Leaves affected by powdery mildew were collected from a Norway maple tree in early October 2007 in Beaver Dam, WI (Dodge County). Diseased leaves were present throughout the crown of this tree, with white mycelium in irregular and often vein-associated spots and often covering as much as 50% of the upper surfaces of leaves. Examination of the lower surfaces revealed necrosis of the areas underlying mycelium. Blades of samaras also bore white mycelium. Chasmothecia were present singly or in groups on the mycelium. Morphology of chasmothecia, including simple and bifid appendages with uncinate to circinate apices, was sufficient to identify the pathogen to the genus Sawadaea (1). Data for nuclear rDNA ITS sequence (546 bp) obtained for a specimen (GenBank Accession No. EU247884) exactly matched sequences for Sawadea tulasnei (GenBank Accession Nos. AB 193363, 478 bp; AB193385, 490 bp; AB193390 and AB193391, 546 bp). This data was 96% similar (528 of 552 nucleotides) to that of another European powdery mildew pathogen, S. bicornis (GenBank Accession No. AB193380), which is also reported to occur on maples in Idaho, Washington, and Wisconsin (2,3). A further survey revealed the same fungus on several additional nearby Norway maples along streets and in yards (including varieties with both darkly colored and variegated leaves), but on these trees very few leaves were affected and usually less than 5% of the upper leaf surfaces bore mycelium. This pathogen was not observed on leaves of either red (A. rubrum) or silver maples (A. saccharinum) examined in the same area. S. tulasnei was previously known in North America only by collections in New York, Ohio, and Montreal, Canada (4), but our observation indicates that the geographic distribution of this pathogen is probably much broader and overlaps with that of S. bicornis. Specimens from Beaver Dam, WI have been deposited in the U.S. National Fungus Collections (BPI 878273).
References: (1) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena-Stuttgart, New York, 1995. (2) C. Nischwitz and G. Newcombe. Plant Dis. 87:451, 2003. (3) G. Stanosz et al. Plant Dis. 91:636, 2007. (4) J. Weiland and G. Stanosz. Plant Dis. 90:830, 2006.
