occur at high soil temperature capable of grafted vegetable seedling pro- develop long-term strategies for pest and (Dropkin, 1969; Williamson, 1998). Nema- duction are located in Canada and Mexico. disease control, particularly for introduction tode isolates capable of breaking Mi resis- Currently, U.S. propagators have limited olgrafting into open-field tomato production. tance have also been identified in many areas experience. Furthermore, a limited number Grafied cucurhit seedling production and of world (Williamson, 1998). loannou (2001) of university extension and research institu- use. Several commercial trials using grafted examined a tomato rootstock for grafting tions are familiar with the specific techniques watermelon and muskmelon seedlings were eggplant and found reduced efficacy of graft- of grafted seedling production. Conse- conducted in multiple locations across the ing against RKN in summer crops, presum- quently, many propagators in North America country to increase yield, quality, or to ably as a result of the high soil temperature. had to learn grafting technologies from prac- demonstrate grafting as a potential partial More information is needed on the effective- titioners in countries such as Korea, Spain, alternative to soil fumigants. Unlike mem- ness of various tomato rootstocks under high The Netherlands, and Israel. bers of the Solanaceae, in which tube grafting soil temperatures (critical temperatures are Limited access to information on the pro- is the single dominant grafting method, generally above 28°C) (Dropkin, 1969). Wild- cess of grafting seedlings and production cucurbit species are grafted using many type-based tomato rootstocks may have other scheduling only available in languages other different methods, including approach graft- resistance genes that are stable at higher soil than English is an additional issue. A short- ing, cotyledon grafting (a type of splice temperatures. Such high temperature-resis- age of well-trained propagators with experi- grafting), cleft grafting, and hole-insertion tant genes were reportedly found in Lycoper- ence in grafted seedling production also grafting (Lee and Oda, 2003). Some propa- sicon spp. (Williamson, 1998). Therefore, limits wider application in North America. gators use unrooted cuttings as rootstocks resistance screening tests need to be con- Education and dissemination of information harvested before grafting, and root the ducted at varied temperatures using RKN through workshops and short courses is impor- grafted cuttings while the grafted unions are species specific to the location in which tant but will reach only a limited number of healed. These nonstandardized procedures grafted plants are to be introduced. Such stakeholders. The authors are in the process for cucurbit grafting also make technology information is particularly critical for apply- of developing an informational web site to transfer challenging in cucurbits. Many ing grafting technology in Mexico and the reach a wider audience and a larger number North American propagators still rely on southern United States, including Florida and of stakeholders. approach grafting, the slowest but most south Texas, to identify overall resistance of Handling large numbers of grafted assured method for cucurbits. rootstocks to multiple pests and diseases seedlings. Another issue unique to North under subtropical conditions. America is the large number of seedlings CURRENT AND FUTURE To our knowledge, cucurbit rootstocks needed in a single shipment. The labor avail- TECHNOLOGY NECESSARY TO with RKN resistance are not commercially able within a propagation operation will limit WIDELY ADOPT GRAFTING IN available, but there are a few species that are the number of plants available for delivery at NORTH AMERICA promising candidates. Igarashi et al. (1987) a given time. One solution is the development examined several wild Cucumis species, of short-term seedling storage techniques, Wider use of grafted vegetable seedlings including Kiwano (Cucumis metuliferus which can distribute available labor input has considerable potential in North America. Naud.), for resistance to RKN and grafting for grafting over time while targeting a However, several issues described subse- compatibility to muskmelon. Considering all narrow shipping window. Tomato and egg- quently have been identified as major limi- aspects, including fruit size and yield, Igarashi plant seedlings can be stored at low temper- tations. et al. (1987) concluded that C. metuli/drus is atures under dim light for up to 4 to 6 weeks Limitation of available rootstock the most suitable rootstock exhibiting good (Kubota, 2003), but storage protocols are not information. Greenhouse tomato constitutes resistance to RKN. Recently, a research well developed for cucurbits. Lowering more than 90% of grafted seedling produc- group in California (Siguenza et al., 2005) temperature can slow the respiration and tion in North America (Kubota, 2008). The reported that C. metuliferus can be used as a metabolic processes and thereby prevent un- majority of these seedlings are currently rootstock for muskmelon to prevent both desirable quality degradation. In postharvest grafted to one or two common rootstocks. plant growth reduction and nematode popu- storage, temperature is generally selected to There is only limited information on the use lation increases in M. incognita-infested soil. be the lowest possible temperature that of other rootstocks, compatibility to open-field Sigüenza et al. (2005) also found that does not cause chilling injury (CI) to the cultivars, and field performance of grafted C. moschata rootstock, a traditional rootstock produce. For storage of seedlings, avoiding seedlings in various climatic conditions. used for cucurbits in Asia, had a high level of Cl and maintaining photosynthetic and Resistance information of rootstocks avail- tolerance to M. incognita. regrowth abilities are necessary. Both tem- able from seed companies may not necessar- Information on resistance/tolerance of perature and light environments are impor- ily contain all the resistance/tolerance commercially available rootstocks and tant to optimize storability of seedlings potentially exhibited in local soil and envi- potential genriplasm usable for breeding (Hems et al., 1992; Kubota, 2003). Grafting ronmental conditions. One example is the new rootstocks needs to be collected based on chilling-tolerant rootstocks may extend recent finding that muskmelons grafted to on locally conducted trials and research. the storability of the seedlings and therefore interspecific hybrid squash (Cucurbita Germplasms that are considered as weeds information should be collected for different maxima x Cucurhita moschata) had resistance elsewhere are sometimes introduced as new graft combinations. to vine decline caused by Mono.sporascus rootstocks. For example, selected lines of The introduction of mechanization and spp. (Cohen et al., 2000: Edelstein et al., Solanuin torvum Swartz (Turkey berry, a automation technology will also help address 1999) and tolerance to charcoal rot (ivIacro- weed widespread in Florida) from germp!asm large-scale production issues. Efficient labor phominaphaselina) ( C. Kubota et al., unpub- collected from Puerto Rico and Thailand management has been recognized as a key lished data). have been used for grafting solanaceous to success in mass production of grafted Other rootstock information missing but plants in Japan (Takii Seeds, personal com- seedlings. Semi- or fully-automated grafting critical in applications to North America is munication). The vigorous growth character- robots were invented by several agricultural efficacy over root-knot nematode (RKN) istics of S. torvum in various ecosystems is machine industries (Kurata, 1994) and some (Meloidogvne spp.). Many tomato rootstocks an attractive feature to introduce for Solana- models are available in East Asia, Europe, contain the tv/i gene and have been incorpo- ceae rootstocks. Introduction of biotechnol- and more recently in the United States. rated into many tomato cultivars for resis- ogy to rootstock development has been also According to Kobayashi (2005), the first tance to RKN as demonstrated using attempted by researchers (e.g., Gal-On et al.. commercial model of a grafting robot Beaufort rootstock by Cao et al. (2005). A 2005). (GR800 series; Iseki & Co. Ltd., Matsuyama,
1666 HORTSCIENCE VOL. 43(6) OCTOBER 2008 Japan) became available for cucurbits in 1993 and there were various semi- and fully w automated grafting robots presented from nine different agricultural machine industries S 4,L at an international horticultural trade show in 4 Tokyo in 1996. Kobayashi (2005) also noted that grafting robots were developed in other countries, and a semiautomated system suit- able for multiple species, including cucurbits. - members of the Solanaceae, and roses, be- came available from Arnabat S.A. (Barcelona. Spain) in 2000. Another semiautomated grafting robot for cucurbits, similar to lseki s GR800, was developed in Korea in 2004 (GR-600; Helper Robotech Co., Gimhae, Korea). For fully automated grafting robots, in 1994, Yanmar Agricultural Equipment Co. (Osaka, Japan) introduced an AGI000 robot for grafting solanaceous plants based on a 128-cell tray system, in which an entire row of eight seedlings could be grafting at one time, achieving a speed of 1200 grafts per hour with one assisting operator. Unfortu- nately. many of these grafting robots intro- duced to commercial propagators in the late 1990s and the early 2000s are under-used in operations. Kobayashi (2005) pointed out several issues, including challenges in pro- Fig. 2. A prototype of a fully automated grafting robot for cucurbits with a capability of 750 grafts per hour ducing scion and rootstock seedlings that (courtesy of BRAIN, Saitama, Japan). consistently meet the size and quality spec- ifications required by grafting robots. Along with internationalization of grafting technol- ogy and emerging demand from counties like the EPA (1997), fumigation with methyl ability without experiencing negative impact the United States where labor management is bromide costs $0.41 to $0.92 per plant, for the 4-d simulated transportation. Seed- a significant issue in agriculture and horticul- almost comparable to the current cost for lings at 18 °C exhibited serious quality tural operations, improving grafting robots buying a young grafted seedling produced in deterioration, delay in early season growth and seedling production technology adopt- Canada ($0.90 per a "double-headed" plant and development, loss of flower buds on the able to automation is an immediate need. or $0.45 per head), although the cost of the first truss, and yield reduction (Kubota and Several groups of researchers and engineers grafted plant does not account for other pest Kroggel. 2006). Transportation conditions in Japan currently work on improvement of control tactics such as effective herbicide need to be modified according to plant the relatively old grafting robot technology or packages required in the absence of methyl growth stage and plant quality at the time of on development of more flexible grafting bromide. Further investigation and economic transportation. Air freight is an alternative robots. One such effort is a fully automated analyses are necessary taking local eco- transportation means for smaller seedlings grafting robot for cucurbits, which was nomic, agronomic, and pathologic situations and plugs but has less flexibility in control- recently developed based on the 1993 version into consideration. The University of Arizona ling temperature than refrigerated trucks. of Iseki s GR800 semiautornated robot also conducted a yield comparison in tomato Trailers temperature can be selected to pre- (K. Kobayashi and K. Shigematsu, personal and found an increase of15°o when grafted vent Cl while minimizing the physiological communication). The prototype model (Fig. plants were compared with nongrafted seed- deterioration of plants resulting from exces- 2) has scion and rootstock feeders, which lings in both a greenhouse (P. Rorabaugh, sive respiration and ethylene accumulation pick, orient, and feed the scion and the unpublished data) and in a small field trial during transportation. The vapor pressure rootstock shoots to the grafting processor, with low disease pressure (C. Kubota, unpub- inside trailers is generally very close to performing 750 grafts per hour with a 90°o lished data). It may be possible to pair saturation (100% relative humidity) as a success rate. The new robot will be commer- grafting with effective, site-specific herbicide result of the floor-to-ceiling loading of seed- cially available in a few years. packages to accomplish production with lings. This high humidity sometimes causes High production costs. Our recent survey yields similar to those achieved in open-field rapid spread of disease. Damage to grafted found that the price of tomato seedlings for production using soil fumigation, which seedlings resulting from an unpredicted event fresh market tomato production is currently would give growers an incentive to use such as an unanticipated prolonged duration $0.03 to $0.40 per seedling (excluding seed grafted seedlings. of transportation or disease epidemic could costs), whereas the current price of grafted Long-distance transportation. As a result result in a long-term impact on plant growth melon and tomato seedlings is $0.60 to $0.90 of the limited availability of grafted and yield. in addition to seed costs. The high cost of seedlings, greenhouse tomato growers often New controlled environment technology. grafted seedlings is the result of intensive purchase grafted seedlings from distant One critical item necessary for successful labor input for propagation, a longer pro- propagators, risking deterioration of trans- grafted seedling production is uniformity of duction period, and the additional costs of the plants during transportation and the conse- seedlings used for scion and rootstock. One rootstock. Those expenses often discourage quential delayed growth or fruit development way to improve the uniformity is the use of potential users of grafted seedlings. Not often (Kubota and Kroggel, 2006). It has been automatic sorting machines equipped with a acknowledged is that growers may be com- observed that lower temperatures and illumi- machine vision system as successfully used pensated for the greater initial cost of buying nation significantly maintained transportabil- in Europe and Canada. Another approach is grafted seedlings by additional benefits of ity of the seedlings. For mature tomato the use of production systems under artificial increased yield and reduced cost of control seedlings at flowering stage, simulated trans- lighting. The latter can contribute to the measures for soilborne pests. According to port at 6 to 13 °C showed the best transport- ability to manipulate production scheduling.
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