A second gene at the tomato Cf-4 locus confers resistance to Cladosporium fulvum through recognition of a novel avirulence determinant

The tomato Cf-4 and Cf-9 genes confer resistance to the leaf mould pathogen Cladosporium fulvum and map at a complex locus on the short arm of chromos ome 1. It was previously shown that the gene encoding Cf-4, which recognize s the Avr4 avirulence determinant, is one of five tandemly duplicated homol ogous genes (Hcr9-4s) at this locus. Cf-4 was identified by molecular analy sis of rare Cf-4/Cf-9 disease-sensitive recombinants and by complementation analysis. The analysis did not exclude the possibility that an additional gene(s) located distal to Cf-4 may also confer resistance to C. fulvum. We demonstrate that a number of Dissociation-tagged Cf-4 mutants, identified o n the basis of their insensitivity to Avr4, are still resistant to infectio n by C. fulvum race 5. Molecular analysis of 16 Cf-4 mutants, most of which have small chromosomal deletions in this region, suggested the additional resistance specificity is encoded by Hcr9-4E. Hcr9-4E recognizes a novel C. fulvum avirulence determinant that we have designated Avr4E.