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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 the fungal tomato pathogen Cladosporium fulvum, in addition to the Cf-9 resistance gene, present on the Lycopersicon pimpinellifolium Cf-9 segment introgressed into L. esculentum, was demonstrated with strains of C. fulvum lacking a functional Avr9 avirulence gene and tomato genotypes 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 not trigger the hypersensitive response-mediated resistance on MM-Cf9 genotypes that is normally induced after recognition of the AVR9 elicitor. However, when these strains are inoculated onto MM-Cf0 and MM-Cf9 genotypes, adult MM-Cf9 plants still show weak resistance. This resistance 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 fungal mycelium and conidiation are poor. Strong accumulation of pathogenesis-related proteins and early leaf chlorosis are associated with this phenotype of weak resistance. A search among natural strains lacking the Avr9 gene revealed that one strain is able to overcome this weak resistance. Possible mechanisms underlying this weak resistance are discussed. The presence of the additional weak resistance gene(s) could explain why the resistance of Cf9 genotypes has not been overcome so far in practice.