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First report and new hosts of the oak pathogen Diplodia corticola in Wisconsin

Denise Smith: University of Wisconsin-Madison

<div>Diseased oaks (<em>Quercus</em> species) exhibit tip blight, branch and stem cankers, and dieback often attributed to <em>Botryosphaeria</em> species or related anamorphic fungi. <em>Diplodia</em> <em>corticola</em> (teleomorph described as <em>Botryosphaeria</em> <em>corticola</em>) has been identified as part of a disease complex of oaks observed in Europe since the 1980’s. Reports of this fungus on the east and west coasts of the United States prompted re-examination of isolates from oaks in Wisconsin. These isolates had been obtained in the late 1990’s and early 2000’s and previously were identified only as <em>Diplodia </em>species. Phylogenetic analysis of rDNA- ITS sequences (including <em>Diplodia</em> and <em>Botryosphaeria</em> sequences from GenBank) confirmed most Wisconsin isolates from northern red oak (<em>Q. rubra</em>), black oak (<em>Q. velutina</em>), white oak (<em>Q. alba</em>), and bur oak (<em>Q. macrocarpa</em>) as <em>D. corticola</em>. Other isolates from oaks in Wisconsin were <em>D. mutila</em> and <em>D. seriata</em>. Wound-inoculation of northern red, white, and bur oak shoots of seedlings in a greenhouse resulted in shoot death and stem lesions, from which the pathogen was reisolated. We conclude that <em>D. corticola</em> has been present in the northcentral United States for at least two decades and report two previously unrecognized hosts of this pathogen: white oak and bur oak. The roles of <em>D. corticola </em>and other <em>Diplodia</em> and <em>Botryosphaeria</em> species in the deterioration of oak health in North America merit additional investigation.</div>