Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2014
Publication Date: 7/1/2014
Citation: Rivera, Y., Rane, K., Crouch, J. 2014. First report of downy mildew caused by Plasmopara halstedii on black-eyed susan (Rudbeckia fulgida cv. ‘Goldsturm’) in Maryland . Plant Disease. 98(7):1005. Interpretive Summary: Downy mildew diseases are caused by fungus-like parasites that cause significant damage to crop plants in the United States. Black-eyed Susans are a popular flowering herbaceous perennial plant, prized for their brilliant colors and ease of care in the landscape. This research describes outbreaks of downy mildew disease on black-eyed Susan plants at two commercial nurseries in Maryland during 2013, the first time this disease was observed in this state. DNA sequences and morphological characteristics were used to accurately identify the pathogen. Knowledge of this disease will be useful to plant regulatory officials working to control the spread of downy mildew diseases in the United States.
Technical 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 plants at two commercial nurseries in the Maryland counties of Howard and Anne Arundel. Over one hundred R. fulgida cv. ‘Goldsturm’ plants were affected by this disease, rendering the plants unmarketable and causing a substantial loss to both nurseries. 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 um; x = 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 ( x = 31 to 28 µm; n = 50). Based on morphological data, the organism was identified as Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc (1). Voucher specimens were deposited in the U.S. National Fungus Collections (BPI 892792-892794). Molecular identification was conducted by extracting genomic DNA from sporangiophores and mycelium tweezed from the surface of three infected plants 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 (2) and sequenced bi-directionally. BLASTn searches of NCBI GenBank showed the rDNA sequences (Accession Nos. KF927152-KF927154) shared 99% nucleotide identity with 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 x 104 sporangia/mL and sprayed until leaves were covered. One negative control plant was sprayed with deioinized water. Plants were placed in clear plastic bags in a growth chamber (20 oC, 12 h photoperiod). Disease symptoms were observed 3 d post inoculation from all plants. Control plants were symptomless. Morphological features of the pathogen on the surface of inoculated plants were identical to those observed from the original infected plants. Although downy mildew on R. fulgida cv. ‘Goldsturm’ was reported in Virginia in 2006 and Florida in 2004, to our knowledge, this is the first report of P. halstedii on R. fulgida cv. ‘Goldsturm’ in Maryland (3). Black-eyed Susans are widely distributed throughout Maryland’s landscape and are an important ornamental crop for the region’s greenhouse and landscape industry. Given the destructive nature of this disease, downy mildew has the potential to change Maryland’s landscape and cause considerable economic losses to the region’s ornamental crop industry.