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Pacific Division -What Alternaria species cause diseases of potato in the Pacific Northwest?
L. TYMON (1), T. F. Cummings (1), T. L. Peever (1), D. A. Johnson (1). (1) Washington State University, Pullman, WA, U.S.A.

Isolations from early blight and brown spot lesions on potato leaves collected in the Pacific Northwest were made between 2008 and 2011 to study the diversity of fungi causing these diseases. Large-spored isolates from Section <i>Porri </i>(<i>As</i>) were isolated less frequently (42% overall) than small-spored isolates in Section <i>Alternaria</i> (<i>Aa</i>) (50% overall) in all years except 2010. Pathogenicity was assessed on detached leaves of 'Russet Norkotah'. Seventy-three % of <i>Aa</i> isolates and 92% of <i>As</i> isolates were pathogenic, respectively. A restriction digest of SCAR marker OPA1-3 was used to distinguish phylogenetic species within the section alternata. Twenty-three of 66 isolates were identified as <i>Aa </i>and 36 isolates as <i>A. arborescens. </i>No correlation was observed between pathogenicity and phylogenetic species (r= 0.12, <i>P=</i> 0.33) Seven isolates had no restriction site and were identified as phylogenetic species <i>A. oregonensis, A. arbusti</i>, or <i>A. metachromatica</i> in section infectoria by sequencing of g3pd. Aggressiveness of<i> As, Aa, </i>and<i> A. arbusti </i>to potato was quantified on non-wounded and wounded detached leaves. Mean infection frequencies (MIF) and area under the lesion expansion curve (AULEC) were significantly greater for <i>As</i> than for <i>Aa</i> or <i>A. arbusti</i> on non-wounded leaves and wounding of tissue significantly increased MIF and AULEC of <i>Aa</i> but only MIF significantly increased for <i>A. arbusti</i> relative to non-wounded tissue.

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