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

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

Introduction

Grafting is a valuable technique to reduce damage caused by biotic and abiotic stressors in agricultural systems. Americans have typically limited the use of this practice to perennial crops such as fruits and ornamentals, where the graft is made during the dormant season. However, the worldwide use of soft-tissue grafting is evident, as increased sustainability as well as productivity has become more important worldwide in fruit-bearing vegetable production systems. The cultivation of grafted vegetable plants began in Korea and Japan at the end of the 1920’s when watermelon plants were grafted onto squash rootstock. Since this time, this technique has spread throughout Asia and Europe. Currently, 81% of Korean and 54% of Japanese vegetable cultivation uses grafting (Rivero, 2003).
Grafting can take place on a number of crops. However, because of the added expense, it is typically associated with melons, cucurbits, and members of the Solanaceae family such as eggplant and tomato. In many Mediterranean countries, this technique is being used to control root-knot nematodes, bacterial wilt, and other soil-borne pathogens, as an alternative to methyl bromide applications for eggplant as well as melon and tomatoes (Ioannou, 2001).
There are a variety of methods for grafting vegetable crops. Tube grafting has caught on as the primary method for vegetable grafting on the farm as it can be easily carried out with small healing chambers, and typical success rates for this technique are between 85 and 90 percent (Oda, 1995). Tube grafting takes place when the scion and rootstock are severed as seedlings and reattached with a small, silicon tube or clip. This technique has been highly effective as it can be carried out when plants are very small, thereby eliminating the need for large healing chambers while increasing throughput. Grafted vegetable crops have been used extensively in high tunnel production systems as a way to increase sustainability. Because these operations typically do not use extensive crop rotation, high levels of soilborne disease can lead to significant yield losses. Conventional “non-grafted” operations rely heavily upon chemical fumigants to decrease soilborne disease levels. As a result, the implementation of grafting can reduce disease levels in operations where fumigants are either unwanted or unavailable (Oda, 1999).
Although grafting has become increasingly important for all soilborne diseases, it is fundamental in reducing damage caused by bacterial wilt (Ralstonia solanacearum) of tomato. This particular disease requires long rotation intervals to successfully eliminate primary inoculum. Furthermore, breeders continue to struggle as resistance often leads to poor fruit quality. Thus far, attempts to uncouple these traits have been relatively unsuccessful. Grafting has been essential in Asian horticultural production for eliminating bacterial wilt incidence in solanaceous crops (Oda, 1999). It has also been used in tropical environments, like Brunei, where bacterial wilt incidence is so high that tomatoes cannot be planted unless the soil is sterilized or resistant rootstocks are implemented (Peregrine, 1982). In India, CRA 66 rootstocks were used to reduce bacterial wilt in tomatoes, and plant survivability rates at 1st harvest increased from 54.5% in the control to 100% (Tikoo, 1979). By the end of the season, none of the control plants had survived while 100% of the grafted plants continued to produce. Furthermore, the yield of the tomatoes with resistant rootstocks was 4 times that of the susceptible lines. This particular line was also identified for use against bacterial wilt in Germany and similar results were found (Grimault, 1994). Several Hawaiian lines (Hawaii 7996-7998) have been identified as suitable candidates for resistance to bacterial wilt (Oda, 1999). This technique could be a very valuable tool for eliminating bacterial wilt in tomato, pepper, and eggplant production systems.
Although the use of grafted vegetables is associated with disease reduction and/or abiotic stress, yield may be increased even without the presence of these identified stressors. Yields increased by as much as 106% compared to the control in watermelon production systems in Australia (Yetisir, 2003). Data which further supports this idea is sparse. However, in cases where lines have been bred specifically to be used as rootstocks, yield increases are evident. This trend indicates the need for further research and development of this technique. By increasing yields even without the presence of specific disease pressure, grafting can be an economically viable method for improving production.
Agriculture in America is changing. Everyday, organic practices are more widely encouraged in US markets. Furthermore, the reappearance of the farmer’s market as a viable marketing tool, has made small-scale sustainable farming ever more popular. Heirloom tomatoes make up a significant portion of these markets, but their production can vary based upon the presence or absence of disease epidemics in the field. The occurrence of bacterial wilt and other soilborne diseases consistently plague heirloom tomato growers in the southeastern United States. This problem will continue to grow as Early Blight, caused by Alternaria solani, has forced many organic tomato production systems to move inside of greenhouses and other tunnel systems. This practice may ultimately lead to increased severity of bacterial wilt and other soilborne diseases as crop rotation is limited in these instances.
Because this technique follows sustainable as well as certified organic practices, it is essential to integrate this method into our organic production systems. Grafting can be used by organic growers as a way to combat bacterial wilt as well as a number of other soilborne pathogens. Using grafted transplants also coincides with economic and social sustainability in agriculture as well as environmental sustainability. With the growing availability and encouragement of local markets, the use of greenhouses and other intensified farming methods has increased in the United States. This system leads to many of the same problems that brought about grafting in other countries, and it is fundamental that we bring these ideas to the forefront in our own.