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First Report of Melampsora larici-populina on Populus spp. in Eastern North America

In September 2002, yellow spots were observed on the leaf surface of a hybrid poplar (Populus maximowiczii Henry × P. balsamifera L.) grown at the Berthier forest nursery (46°2′N, 73°11′W) in the St. Lawrence Valley (Lanaudière Region, Québec, Canada). Disease severity was low, but the pathogen was present on a hybrid that was previously thought to be resistant to Melampsora medusae Thuem, the only reported poplar rust in eastern North America. Uredinia typical of a Melampsora sp. were observed on the abaxial leaf surface. The observed urediniospores were longer (32 to 48 μm) than the expected range for M. medusae (23 to 35 μm) and possessed an apical bald spot; thick paraphyses were also observed. These characteristics are diagnostic of M. larici-populina Kleb (2). Samples were deposited in the National Mycological Herbarium of Canada (DAOM 232107 and 232108) and in the Quebec Forest Biology Herbarium (QFB14703 and 14704). DNA was extracted from uredinia, and the internal transcribed spacer (ITS) of the ribosomal RNA gene was amplified and sequenced (GenBank Accession Nos. AY429656 and AY429657). There was a 100% match between the two sequences obtained and that of M. larici-populina (GenBank Accession No. AY375267), but there was approximately 12% divergence with the ITS sequence of M. medusae (GenBank Accession No. AY375273-5). This is the first report of M. larici-populina in eastern North America. This fungus was reported on P. trichocarpa × P. deltoides hybrids in the western United States in the early 1990s (1). It appears that M. larici-populina can overwinter in Québec because it was observed again at the nursery in September 2003. The occurrence of M. larici-populina in eastern North America has direct implications for the poplar industry since the host specificities of M. medusae and M. larici-populina differ; P. balsamifera and P. maximowiczii are sensitive to M. larici-populina (3). Hybrids with P. balsamifera or P. trichocarpa components may be particularly at risk.

References: (1) G. Newcombe and G. A. Chastagner. Plant Dis. 77:532, 1993. (2) J. Pinon. Eur. J. For. Pathol. 3:221, 1973. (3) J. Pinon. Silvae Genet. 41:25, 1992.