A severe foliage disease was observed on black-eyed Susan (Rudbeckia fulgida cv. Goldsturm) at a commercial nursery in Virginia Beach, VA in early June of 2005 and 2006. In both years, disease symptoms began after plants were transferred from 1-liter (1quart) to 4-liter (1-gallon) containers. Several hundred ‘Goldsturm’ plants were grown at this nursery but none was marketable because of this disease. Typical symptoms were dark, blotchy necrotic areas on the upper leaf surface. Fuzzy, grayish white zoosporangiophores and zoosporangia were commonly produced on the underside of affected leaves. These disease signs also were observed on both sides of new leaves in severely affected plants. Zoosporangiophores were monopodially branched at right angles with acutely tapering termini. Zoosporangia were ovoid to elliptical, hyaline, and measured 19 to 28 × 17 to 21 μm. On the basis of these morphological characters and host specificity, the organism was identified as Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc. Inoculum at 1.4 × 104 zoosporangia/ml was prepared by washing diseased leaves with deionized reverse osmosis water and applied with a hand sprayer onto the foliage of test plants until runoff. Control plants were treated with deionized reverse osmosis water only. Inoculated and control plants were placed in plastic bags to keep moist for 48 h at 20°C. The inoculated plants developed the same symptoms with fresh crops of the zoosporangiophores and zoosporangia of P. halstedii on the underside of leaves, while the control plants remained healthy 14 days after treatment. The source of the primary inoculum is not known. Interestingly, none of 12 other Rudbeckia spp. and cultivars (R. hirta cvs. Autumn Colors, Becky, Cherokee Sunset, Indian Summer, Irish Eye, Prarie Sun, and Sonora, R. laciniata, R. maxima, R. missouriensis, R. occidentalis cv. Green Wizard, and R. speciosa cv. Viette's Little) grown at the same nursery adjacent to those severely diseased ‘Goldsturm’ plants was affected. Although downy mildew was reported on rudbeckias in north Florida previously (1), to my knowledge, this is the first report from Virginia. This pathogen attacks a number of plants in the genera of Centaurea, Coreopsis, Erigeron, Helianthus, and Verbena in addition to Rudbeckia spp. (2). Thus, it could significantly impact the ornamental plant nursery industry.
References: (1) H. Dankers et al. Plant Health Progress. Online publication. doi:10.1094/PHP-2004-0119-01-HN, 2004. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.
