First Report of Impatiens Downy Mildew Caused by Plasmopara obducens in Alabama

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 obt...