Fresh-market tomato (Solanum lycopersicum) production is a vital part of the agricultural economy in Florida and Virginia. The fresh-market tomato industries in Florida and Virginia rank first and fourth nationally, respectively, and have a combined farm gate value of over $583 million (USDA-NASS, 2011). Bacterial wilt of tomato caused by the soilborne bacterium Ralstonia solanacearum race 1 (biovar 1, phylotype II) is widely distributed in the southeastern United States and causes considerable economical losses under ideal conditions for disease incidence (Ji et al., 2007). Once established, R. solanacearum can survive in the soil on the roots of asymptomatic plants, weeds, and plant debris (Elphinstone et al., 1998; Granada and Sequeira, 1983; Hayward, 1991). Although most soilborne pathogens have been traditionally managed with soil fumigants, this strategy has been minimally effective against bacterial wilt (Chellemi et al., 1994; Enfinger et al., 1979). Crop rotation as a disease management strategy is effective but can be difficult because R. solanacearum can infect over 200 plant species (Hayward, 1994). Although resistance is available in tomato cultivars Hawaii 7996, Hawaii 7997, and Hawaii 7998 (Chellemi et al., 1994; Grimault and Prior, 1993; Grimault et al., 1995; Scott et al., 1989), these cultivars have not been widely accepted as a result of poor horticultural traits such as small fruit, a trait linked with bacterial wilt disease resistance (Opena et al., 1990; Wang et al., 1998).
Grafting has been practiced for decades in Asia as a technique to manage soilborne diseases and in 2003 grafted plants accounted for 81% and 54% of the vegetable acreage in Korea and Japan, respectively (Lee, 2003). Grafting has recently been gaining popularity in the United States, partly as a result of the loss of methyl bromide and the increased restrictions of using soil fumigants. Several studies have demonstrated the effectiveness of grafting to manage bacterial wilt in tomato but all have used open-pollinated breeding lines that are not widely available (Lin et al., 2008; Rivard and Louws, 2008). The level of bacterial wilt resistance in available hybrid lines is unknown. It is also unclear as to whether hybrid rootstocks would aid in increasing tomato fruit yield.
Previous research has indicated that resistance to bacterial wilt in tomato does not prevent the pathogen from entering the host root system but reduces its spread into the stem vasculature (Grimault and Prior, 1993; Prior et al., 1996). In the case of ‘Hawaii 7996’, a restriction in the movement of the pathogen from the protoxylem to other xylem tissues has been noted. As a result of this restriction, the incidence of bacterial wilt on a susceptible scion ‘Ponderosa’ grafted onto a ‘Hawaii 7996’ rootstock was significantly less compared with non-grafted and ‘Ponderosa’ grafted to a susceptible scion (Nakaho et al., 2004).
The objectives of this study were to 1) determine the level of bacterial wilt resistance present in new hybrid rootstocks with possible resistance to bacterial wilt; 2) assess the ability of the grafted plants in reducing bacterial wilt in tomato field production; and 3) determine the effect of the grafted plants on tomato fruit yield.
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