AMES, Iowa — Impatiens, the top-selling bedding plants in the United States, are easy to grow, provide color to shady areas throughout the growing season, and have few insect and disease problems. Unfortunately, the disease situation appears to be about to change as impatiens downy mildew has appeared in much of the eastern U.S. in the last two years. This destructive disease may impact many home gardeners over the next few years. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
Symptoms of impatiens downy mildew initially appear as yellowing of infected leaves. The yellow-green foliage may initially be confused with a nutritional deficiency. As the infection progresses, leaves may curl downward and a white, fuzzy growth can be seen on the undersides of the leaves. Severe infections often lead to early defoliation and blossom drop, leaving bare stems with a few tiny, yellow leaves at the tips of the shoots.
Downy mildew is caused by the water mold Plasmopara obducens. This fungus-like pathogen thrives under cool temperatures and moist or humid conditions. Disease spores (zoospores) are produced in spore-bearing structures called sporangia. The sporangia are responsible for the white, fuzzy growth on the undersides of the leaves. The sporangia are easily dislodged and can be splashed by rain or water to other leaves and carried to distant plants by air currents. When provided with a wet leaf surface (either from rain or irrigation water), zoospores released by the sporangia invade the leaf tissue and begin an infection. Impatiens downy mildew also produces a second type of spore (oospore). Oospores form inside infected leaves and stems and are released into the soil as the plant debris decays. These resting spores may survive in the soil for a period of several years.
All varieties of the common garden impatiens (Impatiens walleriana) and interspecific hybrids with an Impatiens walleriana parent (such as Fusion and Butterfly impatiens) are susceptible to impatiens downy mildew. Garden balsam (Impatiens balsamina) is also considered susceptible to impatiens downy mildew. New Guinea impatiens (Impatiens hawkeri) are highly tolerant to this disease. No other annuals are infected by this pathogen.
In landscape plantings, control of downy mildew on impatiens with fungicides is not practical. Sanitation is the best management strategy for this disease. Remove and destroy infected plants as soon as they are noticed to reduce the spread to healthy plants and minimize the amount of over-wintering inoculum (oospores). Do not place infected plant material in home compost piles. Temperatures in home compost piles often don’t get high enough to kill disease pathogens. Avoid planting common garden impatiens in areas that experienced a downy mildew problem the previous year. Plant shade tolerant alternatives, such wax begonias, pansies, lobelia, torenia, caladium and coleus.