Downy mildew of impatiens has been observed recently on impatiens in greenhouses in New York and Massachusetts. This disease is caused by the oomycete pathogen Plasmopara obducens. Garden impatiens (Impatiens walleriana), double impatiens or garden balsam (I. balsamina), and the jewelweed species I. pallida and I. capensis are all highly susceptible to downy mildew; however, New Guinea impatiens (I. hawkeri) is highly resistant.
Young plants and immature tissues are most susceptible to infection. Symptoms typically start with a few leaves that appear slightly chlorotic or stippled and become completely yellow over time. White, fuzzy growth eventually appears on the undersides of leaves. Plants may be stunted. Leaves eventually drop, leaving bare stems.
Spores are produced by the downy white growth on the undersides of infected leaves. These spores are spread to other plants by wind currents and splashing water. All downy mildew pathogens require a film of moisture on plant leaves in order to germinate and infect the host, so wet foliage and high humidity are conducive to disease development. For more information, please see our freshly revised Downy Mildew of Impatiens fact sheet at https://ag.umass.edu/greenhouse-floriculture/fact-sheets/downy-mildew-of....
Impatiens downy mildew is best prevented by implementing good cultural management practices, including the following
- Inspecting incoming plants for signs and symptoms of disease, and not allowing unhealthy plants into the greenhouse.
- Decreasing relative humidity and leaf wetness in the greenhouse by heating and ventilating, along with increasing horizontal air flow.
- Spacing plants appropriately to decrease humidity in the canopy and promote rapid drying of leaves.
- Avoiding overhead watering if possible.
- Keeping seed and vegetative impatiens in separate greenhouses (Impatiens downy mildew is not known to be seed borne).
- Removing jewelweed, if present, from around greenhouses.
- Scouting regularly for signs and symptoms of disease.
- Removing and disposing of any infected plants, as well as asymptomatic plants adjacent to infected plants(when discarding plants, bag and remove the entire plant including the soil and any plant or soil or leaf debris, and do not compost).
Growing downy mildew resistant cultivars such as the SunPatiens, Bounce, and Imara series. For a full list of registered fungicides, please see the New England Greenhouse Floriculture Guide: http://ag.umass.edu/greenhouse-floriculture/publications-resources/new-e...
- Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab