During spring 2012, potted impatiens (Impatiens walleriana Hook.f.) plants with symptoms of a foliar disease were found in several commercial greenhouses in Mobile County, Alabama. Symptomatic leaves were chlorotic with no distinct lesions, and quickly wilted and abscised from erect green stems. In summer 2012 and 2013, numerous landscape impatiens plants with similar symptoms were observed in a large area from Mobile County north to Lee County, Alabama. A downy mildew was observed on the lower surfaces of symptomatic and abscised leaves from all locations. It consisted of hyaline, monopodial sporangiophores and ovoid, hyaline sporangia. Sporangiophores, which emerged from stomata, consisted of apical branches arranged at right angles to the supporting branches; they measured 69 to 90 μm long with individual branches measuring 7 to 14 μm long. Sporangia were borne on the tips of sporangiophore branches and measured 10 to 16.5 × 17 to 22.5 μm. No oospores were observed. In 2013, symptomatic plants were obtained from two separate locations in Alabama (Mobile and Tallapoosa counties). Total genomic DNA was extracted directly from symptomatic plant tissue and the large ribosomal subunit DNA was amplified by PCR using primers NL-1 and NL-4 (1). From both isolates, amplicons of 600 and 775 bp were obtained. DNA from each amplicon of both isolates was purified, sequenced, and the sequences were deposited in GenBank (Accession Nos. KF956518 to 21). The sequences of the 600-bp amplicons were 99% similar to that of I. walleriana (JX142135); the sequences of the 775-bp amplicons were 99% similar to Plasmopara obducens isolates from Florida (JX217746), Ohio (JX142134), Serbia (HQ246451), and the United Kingdom (AY587558). In pathogenicity tests, 10 potted impatiens plants, I. walleriana‘Super Elfin,’ were inoculated with a sporangial suspension (1 × 105 sporangia/ml washed from infected leaves) from the Mobile County isolate, by spraying until runoff. Controls were inoculated with sterile water. Plants were incubated in a moist chamber at 21°C for 48 h and then maintained in a greenhouse at 22 to 25°C until symptom development. All inoculated plants developed symptoms of downy mildew within 10 days. Microscopic examination of the symptomatic tissue revealed sporangiophores and sporangia similar to those observed in naturally infected plants. Control plants showed no symptoms. To our knowledge, this is the first report of downy mildew caused by P. obducens on impatiens in Alabama. This disease has been reported to have a significant economic impact for growers throughout the United States (2,3). Impatiens downy mildew is likely to be a recurring problem in Alabama.
References: (1) K. O'Donnell. Curr. Genet. 22:213, 1992. (2) A. Palmateer et al. Plant Dis. 97:687, 2013. (3) S. Wegulo et al. Plant Dis. 88:909, 2004.
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