One of the standbys of the shade garden, impatiens has in the past been a pretty trouble-free plant in most situations. Unfortunately, a new disease has arrived on the scene that threatens impatiens in many parts of the country. Although downy mildew of impatiens was first observed in the U.S. in 2004, it reached epidemic proportions by last year.
Caused by the water mold Plasmopara obducens, the first signs of the disease are stunted, yellowing plants, with leaves that may curl downward as if they were suffering from drought. The undersides of the leaves may be covered with a fluffy white coating. Eventually the leaves and flowers drop, leaving bare stems with only a few small, yellow leaves left clinging to them. If you are unsure of your diagnosis, contact your local Extension Service for assistance.
Cool temperatures, especially at night, wet foliage, and high humidity all favor the development of this destructive disease. It infects all types of garden impatiens (Impatiens walleriana), as well as balsam impatiens (I. balsamina). However, New Guinea impatiens (I. hawkeri), including SunPatiens®, are not affected.
Infected plants will not recover, so if any develop symptoms, pull them up, bag them in plastic, and put them in the trash, not in your compost pile. Because the disease can persist in the soil for years, if you have infected impatiens in a bed, it's a good idea to choose different plants that are not susceptible to this disease for that bed the next season, such as New Guinea impatiens, coleus, or begonias.
For more information on downy mildew of impatiens in home gardens, including color pictures of symptoms, go to: UMass Extension.