Green Thumbs: Downy mildew fungus strikes popular impatiens bedding plants

Impatiens, a popular bedding plant, brighten shady areas. Plants infected by downy mildew do not recover and soon die.

Question: Last year I had a problem with my impatiens. Most of my neighbors had the same problem. I would like to know if I should plant impatiens again and if you have any information on what was going on with those plants?

Answer: Impatiens are one of the most popular bedding plants for home gardeners. They are easy to grow, provide wonderful color to shady areas throughout the season, and usually have few insect and disease problems. In the Eastern United States for the past two years there has been a very destructive disease that has impacted the use of impatiens in our gardens. A fungus known as downy mildew affects impatiens walleriana, the common bedding plant as well as double flowered, mini-impatiens and fusion and butterfly impatiens. Other varieties such as balsam impatiens show milder symptoms in the form of mostly yellow spots. New guinea impatiens are not affected by this disease.

A downy mildew infection begins as a mild yellowing of the leaf. This subtle yellowing may not seem too worrisome or can be mistaken for a nutrient or insect problem. Infected plants may not show any symptoms for a period of time but when conditions are right the disease becomes apparent. Once the susceptible plants are infected, they will not recover.

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Infected plants may look healthy from a distance but as you move closer the symptoms are more obvious. The leaves progressively begin to curl downward, looking as if the plants need a good watering. If conditions are humid you may see a white coating on the undersides of the leaves. This is the most indicative sign of downy mildew impatiens but usually goes unnoticed because of the location. Eventually the flowers will fall off, followed by the leaves and then the stem collapses. Nutrient-stressed plants will show the disease symptoms earlier than well-fertilized plants.

The impatiens become infected either by spores from nearby infected plants, wind or from the soil. When conditions are cool, humid or moist the disease is more common. Infections occur when there is a layer of moisture on the leaves for a few hours. So rain, overhead irrigation, crowding or shade will encourage infection. Once infected plants are discovered they should be pulled, roots included, and disposed of properly. Pulled plants should not be composted or left on the ground.

Since it is possible for the disease to overwinter, susceptible impatiens should not be planted in previously infected areas. New guinea impatiens, begonias, pansies, lobelia, caladium or coleus would make wonderful, shade-tolerant alternatives.

Mona Bawgus is a certified master gardener and consumer horticulturist with Rutgers Cooperative Extension of Atlantic County. Write to her c/o Rutgers Cooperative Extension, 6260 Old Harding Highway, Mays Landing, NJ 08330. Email:

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