In August 2012, Norway maple ‘Crimson Century’ trees affected by powdery mildew were observed in Portland, OR. A majority of leaves throughout the crowns on each of the five trees planted in the street terrace on one side of a single city block were affected. White mycelium was present in irregular, discrete, scattered spots or almost continuously on the upper surfaces of leaves and also was present on some petioles. The affected area of the upper surface of leaf blades was estimated to typically be >50%. Chasmothecia were present singly or in groups on the mycelium, and bore simple and bifid appendages (originating from the upper half of the chasmothecia) including some with uncinate or circinate apices that allowed tentative identification to the genus Sawadaea (1). Mean diameter of the chasmothecia was 160 (standard error [SE] = 3.9) μm, and mean appendage length was 72 (SE = 4.9) μm. Asci averaged 77 (SE = 1.2) × 52 (SE = 1.2) μm and ascospores averaged 24 (SE = 0.4) × 12 (SE = 0.3) μm. Braun (1) reported ranges in dimensions of these features as: chasmothecia, (min. 125) 140 to 190 μm; appendages, 40 to 80 μm; asci, 60 to 80 (max. 100) × (min. 30) 35 to 50 (max. 55) μm; and ascospores 15 to 25 (max. 30) × 10 to 15 μm. Nuclear rDNA sequence data was obtained for ITS1, 5.8S, and ITS2 regions using primers PMITS1 and PMITS2. A BLASTn search revealed that the data (430 bp) obtained (GenBank Accession No. KF258718) exactly matched respective sequences of Sawadaea tulasnei (AB193363, AB193385, AB193390, AB193391, and EU247884), and differed from respective sequence data for S. bicornis (AB193380) by 22 nucleotides. S. tulasnei is a European species previously found on maples including Acer platanoides. However, this pathogen has been reported only rarely in North America in the states of New York, Ohio, and Wisconsin, and the province of Quebec (2,3,4). Although not likely to seriously affect the growth or survival of established trees, severe powdery mildew can detract from the aesthetic value of trees. Further, S. tulasnei might seriously damage small seedlings, and detection of this exotic pathogen in nurseries should restrict movement of stock. To our knowledge, this is the first report of S. tulasnei in western North America, and it indicates the distribution of this pathogen is transcontinental. Specimens have been deposited in the U.S. National Fungus Collections (BPI 892675).
References (1) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena-Stuttgart-New York, 1995. (2) S. Hirose, et al. Mycol. Res. 109:912, 2005. (3) B. Hudelson, et al. Plant Dis. 92:485, 2008. (4) J. Weiland and G. Stanosz. Plant Dis. 90:830, 2006.
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