The Cladosporium fulvum resistance gene Cf-ECP3 is part of the Orion cluster on the short arm of tomato Chromosome 1

The resistance of tomato to the pathogenic fungus Cladosporium fulvum complies with the gene-for-gene relationship. Race specific resistance is based on Cf-gene mediated recognition of secreted avirulence products, resulting in a hypersensitive response (HR). Besides the avirulence gene products, C. fulvum secretes a number of extra cellular proteins (ECPs) into the apoplast. Two L. esculentum accessions have previously been identified that reacted with a HR upon injection with purified ECP3. The corresponding resistance gene designated Cf-ECP3 was mapped by using an F-2 population composed of 192 plants from the cross of susceptible MoneyMaker to resistant L. esculentum G1.1153. Cf-ECP3 inherited monogenically, cosegragated with the Chromosome 1 Cleaved Amplified Polymorphic Sequence (CAPS) marker CT116 and was mapped accurately at Orion, a locus harbouring Cf-ECP2 in other genotypes. RFLP anaysis with a Cf-9 probe further demonstrated cosegregation of Cf-ECP3 with an Hcr9 (Homologue of Cladosporium fulvum resistance gene Cf-9) indicating that this gene is likely an Hcr9. Thus in addition to the Milky Way locus harbouring the Cf-4, Cf-4A and Cf-9 resistance genes targeted against AVR4, AVR4A and AVR9, Orion is another complex locus on the short arm of Chromosome 1 that harbours at least two functional Cf-genes, Cf-ECP2 and Cf-ECP3, targeted against the fungal excreted proteins ECP2 and ECP3.