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First Report of Powdery Mildew Caused by Podosphaera pannosa on Prunus cerasus in France

Podosphaera pannosa (Wallr.:Fr.) de Bary (anamorph Oïdium leucoconium Desm.) is described as the most frequent species causing powdery mildew of members of the Rosaceae family, especially on Rosa spp. and Prunus spp. P. pannosa is reported as cosmopolitan, but its occurrence on Prunus cerasus (cherry) is limited to Hungary (3). During spring 2011, typical symptoms of powdery mildew were observed in a Prunus cerasus orchard located in La Roche de Glun (southeastern France). On average, 25% of the shoots per individual tree were affected by this disease. Although several different cultivars were grown in the orchard, cultivar Bigalise alone displayed powdery mildew symptoms. The lower surface of the leaves was covered with superficial, white, dense mycelium, whereas the upper side showed discoloration, necrosing patches, and blisters. Microscopic slides were prepared from fresh material by gently pressing a clear adhesive tape onto the lower surface covered by mycelium, which was further stained with lactic acid/methyl blue. The presence of powdery mildew was confirmed by the observation of typical microscopic features of the anamorphic stage of the fungus (2). Conidiophores were erect. Conidia (oïdia) were hyaline and keg-shaped, and developed basipetally in chains of six to eight conidia. Conidial dimensions were 17 to 29 (23) × 9 to 17 (14) μm. No cleistothecia (teleomorphic state of the fungus) were observed. Species identity was determined by sequencing the ITS region of the rDNA followed by comparison with reference sequences available on GenBank (1). Fungal material was collected from infected leaves by scraping the mycelium with a sterile needle, and was transferred into 2-ml microtubes. Fungal total DNA was then extracted using a commercial plant DNA extraction kit and the ITS region was amplified by PCR using the ITS1-ITS4 primer pair (4). Nucleotide sequence was determined and deposited in GenBank (Accession No. JN654341). Analysis of the sequence by BLAST showed 100% identity with Podosphaera pannosa. To our knowledge, this is the first report of Podosphaera pannosa on Prunus cerasus in France. This species was hitherto scarcely reported on cherry trees, and may deserve more attention in the future.

References: (1) M. Gàbor et al. Eur. J. Plant Pathol. 131:135, 2011. (2) G. Grove et al. Page 12 in: Compendium of Stone Fruit Diseases. American Phytopathological Society, St Paul, MN, 1995. (3) L. Vajna et al. New Dis. Rep. 12:15, 2005. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications, 1990.