Downy Mildews

by Elizabeth H. Roche, Nancy J. Taylor & Francesca Peduto Hand

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Downy Mildews are some of the most destructive diseases of ornamental crops. Contrary to Powdery Mildews, which are caused by true Fungi, downy mildews are caused by a distinct group of organisms called Oomycetes. Members of this group of pathogens also include water molds such as Pythium and Phytophthora species. Most of the downy mildew pathogens are host specific, being able to infect only one plant family. Pathogen species include Peronospora, Bremia, Plasmopara and Basidiophora. They infect a wide variety of greenhouse crops including Pansy, Snapdragon, Salvia, Primula, Verbena as well as cut flowers, including Scabiosa and Stock. In recent years, new destructive downy mildews have been reported on Coleus and garden Impatiens. Symptoms of the disease include severe defoliation, flower drop, and eventual plant death. The disease can only be managed if caught early so careful scouting is a key component of downy mildew management programs.