First Report of Impatiens Downy Mildew in Utah
Impatiens infected with impatiens downy mildew will not survive. The "green stick syndrome" is a common appearance of infected plants, but the surest diagnostic characteristic is the white fruiting structures on the undersides of the leaves.
In fall 2012, fall downy mildew was discovered for the first time on garden impatiens (Impatiens walleriana) in Utah. The disease is relatively new to the U.S. It was first found in California in 2004 and then in Delaware in 2009. It is now present along the east coast from New England to Florida, and in several midwest states. Downy mildew of impatiens is caused by the fungal-like organism Plasmopara obducens. The disease is specific to I. walleriana and balsam impatiens (I. balsamina) and will not affect other ornamentals.
After impatiens are infected, the disease progresses quickly, killing the plants within 3-4 weeks. The initial symptom is yellowing of the leaves, which can be overlooked as a disease indicator because nutrient deficiency looks similar. However, white fluffy mycelium and spores will be visible on the undersides of the leaves. In the advanced stage of infection, leaves will drop from the plant, seemingly overnight, leaving behind green stems.
The plant damage may be mistaken for deer or rabbit feeding, but it is important to confirm the damage before replanting new plants in the same soil. This can be done by looking for fallen leaves on the ground and inspecting them with a dissecting microscope or a strong hand lens. The mycelium and spores on the fallen leaves will infect new impatiens plants that are planted in the same spot, repeating the disease cycle.
Plasmopara obducens thrives in humidity and is spread by splashing water and wind. The spores of the pathogen overwinter in the soil where infected impatiens grew. The disease may also be spread by infected plants purchased at garden centers. Symptoms usually appear 7-14 days after infection, so plants may initially appear healthy.
Management of downy mildew is difficult. Treatment with fungicides in landscape settings has yielded mixed results. (Fungicides are effective in greenhouses.) The best method is to remove infected plants from the garden bed or greenhouse and throw them in the trash or burn them. (The spores will survive composting. ) Also gather fallen impatiens leaves because the pathogen can survive there as well. New impatiens plants should not be planted in the same location where infected plants have been for 2 to 3 years. Due to the host specificity of P. obducens, other ornamentals can be planted in the location without any problems. There are no resistant I. walleriana or I. balsamina varieties. The only commonly planted impatiens that have not been affected are New Guinea impatiens and SunPatiens.
-Claudia Nischwitz, Extension Plant Pathologist