Grafting Tomato to Manage Soilborne Diseases and Improve Yield in Organic Production Systems

Abstract

The use of grafted tomato for commercial production has been implemented worldwide, where soilborne disease pressure is high. Grafting has been 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. With the increased direct-marketing avenues available to small, sustainable farmers, demand for vine-ripened organic heirloom varieties has also increased. 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 a major component in an integrated approach to reduce soilborne disease and increase crop productivity in organic heirloom tomato production. Because this research relies heavily on well-developed international techniques and practices, an extension objective was declared to better disseminate information regarding grafting benefits and technique, and to facilitate local adoption of this technology.

During 2005 and 2006, field trials were implemented 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. 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. Rootstock-specific cultivar, 'Maxifort', showed no symptomatic plants for fusarium wilt (caused by Fusarium oxysporum f.sp. lycopersici) in organic production, and non- and self-grafted controls had 45-50% disease incidence. Verticillium wilt is a severe endemic problem in the mountain growing regions of North Carolina due to the lack of genetic resistance against race 2. 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 (P=0.005).

Grafting could be a vital component in commercial organic production of heirloom tomato. The unification of heirloom scion with rootstock that confers disease resistance, tolerance to abiotic stressors, and enhanced vigor may be a valuable tool for organic and conventional growers in the United States.