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Title: First report of downy mildew disease caused by Plasmopara halstedii on the native Rudbeckia fulgida Aiton var. speciosa (Wender.)

item RIVERA, YAZMIN - Rutgers University
item CRESWELL, THOMAS - Purdue University
item Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2015
Publication Date: 4/14/2015
Citation: Rivera, Y., Creswell, T.C., Crouch, J. 2015. First report of downy mildew disease caused by Plasmopara halstedii on the native Rudbeckia fulgida Aiton var. speciosa (Wender.). Plant Disease.

Interpretive Summary: This research describes a novel plant disease outbreak affecting crops of a native perennial wildflower form of the popular black eyed Susan plant, Rudbeckia fulgida Aiton var. speciosa. The disease is referred to as a downy mildew, named for the cushiony white appearance of the destructive fungal-like parasite that causes the disease. This downy mildew was identified from a commercial nursery in Indiana, and resulted in the loss of over 300 black eyed Susan plants. This the first time that downy mildew disease has ever been observed from this plant. Knowledge of this disease will be useful to plant regulatory officials working to control the spread of impatiens downy mildew in the United States.

Technical Abstract: The showy black-eyed Susan Rudbeckia fulgida Aiton var. speciosa (Wender.) is an important perennial wildflower native to the Northeast and Midwest regions of the United States. Besides its aesthetic value in the landscape, this native plant attracts pollinators and provides seeds for birds during the winter months. During the summer of 2014, disease symptoms characteristic of downy mildew were observed on approximately 300 small potted plants (32 cu in) at one commercial nursery in the Delaware County of Indiana. Plants affected exhibited dark lesions on the adaxial leaf surface and severe white sporulation on the abaxial surface of leaves. Microscopic examination revealed sporangiophores (up to 900 'm in length) monopodially branched with several terminal branchlets at right angles. Observed sporangia were hyaline, mostly ovoid with smooth surfaces ( = 24 x 20 'm, n = 50). The morphological characteristics of this organism associated with the downy mildew signs and symptoms were consistent with the biotrophic oomycete Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc. Diseased leaves were dried and deposited as voucher speciments in the U.S. National Fungus Collection. In order to confirm morphological identification, genomic DNA from sporangial and sporangiophore masses was extracted using the Qiagen Plant DNA kit (Qiagen, Gaithersburg MD). The large subunit of the nuclear rDNA was PCR amplified by PCR using the primers LROR and LR7 and sequenced bidirectionally. The generated sequences were compared to those in the NCBI GenBank database using a BLASTn search, revealing a 99% nucleotide identity with curated P. halstedii sequences. This finding agreed with the morphological identification. Plasmopara halstedii is a highly destructive disease on sunflowers and more recently a problem on the widely-planted perennial Rudbeckia fulgida cv. ‘Goldsturm’. To our knowledge, this is the first report of P. halstedii on the native perennial R. fulgida var. speciosa in the United States. This report expands the host range of this pathogen within the genus Rudbeckia and presents a potential threat to natural settings and nursery producers.