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Occurrence of a New Subclade of Leptosphaeria biglobosa in Western Australia

First, second, sixth, and seventh authors: INRA, UMR 1290 (BIOGER-CPP), Route de Saint Cyr, 78026 Versailles Cedex, France; third and fifth authors: School of Earth and Geographical Sciences; fourth author: School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley, WA 6009, Australia; sixth author: AgroParisTech, Protection des Plantes, 78850 Thiverval Grignon, France.

Stem canker of crucifers is caused by an ascomycete species complex comprising of two main species, Leptosphaeria maculans and L. biglobosa. These are composed of at least seven distinct subclades based on biochemical data or on sequences of internal transcribed spacer (ITS), the mating type MAT1-2 or fragments of actin or β-tubulin genes. In the course of a wide-scale characterization of the race structure of L. maculans from Western Australia, a few isolates from two locations failed to amplify specific sequences of L. maculans, i.e., the mating-type or minisatellite alleles. Based on both pathogenicity tests and ITS size, these isolates were classified as belonging to the L. biglobosa species. Parsimony and distance analyses performed on ITS, actin and β-tubulin sequences revealed that these isolates formed a new L. biglobosa subclade, more related to the Canadian L. biglobosa ‘canadensis’ subclade than to the L. biglobosa ‘australensis’ isolates previously described in Australia (Victoria). They are termed here as L. biglobosa ‘occiaustralensis’. These isolates were mainly recovered from resistant oilseed rape cultivars that included the Brassica rapa sp. sylvestris-derived resistance source, but not from the susceptible cv. Westar. The pathogenicity of L. biglobosa ‘occiaustralensis’ to cotyledons of most oilseed rape genotypes was higher than that of L. biglobosa ‘canadensis’ or L. biglobosa ‘australensis’ isolates.

Additional keywords:blackleg disease, B. napus, Phoma lingam.