INRA, Unite de Pathologie Vegetale, Route de St Cyr, 78026 Versailles Cedex, France

Background and objectives
Leptosphaeria maculans causes blackleg of oilseed rape (Brassica napus), a disease of major economic incidence worldwide. The fungus displays specificity of interaction, i.e. virulence or avirulence, when inoculated on cotyledons of differential genotypes of B. napus. The original differential set comprised three cultivars of B. napus, Westar (susceptible check), Quinta and Glacier, that discriminated three 'pathogenicity groups' - PG2, avirulent on Quinta and Glacier; PG3, avirulent on Quinta; and PG4, virulent on all three cultivars. In the case of these interactions, genetic control of the specificity of interaction was studied on both the plant and fungus side and found to be governed by pairs of matching avirulence-resistance genes. The AvrLm1-Rlm1 pair is responsible for incompatibility of PG3 isolates towards Quinta, whereas the linked avirulence gene AvrLm2 matches the Rlm2 resistance gene in Glacier to specify incompatibility of PG2 isolates towards Glacier [1].

The objective of the present study was to investigate the genetic control of additional specificities of interaction and to investigate the possible allelism of new avirulence genes with previously evidenced AvrLm1 and AvrLm2.

Material and methods
Specificity of interaction was identified on the basis of the interaction phenotype observed following a cotyledon-inoculation test performed with conidial suspensions [2]. Isolates displaying differential interactions were crossed in vitro, and tetrads and/or random ascospores were recovered according to [2]. In the present study, crosses were performed to analyse, on the fungal side, the genetic control of the following interactions: (i) avirulence towards B. napus cvs Vivol, Columbus and Capitol; (ii) avirulence towards B. napus cv Jet Neuf; and (iii) virulence towards two cultivars of the related Brassica species, B. juncea.

Results and conclusions
Analysis of the progeny of a cross between one isolate avirulent towards cvs Quinta, Vivol, Columbus and Capitol and one isolate virulent towards all these cultivars did not enable us to recover recombinant genotypes: all isolates avirulent towards Quinta were avirulent towards the three other cultivars, supposedly unrelated to Quinta. As a consequence, AvrLm1 was found to specify incompatibility towards all these cultivars, suggesting that they all may possess Rlm1, or alleles displaying the same recognition spectrum. One can also consider that AvrLm1 is part of a cluster of avirulence genes specifying avirulence towards these unrelated cultivars.

Incompatibility of one German isolate of L. maculans towards the Jet Neuf genotype was found to be under a simple genetic control. The avirulence locus was found to be genetically unlinked to AvrLm1 and the corresponding avirulence gene was termed AvrLm4. Finally, two independent loci were found to control avirulence towards B. juncea.

These data indicate that both narrow host-range specificity, i.e. cultivar specificity, and broad host-range specificity, at the species level, can be determined by avirulence genes in L. maculans. As more avirulence genes are likely to be uncovered in the forthcoming year, race nomenclature of the pathogen has now to be worked out in detail.

1. Ansan-Melayah D, Balesdent MH, Delourme R et al., 1998. Plant Breeding (in press).
2. Ansan-Melayah D, Balesdent MH, Bude M, Rouxel T, 1995. Phytopathology 85, 1525-1529.