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

The genus Tubakia is revised on the basis of morphological and phylogenetic data. The phylogenetic affinity of Tubakia to the family Melanconiellaceae (Diaporthales) was recently postulated, but new analyses based on sequences retrieved from material of the type species of Tubakia, T. dryina, support a family of its own, viz. Tubakiaceae fam. nov. Our phylogenetic analyses revealed the heterogeneity of Tubakia s. lat. which is divided into several genera, viz., Tubakia s. str., Apiognomonioides gen. nov. (type species: Apiognomonioides supraseptata), Involutiscutellula gen. nov. (type species: Involutiscutellula rubra), Oblongisporothyrium gen. nov. (type species: Oblongisporothyrium castanopsidis), Paratubakia gen. nov. (type species: Paratubakia subglobosa), Racheliella gen. nov. (type species: Racheliella wingfieldiana sp. nov.), Saprothyrium gen. nov. (type species: Saprothyrium thailandense) and Sphaerosporithyrium gen. nov. (type species: Sphaerosporithyrium mexicanum sp. nov.). Greeneria saprophytica is phylogenetically closely allied to Racheliella wingfieldiana and is therefore reallocated to Racheliella. Particular emphasis is laid on a revision and phylogenetic analyses of Tubakia species described from Japan and North America. Almost all North American collections of this genus were previously referred to as T. dryina s. lat., which is, however, a heterogeneous complex. Several new North American species have recently been described. The new species Sphaerosporithyrium mexicanum, Tubakia melnikiana and T. sierrafriensis, causing leaf spots on several oak species found in the North-Central Mexican state Aguascalientes and the NorthEastern Mexican state Nuevo León, are described, illustrated, and compared with similar species. Several additional new species are introduced, including Tubakia californica based on Californian collections on various species of the genera Chrysolepis, Notholithocarpus and Quercus, and T. dryinoides, T. oblongispora, T. paradryinoides, and Paratubakia subglobosoides described on the basis of Japanese collections. Tubakia suttoniana nom. nov., based on Dicarpella dryina, is a species closely allied to T. californica and currently only known from Europe. Tubakia dryina, type species of Tubakia, is epitypified, and the phylogenetic position and circumscription of Tubakia are clarified. A revised, supplemented key to the species of Tubakia and allied genera on the basis of conidiomata is provided.

Leptosphaeria maculans, a dothideomycete fungus causing stem canker on oilseed rape, develops gene-for-gene interactions with its host plants. It has the ability to rapidly adapt to selection pressure exerted by cultivars harbouring novel resistance genes as exemplified recently by the 3-year evolution towards virulence at the AvrLm1 locus in French populations. The AvrLm1 avirulence gene was recently cloned and shown to be a solo gene within a 269 kb non-coding, heterochromatin-like region. Here we describe the sequencing of the AvrLm1 genomic region in one avirulent and two virulent isolates to investigate the molecular basis of evolution towards virulence at the AvrLm1 locus. For these virulent isolates, the gain of virulence was linked to a 260 kb deletion of a chromosomal segment spanning AvrLm1 and deletion breakpoints were identical or similar. Among the 460 isolates analysed from France, Australia and Mexico, a similar large deletion was apparent in > 90% of the virulent isolates. Deletion breakpoints were also strongly conserved in most of the virulent isolates, which led to the hypothesis that a unique deletion event leading to the avrLm1 virulence has diffused in pathogen populations. These data finally suggest that retrotransposons are key drivers in genome evolution and adaptation to novel selection pressure in L. maculans.

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The race structure and genotypic diversity of four Leptosphaeria maculans populations isolated from Brassica oleracea (broccoli, cauliflower, cabbage, etc.) in central Mexico (Aguascalientes, Guanajuato and Zacatecas states) were analysed. Race structure was characterized by an unusually low diversity at three locations out of four. Fourteen minisatellite markers revealed a high proportion of repeated multilocus genotypes in these populations, combined with a significant linkage disequilibrium and strong clonal fraction (65-87%). The occurrence of the mating-type idiomorphs always significantly departed from the 1:1 proportion expected in the case of random mating. Each population thus consists of a few (four to nine) multilocus genotypes which are specific to each location. These data strongly support the hypothesis of exclusive, or a high rate of, clonal multiplication. Comparison of cropping practices between B. oleracea and B. napus indicate that the shift in reproductive behaviour of the fungus is chiefly man-mediated.