| Summary: | Blackleg (Phoma stem canker) of crucifers is a globally important disease caused by the ascomycete species complex comprising of <i>Leptosphaeria maculans</i> and <i>Leptosphaeria biglobosa</i>. Six blackleg isolates recovered from <i>Brassica rapa</i> cv. Mizspoona in the Willamette Valley of Oregon were characterized as <i>L. biglobosa</i> based on standard pathogenicity tests and molecular phylogenetic analysis. These isolates were compared to 88 characterized <i>L. biglobosa</i> isolates from western Canada, 22 isolates from Australia, and 6 <i>L. maculans</i> 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 <i>actin</i> and <i>β-tubulin</i> gene sequences. The <i>L. biglobosa</i> isolates derived from <i>B. rapa</i> 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 <i>B. napus</i> cultivars, but caused severe symptoms on three <i>B. rapa</i> cultivars (Accession number: UM1113, UM1112, and UM1161), a <i>B. oleracea</i> var. <i>capitata</i> (cabbage) cultivar (Copenhagen Market), and two <i>B. juncea</i> cultivars (CBM, a common brown Mustard, and Forge). These findings demonstrated that the <i>L. biglobosa</i> isolates derived from a <i>B. rapa</i> crop in Oregon were genetically distinct from existing species of <i>L. biglobosa</i>, and constitute a new subclade, herein proposed as <i>L. biglobosa</i> ‘americensis’.
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