Single and double flowered impatiens (Impatiens walleriana Hook.f.) plants with symptoms of downy mildew were found in commercial greenhouses in Delaware, Wayne, and Holmes counties, Ohio, in April 2012. Plants were stunted and defoliated. Symptoms on remaining leaves included general chlorosis without discrete spots and downward curling of leaves. A downy white growth was observed on the lower surface of infected leaves. The disease was widespread in affected greenhouses and incidence in cvs. Shimmer Coral, Accent Mix, and Super Elfin was nearly 90%. The downy growth consisted of coenocytic mycelia, monopodial sporangiophores, and ovoid, hyaline sporangia typical of Plasmopara obducens (J. Schröt.) J. Schröt in Cohn (1,2,4). Sporangia were borne on branchlets measuring 5 to 15 μm long (average 10 μm) at right angles to the main axis of the sporangiophore. Sporangia were 9.4 to 17.5 × 12.8 to 16.3 μm. No oospores were observed. Total DNA was extracted directly from plant tissue with the Wizard SV Genomic DNA Purification System (Promega, Madison, WI) following the manufacturer's instructions. Large ribosomal subunit DNA was amplified by PCR using primers NL-1 and NL-4 (3). Amplicons of 690 bp and 834 bp were produced from each diseased sample, while only the 690-bp amplicon was produced from healthy tissue. DNA from each amplicon of sample IDM041712 was purified using the Wizard SV Gel and PCR Clean-Up System (Promega), sequenced, and the sequence of the diagnostic 834-bp amplicon was deposited in GenBank (JX142134). The sequence of the 834-bp amplicon was 99% similar to those of P. obducens isolates from Serbia (HQ246451) (1), the UK (AY587558), and Austria (EF196869). The sequence of the 690-bp amplicon (JX142135) was 99% similar to that of I. walleriana (HQ223336). Twelve young impatiens ‘Shimmer Coral’ plants were inoculated with sporangia washed from infected leaves (1 × 104 sporangia/ml). Plants were incubated at room temperature for 24 h in a moist chamber and then maintained in a greenhouse (21 to 23°C) until symptoms appeared. Control plants were sprayed with sterile water and maintained in the same environment. After 12 to 14 days, typical symptoms of downy mildew developed on the inoculated plants and microscopic examination revealed the same pathogen morphology as the original isolate. All non-inoculated control plants remained disease free. To our knowledge, this is the first report of downy mildew on impatiens in Ohio. This disease caused considerable economic losses in Ohio in 2012 and is likely to be a recurring problem requiring intensive preventative management.
References: (1) A. Bulajic et al. Plant Dis. 95:491, 2011. (2) O. Constantinescu. Mycologia 83:473, 1991. (3) W. Maier et al. Can. J. Bot. 81:12, 2003. (4) S. N. Wegulo et al. Plant Dis. 88:909, 2004.
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