Volume 60 - Tomato Genetics Cooperative - University of Florida

esistance, and breeding in geographical areas where different strains of bacterial wilt
are prevalent, the breeders exploited the genetic potentialities at their disposal, and
created material resisting a wide range of bacterial wilt strains as exemplified by the
results obtained in the worldwide trial carried out by Wang et al. (1998). Indeed, the top
nine resistant accessions which had high levels of resistance in almost all 12 locations
tested (>90% survival on average) were developed in Hawaii („H7996‟, „H7997 S and
L‟, „H7998 S and M‟), Philippines („TML46‟ and „TML114‟, „R3034‟), and Japan („BF-
Okitsu‟).
Other sources of resistance in wild tomatoes have been described sporadically in
the literature in accessions of the same species (S. pimpinellifolium, cherry and pear S.
lycopersicum) as well as in other wild relatives of tomato (Laterrot & Kaan, 1977;
Jaworski et al., 1987; Anaïs, 1997; Mohamed et al., 1997; Carmeille et al. 2006b; Hai et
al., 2008). From these results, it seems that resistance to bacterial wilt is not that
frequent in tomato germplasm. The high genetic diversity displayed by Ralstonia
solanacearum complex (Fegan & Prior, 2005) and the strong interactions between
strains and resistant material (Lebeau et al., 2011) suggest that various resistance
mechanisms, including strain specific ones, exist in tomato resistant germplasm.
Therefore, breeders have some opportunities at their disposal to enlarge the relatively
narrow range of resistance sources primarily used so far, and to continue accumulating
different mechanisms of resistance in tomato genotypes to obtain better stability of
resistance in different environments. However, they might be limited by the fact that
some bacterial strains are not controlled by any resistant accession (see section 5.
below).
Inheritance studies have focused mostly on F2, F3 and RILs progenies of „Hawaii
7996‟ crossed with the susceptible „WVa700‟ (S. pimpinellifolium). Several QTLs of
resistance have been identified, including a major QTL on chromosome 6 effective
towards „GMI8217‟, an isolate of race 1 biovar 1 (Thoquet et al. 1996a & b); of „Pss4‟,
an isolate of race 1, biovar 3, phylotype 1 (Wang et al., 1998); and „JT516‟, an isolate
representative of race 3-phylotype II (Carmeille et al., 2006a). Wang et al. (2000)
identified another major QTL of resistance of „Hawaii 7996‟ effective towards „Pss4‟ and
located on chromosome 12. Several minor QTLs located on chromosomes 3, 4, 8,
some of which having a season dependent expression (Carmeille et al., 2006a) have
also been identified. Mejia et al (2009) confirmed the QTLs on chromosome 6 and 12
were associated with resistance in „Hawaii 7996‟ observed in Guatemala field evaluation
against local phylotype I strains. Work is ongoing at AVRDC for adding markers to the
QTLs regions of „Hawaii 7996‟ associated to resistance to bacterial strains belonging to
phylotype I, in order to develop tools for marker assisted selection.
QTLs of resistance of the resistant line „L285‟ effective towards „UW364‟ an isolate
of race 1, biovar 4, have also been located on chromosomes 6, as well as on
chromosomes 7 and 10 (Danesh et al., 1994). Pattern of resistance derived from
CRA66 has been described as polygenic (Prior et al., 1994) but no molecular data are
available for this source.
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