First Report of Downy Mildew Caused by Plasmopara halstedii on Black-eyed Susan (Rudbeckia fulgida cv. 'Goldsturm') in Maryland.

Abstract

The North American perennial black-eyed Susan (Rudbeckia fulgida cv. Goldsturm) is an important nursery crop, prized by gardeners and landscapers for its persistent bloom and ease of cultivation. In September 2013, disease symptoms characteristic of downy mildew were observed from multiple R. fulgida plants at two commercial nurseries in the Maryland counties of Howard and Anne Arundel. Over 100 R. fulgida were affected by this disease in both nurseries, rendering the plants unmarketable and causing a substantial financial loss. Plants exhibited dark necrotic lesions on the adaxial leaf surface, and sporulating masses of white mycelium on the abaxial leaf surface and on the adaxial in extreme infections. Plants were stunted with a reduced number of blooms. Microscopic visualization showed coenocytic mycelium, hyaline sporangiophores (length 261 to 904 μm; = 557 μm; n = 20) that were straight and monopodially branched at right angles with several terminal branchlets. Sporangia were hyaline, ovoid to elliptical with smooth surfaces ( = 31 × 28 μm; n = 50). Based on morphological data, the organism was identified as Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc (2). Voucher specimens were deposited in the U.S. National Fungus Collections (BPI 892792 to 892794). Molecular identification was conducted by extracting genomic DNA from sporangiophores and mycelium tweezed from the surface of three infected plants, with extractions performed using the QIAGEN Plant DNA kit (QIAGEN, Gaithersburg, MD). The large subunit of the nuclear rDNA was amplified by PCR using primers LROR and LR7 (3) and sequenced bidirectionally. BLASTn searches of NCBI GenBank showed that the resultant rDNA sequences (accessions KF927152 to KF927154) shared 99% nucleotide identity with curated P. halstedii sequences, consistent with morphological identification. To confirm pathogenicity, three 3.78-liter (1 gallon) containerized R. fulgida cv. Goldsturm plants were inoculated with a sporangial suspension of 2.4 × 104 sporangia/ml and sprayed until both the upper and lower surface of the leaves were completely covered. One negative control plant was sprayed with deionized water. Plants were placed in clear plastic bags in a growth chamber (20°C, 12-h photoperiod). Disease symptoms were observed 3 days post inoculation on all plants. The control plant was symptomless. Morphological features of the pathogen on the surface of inoculated plants were identical to those observed from the original infected plants. Although P. halstedii on R. fulgida cv. Goldsturm has been previously reported in Virginia in 2006 and Florida in 2004, to our knowledge, this is the first report on R. fulgida cv. Goldsturm in Maryland (1). Black-eyed Susans are widely distributed throughout Maryland's landscape and are a staple plant for gardeners, nurserymen and landscape professionals. Given the destructive nature of this disease, downy mildew has the potential to cause considerable economic losses to the state's ornamental crop industry.