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Field experiments were conducted in Versailles, France, to assess blackleg resistance of Brassica napus cultivars Quinta and Glacier under natural infection conditions. Blackleg disease severity was assessed twice during growth of B. napus. Quinta resistance was highly expressed as only 13% to 18% of the plants exhibited leaf symptoms in December, whereas Glacier and other cultivars displayed more than 80% of infected plants. In June (harvest), 70% (first year) to 41.5% (second year) of Quinta plants were canker-free. In contrast, Glacier was as infected as the susceptible control cultivars, with more than 88% of plants displaying canker. The Leptosphaeria maculans population structure was examined in parallel. Based on soluble protein patterns, 9% of the 299 fungal isolates collected were characterized as Tox0 species, and belonged to the NA1 sub-group. All but two of Tox0 isolates were isolated from atypical dark necrotic leaf lesions, mainly occurring on Quinta. In contrast, the Tox+ isolates were recovered from typical leaf lesions. Following a cotyledon inoculation test on the differential set Westar, Quinta and Glacier, 92 to 95% of Tox+ isolates collected on susceptible cultivars were characterized as PG3 isolates, i.e. avirulent on Quinta. The remaining Tox+ isolates belong to PG4, i.e. virulent on the three cultivars. No PG2 isolate, i.e. avirulent on both Quinta and Glacier, was identified in the sampling. The present study suggests that specific resistance expressed at the cotyledon level can be efficient under field conditions where the corresponding avirulent races of the pathogen are prevalent.

Leptosphaeria maculans causes blackleg of oilseed rape. Gene-for-gene interactions between race PG3 and Brassica napus cv. Quinta were related to interaction between the fungal avirulence (Avr) gene AvrLm1 and the corresponding resistance gene Rlm1. AvrLm1 isolates were aviru-lent on cvs. Doublol, Vivol, Columbus, and Capitol, and no recombinant phenotypes were observed in the progeny of two AvrLm1 × avrLm1 crosses, suggesting that all of these cultivars may possess Rlm1 or genes displaying the same recognition spectrum, or that a cluster of Avr genes is present at the Avrlm1 locus. In one cross, segregation distortion was observed at the AvrLm1 locus that could be explained by interaction between AvrLm1 and one unlinked deleterious gene, termed Del1. Incompatibility toward cvs. Jet Neuf and Darmor.bzh was governed by a single gene, unlinked to AvrLm1 or Del1. This avirulence gene was termed AvrLm4. Preliminary plant genetic analysis suggested the occurrence of a corresponding dominant resistance gene, termed Rlm4, present in the Quinta line analyzed and linked to Rlm1.

Specificity of interaction at the cotyledon stage was recently demonstrated between the blackleg pathogen, Leptosphaeria maculans, and Brassica napus. Three pathogenicity groups were distinguished, PG2 avirulent towards 'Quinta' and 'Glacier', PG3 avirulent towards 'Quinta', and PG4 virulent on the two cultivars. The genetic control of the interactions was investigated on both the pathogen and the plant. Tetrad analysis was performed following PG3 × PG4 and PG2 × PG4 crosses. 'Quinta' and 'Glacier' were crossed with the susceptible winter oilseed rape cultivar 'Score'. The analysis of F 1 , F 2 and testcross populations suggested that the incompatible interaction between 'Quinta' and PG3 isolates is conditioned by the presence of the dominant single resistance allele Rlm1 in 'Quinta' and the matching avirulence gene Avr Lm1 in L. maculans. Race-specific resistance of 'Glacier' to PG2 isolates was conditioned by the matching gene pair Rlm2/Avr Lm2. Finally, the data suggest that two avirulence genes matching two dominant loci control the 'Quinta'-PG2 interaction. The consequences of the occurrence of race-specific resistance in B. napus are discussed with respect to future breeding for blackleg resistance.

All Leptosphaeria maculans field isolates displayed a minichromosome (MC) clearly separated from the overall electrokaryotype following pulsed-field gel electrophoresis. MCs exhibited a length polymorphism ranging from 650 to 950 kb. Tetrad analyses revealed the parental inheritance of MC length polymorphism (50% of the tetrads) or else the generation of novel-sized MCs (27%), which suggested that recombination occurred between MCs. Nineteen percent of the tetrads displayed a lack of the MC band in the electrokaryotype for one or two of the four resulting genotypes. Crosses between isolates carrying or lacking MCs revealed non-Mendelian segregation and suggested that some isolates could display at least two copies of the MC. Only repeated sequences hybridising to all chromosomes were isolated from the MC. Finally, saprophytic or parasitic fitness was not modified when isolates apparently lacked the MC. All these data suggested that the L. maculans MC behaves like a 'B' chromosome.