Sawadaea Powdery Mildew of Box Elder (Acer negundo) in Wisconsin

Box elder leaves affected by powdery mildew were collected from a single tree in an urban yard in the fall of 2006 in Madison, WI. As much as half of the area of the leaf blades, when viewed from above or below, was pale green-to-tan and necrotic. Mycelium was not visible to the eye on either the upper or lower leaf surfaces, but very sparse white mycelium was observed on the lower surface of leaves with the aid of a dissecting microscope. Chasmothecia were present singly or in groups of a few on the lower surface of leaves. Morphology of the chasmothecia, including simple and bifid appendages with uncinate to circinate apices, was sufficient to identify the pathogen to the genus Sawadaea (1). Other characters were not sufficiently distinct to make an identification of the species. However, DNA was extracted from the chasmothecia, and analysis of a 542-bp sequence of nuclear rDNA ITS (GenBank Accession No. EF122238) revealed a 100% match with the respective sequence obtained from Genbank for S. bicornis (Accession No. AB193380). A specimen from which these chasmothecia were obtained has been deposited in the U.S. National Fungus Collections (BPI 877328). S. bicornis is a European pathogen of maples, including box elder when grown there (1). Knowledge of the geographic distribution and hosts of Sawadaea powdery mildews in North America is extremely limited. S. bicornis was first reported in North America only recently, occurring in the states of Idaho and Washington on Norway maple (Acer platanoides) (3). The morphologically very similar European powdery mildew pathogen of maples, S. tulasnei, is known only from New York, Ohio, and Montreal, Canada (4), and an unidentified Sawadaea species on box elder has been reported from California (2). 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) C. Nischwitz and G. Newcombe. Plant Dis. 87:451, 2003. (4) J. Weiland and G. Stanosz. Plant Dis. 90:830, 2006.