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Impatiens downy mildew is a destructive foliar disease of garden Impatiens (Impatiens walleriana). While downy mildew of Impatiens downy mildew is parasitic on Impatiens, I. balsamina (balsam impatiens, garden balsam, or rose balsam) I. pallida and I. capensis  (native wild impatiens known as jewelweed) are also susceptible;  it does not threaten other flower or vegetable crops. Impatiens x New Guinea Hybrids (New Guinea impatiens) are not affected.

While there have been sporadic reports of this disease in production greenhouses in the United States since 2004, widespread regional outbreaks of impatiens downy mildew were observed for the first time in North American landscapes in 2011.  The first reports of Impatiens downy mildew in New Jersey landscapes came in late June 2012, largely in Monmouth and Ocean counties.

The organism that causes downy mildew is a type of “water mold” or oomycete, formerly referred to as a fungus. Other similarly classified pathogens include Phytophthora spp. And Pythium spp. Downy mildew can spread by two different types of spores. One type is easily airborne and remain viable for just a short time; and the other type, a zoospore, which moves through a film of water.

The spores develop and infect Impatiens when a film of water is present on the plant tissue, and the relative humidity in the air is high, during cool or warm periods. Sporulation and infection will not occur under hot or dry conditions.

Downy mildew symptoms on infected plants begin with leaf stippling, downward curling of leaves and leaf yellowing.  A white, downy-like growth may be present on the underside of yellow leaves, but can also be found on the underside of green leaves. As the disease progresses, leaf drop occur resulting in bare, leafless stems.

Remove and dispose of infected plants (roots included) immediately.  Do not compost the infected plant material. A wide range of commercial fungicides can offer short‐term protection (check Rutgers Recommendations) need to be reapplied regularly throughout the season, but products available to homeowners provide little control.  Avoid overhead irrigation (especially night-time irrigation) and any conditions that result in long periods of leaf wetness.

Plant growers in New Jersey have applied fungicides to protect the plants from downy mildew throughout their production cycle , but that protection only lasts a few weeks after the plants leave the greenhouse or garden center.

Ball Horticulture and Syngenta have also released fact sheets on impatiens downy mildew with detailed information on the disease cycle and management:

Click here for the Ball Horticulture fact sheet.
Click here for the Syngenta fact sheet.

Click here for more information and photos.