Cladosporium fulvum is the causal agent of tomato leaf mold. The interaction between C. fulvum and tomato is used as a model system to study the gene-forgene hypothesis in which fungal avirulence gene products interact with resistance gene products of the plant. Different races of C. fulvum have arisen as a result of adaptation of the fungus to resistance genes present in cultivated tomato. These new races have overcome their initial avirulence by evading recognition by the plant. In this way, several fungal races have overcome resistance gene Cf9 of tomato by a deletion of the avirulence gene avr9 encoding the corresponding race-specific elicitor (van Kan el al., 1991, Mol. Plant-Microbe Interact. 4: 52-59).