The Leptosphaeria maculans - Leptosphaeria biglobosa species complex in the American continent

Stem canker of oilseed rape (canola, Brassica napus) is associated with a species complex of two closely related fungalspecies, Leptosphaeria maculans and L. biglobosa. Of these, L. maculans is the most damaging and develops gene-for-generelationships with the host. Here, a wide scale analysis of the L. maculans - L. biglobosa species complex was performedthroughout the American continent (23 locations from Chile to Canada) plus several locations inWestern Australia for comparisonpurposes, based on a collection of 1132 isolates from infected tissues of a susceptible cultivar. Fungal species werediscriminated on the basis of morphological, phytopathological and molecular criteria and showed that L. biglobosa wasclosely associated with L. maculans in most of the locations. Multiple gene phylogeny using sequences of ITS, actin andb-tubulin confirmed the prevalence of the L. biglobosa ‘canadensis’ sub-clade in Canada, whereas up to three differentsub-clades of L. biglobosa were found in Georgia (USA). Race structure of L. maculans was investigated using a combinationof pathogenicity tests and PCR amplification of avirulence alleles AvrLm1, AvrLm4 and AvrLm6. Three contrasting situationswere observed: (i) race structure in Ontario, Chile and Georgia was related to that of European and WesternAustralian populations, with a low race diversity; (ii) only one race was found in Mexico, and not found outside of thiscountry; (iii) a large diversity of races was observed in central Canada (Manitoba, Alberta and Saskatchewan) with veryspecific features including maintenance of avirulence alleles absent from Europe, absence of the AvrLm7 allele common inEurope (or eastern Canada) and wide location-to-location variability.