Mapping strategy for resistance genes against Cladosporium fulvum on the short arm of Chromosome 1 of tomato:: Cf-ECP5 near the Hcr9 Milky Way cluster

In the past, numerous Lycopersicon accessions have been described that harbor resistance genes to Cladosporium fulvum (Cf genes). Several Cf genes have been isolated, like Cf-4, Cf-4A and Cf-9, which are present on the short arm of Chromosome 1, and Cf-2 and Cf-5, which reside on Chromosome 6. To identify Cf genes linked to the Hcr9 cluster "Milky Way" on the short arm of Chromosome 1, we test-crossed 66 resistant Lycopersicon accessions to the near-isogenic line Moneymaker-Cf4, and the F(1)s were crossed to the susceptible tomato cultivar Moneymaker. Putative linkage between an unknown Cf gene and Cf-4 was concluded based on small-scale allelic tests from an under-representation of susceptible genotypes in the progenies of 24 plants after inoculation with race 0 of C. fulvum. In this way, of the 21 resistant lines tested, 10 harbored a Cf gene that was linked to the Hcr9 Milky Way cluster. Moreover, one of the lines harboring a Cf gene closely linked to Cf-4 specifically recognizes the extracellular protein ECP5 of C. fulvum and was designated Cf-ECP5. Using a testcross population of 338 plants, we mapped Cf-ECP5 more accurately at 4 cM proximal to the Hcr9 Milky Way locus. This report shows that the method of small-scale allelic tests provides a useful tool to rapidly screen for Cf genes on the short arm of Chromosome 1. Further analysis of these Cf genes will elucidate the complex genetic organization of Cf genes on Chromosome 1 of tomato.