Abstract
Blackleg (Phoma stem canker) of crucifers is a globally important disease caused by the ascomycete species complex comprising of Leptosphaeria maculans and Leptosphaeria biglobosa. Six blackleg isolates recovered from Brassica rapa cv. Mizspoona in the Willamette Valley of Oregon were characterized as L. biglobosa based on standard pathogenicity tests and molecular phylogenetic analysis. These isolates were compared to 88 characterized L. biglobosa isolates from western Canada, 22 isolates from Australia, and 6 L. maculans isolates from Idaho, USA using maximum parsimony and distance analysis of phylogenetic trees generated from the ITS rDNA (internal transcribed spacer rDNA) sequence, and the actin and β-tubulin gene sequences. The L. biglobosa isolates derived from B. rapa collected in Oregon formed a separate subclade based on concatenated gene sequences or a single gene sequence, regardless of the analyses. Pathogenicity tests showed that these isolates failed to infect either resistant or susceptible B. napus cultivars, but caused severe symptoms on three B. rapa cultivars (Accession number: UM1113, UM1112, and UM1161), a B. oleracea var. capitata (cabbage) cultivar (Copenhagen Market), and two B. juncea cultivars (CBM, a common brown Mustard, and Forge). These findings demonstrated that the L. biglobosa isolates derived from a B. rapa crop in Oregon were genetically distinct from existing species of L. biglobosa, and constitute a new subclade, herein proposed as L. biglobosa ‘americensis’.
Highlights
Leptosphaeria maculans and L. biglobosa are two closely related fungal species that together form a species complex that causes blackleg or Phoma stem canker of crucifers, including Brassica napus, B. juncea, B. oleracea, and B. rapa [1,2]
L. maculans has been categorized into two subclades: ‘Brassicae’ and ‘lepidii’; while L. biglobosa isolates have been categorized into six subclades: ‘Brassicae’, ‘canadensis’, ‘thlaspii’, ‘erysimii’, ‘australensis’, and ‘occiaustralensis’ [6,7]
Since all the B. rapa-derived isolates were avirulent on all the B. napus cultivars and lines tested, but virulent on several B. juncea (CBM and Forge) and B. rapa (UM1113, UM1112, and UM1161) cultivars and lines, it was deemed necessary to assess whether these isolates were L. maculans or L
Summary
Leptosphaeria maculans and L. biglobosa are two closely related fungal species that together form a species complex that causes blackleg or Phoma stem canker of crucifers, including Brassica napus, B. juncea, B. oleracea, and B. rapa [1,2]. L. maculans–L. biglobosa species complex had been separated into seven distinct groups using the sequence of the ITS region In these studies, ITS rDNA, and actin and β-tubulin sequences have been widely used for the phylogeny analysis of the Leptosphaeria complex and new subclade identification. In 2008, Vincenot et al identified a new subclade of L. biglobosa (L. biglobosa ‘occiaustralensis’) using ITS rDNA, actin, and β-tubulin sequences in western Australia [7]. Six isolates of Leptosphaeria were recovered from the Brassica rapa seed crop, Mizspoona, and were characterized using standard pathogenicity tests and sequence analysis of the ITS rDNA and actin and β-tubulin genes to determine the species of these isolates
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