Resistance to Cladosporium fulvum, the causal organism of leaf mold of tomato, is controlled by a race-specific mechanism. Cf-resistance genes have been introduced from different Lycopersicon-species into commercial lines, and many not yet exploited Cf-genes are still available. The most complex race found in the Netherlands until now is race 2.4.5.9.11 (Lindhout et al., 1989), which has been identified once. The resistance gene Cf-9, which originates from L. pimpinellifolium, is still effective in the Netherlands, and has been introduced in many modern hybrids. It is not known on which chromosome Cf-9 is located. The present study was undertaken to map Cf-9 on the basis of linkage with molecular markers, developed by Dr. S.D. Tanksley, Cornell University, USA.
Restriction fragment length polymorphisms (RFLPs) have been found between the tomato cultivars Sonatine (Cf-9/cf-9) and Moneymaker (cf-9/cf-9). DNA-clones corresponding with these RFLPs have been used as probes to detect polymorphisms between a near-isogenic line for Cf-9 (with 'Moneymaker' used as the recurrent parent during 5 backcross generations) and 'Ontario 7719' (PI 126933) which is the donor genotype for Cf-9. A polymorphism has been found with clone TG 301, located on the short arm of chromosome 1. This suggests linkage between Cf-9 and TG 301. It is possible, however, that even after several backcross generations, parts of chromosomes of the L. pimpinellifolium donor parent are still scattered over the genome. Therefore RFLPs could be found on chromosomal segments not containing the gene Cf-9. In our study 27 commercial tomato varieties, resistant or susceptible to races of C. fulvum, have been analyzed for DNA polymorphism probed by TG 301. All cultivars containing the Cf-9 gene exhibited the TG 301 allele of the Cf-9 donor. Consequently there is firm evidence that Cf-9 is located on the short arm of chromosome 1. The strength of this linkage will be studied by segregation analysis on an F2-population of 'Sonatine'.
Literature Cited:
Lindhout, P., W. Korta, M. Cislik, I. Vos & T. Gerlagh, 1989. Neth. J. Pl. Path. 95: 143-148.