Shoots affected by powdery mildew were collected from Siberian pea trees in July 2009 on the University of Wisconsin--Madison campus and on the campus of Université Laval, Quebec City, Quebec. This exotic shrub or small tree is infrequently planted in Wisconsin and three shrubs in a group that were affected are the only examples known on the UW--Madison campus. In Quebec City, Siberian pea tree is more commonly used as an ornamental, often in hedges (as is the case of the affected plants on the Université Laval campus). In both locations, <10% of foliage was visibly affected, but incidence was greater on shoots closer to the ground than on higher shoots. White-to-grayish mycelium was present on leaves and young stems and sometimes completely covered both upper and lower leaf surfaces. Dark brown-to-black chasmothecia were numerous on leaf blades, petioles, and young stems, but were most abundant on lower surfaces of leaves. Morphology of chasmothecia, including appendages with distinctive terminal dichotomous branching, (1) was consistent with descriptions and illustrations of the fungus Erysiphe palczewskii Jacz. (synonym Microsphaera palczewskii) (1--4) thought to be native to Asia, but also known as an invader of Europe where it occurs on the same host. For a sample from Université Laval, mean diameter of chasmothecia was 113 μm, mean appendage length was 185 μm, and barrel-shaped conidia that lacked fibrosin bodies averaged 30 × 14 μm. Asci contained oval, yellow ascospores with mean dimensions of 20 × 12 μm. DNA was extracted from chasmothecia, and nuclear rDNA sequences (633 nucleotides) of the Wisconsin (GenBank Accession No. GQ497277) and Quebec (GenBank Accession No. GQ497276) specimens differed by only one nucleotide. The sequences that were obtained most closely matched GenBank sequences for Oidium spp. (98%) and Erysiphe spp. (97%). Further observations indicated that the same pathogen affected Siberian pea trees planted as ornamentals at several locations separated by ≥15 km in the metropolitan Quebec area. This report extends the eastern known limit of E. palczewskii in the United States, previously known from collections in Alaska (2), Washington (4), Idaho (4), North Dakota (3), and Minnesota (3). To our knowledge, this is the first report of this disease in Canada, and it indicates that the distribution of E. palczewskii is transcontinental. Specimens from Madison, WI and Quebec, QC have been deposited in the U.S. National Fungus Collections (BPI 879152) and the Rene Pomerleau Herbarium of the Canadian Forest Service Laurentian Forestry Centre (QFB-22601).
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) D. A. Glawe and G. A. Laursen. Online publication. doi:10:1094/PHP-2005-1017-01-BR. Plant Health Progress, 2005. (3) D. A. Glawe et al. Online publication. doi:10.1094/PHP-2006-0117-01-BR. Plant Health Progress, 2006. (4) C. Nischwitz and G. Newcombe. Plant Dis. 87:451, 2003.