ADDITIONAL RESISTANCE GENE(S) AGAINST CLADOSPORIUM-FULVUM PRESENT ON THE CF-9 INTROGRESSION SEGMENT ARE ASSOCIATED WITH STRONG PR PROTEIN ACCUMULATION

The existence of a gene or genes conferring weak resistance against th e fungal tomato pathogen Cladosporium fulvum, in addition to the Cf-9 resistance gene, present on the Lycopersicon pimpinellifolium Cf-9 seg ment introgressed into L. esculentum, was demonstrated with strains of C. fulvum lacking a functional Avr9 avirulence gene and tomato genoty pes lacking a functional Cf-9 gene, respectively, Two mutant strains, obtained by disruption of Avr9 in race 4 and race 5 of C. fulvum, do n ot trigger the hypersensitive response-mediated resistance on MM-Cf9 g enotypes that is normally induced after recognition of the AVR9 elicit or, However, when these strains are inoculated onto MM-Cf0 and MM-Cf9 genotypes, adult MM-Cf9 plants still show weak resistance, This resist ance is not related to the Cf-9 gene, as ethyl methanesulfonate (EMS)- generated Cf-9 mutants retained weak resistance, Growth of the fungus in the leaf mesophyll is strongly inhibited, whereas re-emergence of f ungal mycelium and conidiation are poor, Strong accumulation of pathog enesis-related proteins and early leaf chlorosis are associated with t his phenotype of weak resistance. A search among natural strains lacki ng the Avr9 gene revealed that one strain is able to overcome this wea k resistance, Possible mechanisms underlying this weak resistance are discussed. The presence of the additional weak resistance gene(s) coul d explain why the resistance of Cf9 genotypes has not been overcome so far in practice.