Impatiens are one of the most popular bedding plants for shade gardens worldwide. In 2011, impatiens downy mildew was an unpleasant surprise for greenhouse growers and gardeners. Sometimes pathogens can seem cyclic and reoccur following several years of being absent.
Seed and vegetatively propagated Impatiens spp. including common seed impatiens (Impatiens walleriana), double impatiens and garden balsam (Impatiens balsamina) are susceptible. Fortunately, New Guinea impatiens (Impatiens hawkeri) appear to be tolerant or resistant to downy mildew. The downy mildew disease that affects Impatiens spp. is caused by a fungal-like microscopic organism called Plasmopara obducens. This downy mildew is unique to impatiens. Snapdragons, roses, impatiens, coleus, basil and other crops are susceptible to downy mildew, but each have a unique downy mildew pathogen. While the disease may look similar on the different crops, they are quite different. The downy mildew on snapdragons cannot blight roses nor can the downy mildew on impatiens spread to any plant outside of Impatiens.
In some cases before the pathogen is halted, the loss of plant material can be severe. Given these recent problems, growers should review the downy mildew basics because this can be an explosive, destructive and costly pest if it is not managed.
Read more at Michigan State University (Mary Hausbeck, Blair Harlan and Roberto Lopez)