Sawadaea tulasnei Powdery Mildew of Norway Maple (Acer platanoides) in North America

Norway maple leaves bearing powdery mildew were collected from one location in the fall of 2003 and four locations (as much as 1.5 km apart) in the fall of 2005 in Buffalo, NY. No powdery mildew was observed on leaves collected from sugar maples (Acer saccharum) that were present in the vicinity of affected Norway maples at two locations. Trees were located along streets and in yards. Diseased leaves were present throughout tree crowns but lower leaves were more commonly affected. White mycelium was present in irregular, discrete, scattered spots only on the upper surface of leaves and on both sides of wings of samaras. Typically, <10% of the upper leaf area bore visible mycelium. Cleistothecia were present singly or in groups on the mycelium. Morphology of cleistothecia on leaves collected each year, including simple and bifid appendages with uncinate to circinate apices, was sufficient to identify the pathogen to the genus Sawadaea (1). Other characteristics were not sufficiently distinct to make an identification of S. bicornis or S. tulasnei (1), each a European species found on Acer spp. However, a sample from 2003 was supplied by the authors for use in a study of phylogeny of the genus (2) that served as a first report of the species in the United States. Analysis of nuclear rDNA ITS sequence of this specimen (GenBank Accession No. AB193390) placed the sample in a clade with S. tulasnei specimens from Europe. In the same study, powdery mildew samples from Acer spp. in Ohio and Montreal, Canada also were placed in this clade. Thus, occurrence of S. tulasnei in North America is confirmed. S. bicornis was recently identified (based on morphology) on Norway maple in the western United States (3). Specimens from Buffalo, NY have been deposited in the U.S. National Fungus Collections (BPI 871210). 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.