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Powdery mildew (Sawadaea bicornis) on Rocky Mountain maple (Acer glabrum)

Chelsea Jenkins: Utah State University

<div>The Rocky Mountain maple (<em>Acer gl</em>abrum) is a deciduous tree indigenous to North America. It is found in many western states including Utah, New Mexico, California and Alaska where it provides prevention of slope erosion and wildlife habitat. Trees in Cache Valley in northern Utah began displaying symptoms of powdery mildew infection in the fall of 2017. Thus far, powdery mildew on A. glabrum has only been recorded in Iowa (<em>Uncinula circinata</em>) in 1923 and Idaho (<em>Phyllactinia guttata</em>) in 1973 but the species descriptions did not fit the powdery mildew found in Utah. Based on morphological characteristics (dimensions of chasmothecia and asci, number of appendages) using light microscopy and molecular identification using PCR and DNA sequencing, the powdery mildew was identified as Sawadaea bicornis. <em>Sawadaea bicornis</em> is a European powdery mildew that was first found in the USA on Norway maple (<em>Acer platanoides</em>) and box elder (<em>Acer ne</em>gundo). To our knowledge this is the first report of Sawadaea bicornis infecting Acer glabrum in North America.</div>