Volume 60 - Tomato Genetics Cooperative - University of Florida

acteria (vascular or not) and even fungi (vascular) within single tomato genotypes.
Hence, Scott (1997) advised that when searching for resistance to a given bacterial
pathogen, breeders should not overlook genotypes resistant to other bacteria. In this
regard, resistance genes can emerge by combining genotypes with some reported
bacterial resistance even when one of the parent lines does not show resistance to a
particular disease/race. For instance, Hutton et al. (2010) reported that an allele from
„Hawaii 7998‟ was associated with resistance to bacterial spot race T4 even though this
line was susceptible to race T4, suggesting epistasis with genes from the other parent
that was T4 resistant. Another possibility is that the pleiotropic nature of resistance in
tomato against several diseases could be associated with higher level of expression of
systemic acquired resistance (SAR). Lin et al (2004) found over-expression of
Arabidopsis NPR1 (non-expresser of PR genes) gene in a susceptible tomato line could
enhance resistance to both Fusarium wilt race 2, bacterial wilt, as well as bacterial spot
and gray leaf spot. In this study, they did not test the enhanced resistance against the
bacterial canker pathogen.
Lastly, Rouamba et al. (1988) tested material resistant to bacterial wilt towards a
fourth vascular disease, the Verticillium wilt caused by Verticillium race 2, but they found
only a loose relationship between both resistances since only two („IRAT L3‟ and
„Farako-Ba‟) out of ten lines tested, were resistant to both diseases.
5. Grafting of susceptible cultivars on resistant rootstocks, an alternative to
resistant cultivars
Given the difficulty to create highly resistant lines with good commercial quality,
grafting susceptible scions on resistant rootstocks remains an alternative to the
cultivation of resistant cultivars. As early as 1969, Gilbert and Chin pointed out that
highly resistant tomato lines with poor fruit quality, could be efficiently used as
rootstocks on which susceptible scions of good fruit quality could be grafted. These
authors reported the bacterial wilt resistance of the root system as being effective even
when completely susceptible scions are used. This technique is still used nowadays
(Cardoso et al., 2006; Wang et al., 2009), though the protection provided by the
rootstock is sometimes incomplete (Nakaho et al., 2004). Indeed, tomato resistant
material harbours the bacteria symptomlessly and the resistance is associated with the
ability of the plant to restrict bacteria invasiveness (Grimault et al., 1993). The absence
of incompatible interactions in tomato resistant lines (no symptoms, no latent infection)
has been confirmed by Lebeau et al. (2011) by testing a core collection of bacterial wilt
resistant accessions with a core collection of bacterial strains.
Eggplant is an alternative rootstock for cultivating susceptible tomatoes in
contaminated conditions. It was shown to provide a better protection than tomato
rootstocks (AVRDC, 1998). This result was confirmed and extended by Lebeau et al.
(2011) who found that apart from common cases of latent infection for some eggplant
accessions and all tomato accessions, there also exist incompatible interactions
between some eggplant resistant lines and some bacterial strains. Further, some
eggplant lines control bacterial strains that are not controlled by any of the tomato
resistant lines tested so far, as exemplified by Carmeille et al. (2006b), Wicker et al.
(2007) and Lebeau et al. (2011).
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