University of Groningen Inheritance and genetic mapping of resistance to Alternaria alternata f . sp . lycopersici in

The fungal pathogen Alternaria alternata f. sp. lycopersici produces AAL-toxins that function as chemical determinants of the Alternar ia stem canker disease in the tomato (Lycopersicon esculentum). In resistant cultivars, the disease is controlled by the Asc locus on chromosome 3. Our aim was to characterize novel sources of resistance to the fungus and of insensit ivity to the host-selective AAL-toxins. To that end, the degree of sensitivity of wild tomato species to AAL-toxins was analyzed. Of all members of the genus Lycopersicon, only L. cheesmanii was revealed to be sensitive to AALtoxins and susceptible to fungal infection. Besides moderately insensitive responses f rom some species, L. penneIlii and L. peruvianum were shown to be highly insensitive to AAL-toxins as well as resistant to the pathogen. Genetic analyses showed that high insensit ivity to AALtoxins f rom L. penneIlii is inherited in tomato as a single complete dominant locus. This is in contrast to the incomplete dominance of insensit ivity to AAL-toxins of L. esculentum. Subsequent classical genetics, RFLP mapping and allelic testing indicated that high insensitivity to AAL-toxins f rom L. pennellii is conferred by a new allele of the Asc locus.