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Genetic Characterization of AvrLm1, the First Avirulence Gene of Leptosphaeria maculans. D. Ansan- Melayah, Pathologie Végétale, INRA, Route de Saint Cyr, 78026 Versailles Cedex, France; M. H. Balesdent, M. Buée, and T. Rouxel. Pathologie Végétale, INRA, Route de Saint Cyr, 78026 Versailles Cedex, France. Phytopathology 85:1525-1529. Accepted for publication 13 September 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1525.

Specific interactions of the fungal pathogen Leptosphaeria maculans with Brassica napus cultivars were observed when the cultivars were inoculated with isolates belonging to pathogenicity groups (PG) PG3 and PG4. PG3 isolates induced resistance responses on cotyledons or leaves of cv. Quinta, whereas PG4 isolates caused sporulating lesions on this cultivar. In contrast, both pathotypes caused disease symptoms on cvs. Westar and Glacier. The genetic basis of cultivar specificity was studied using tetrad analysis after in vitro crosses between one PG3 and one PG4 isolate. For the genetic study, the use of random amplified polymorphic DNA (RAPD) as genetic markers was assessed. Of 61 primers, 10 generated reproducible polymorphisms. Of these, 9 generated 18 RAPD markers displaying a 2:2 segregation ratio within the 10 analyzed tetrads. A 2:2 segregation ratio for avirulence/virulence to cv. Quinta also was observed in the progeny. Consequently, the single genetic locus controlling cultivar specificity on Quinta was considered the first avirulence gene described in L. maculans and was designated AvrLm1.

Additional keywords: blackleg disease, gene-for-gene relationship, Phoma lingam.