The town of Saratoga, N.Y., has observed downy mildew on Impatiens walleriana in public and private gardens for about three years. So it was only a matter of time, Margery Daughtrey thought, before downy mildew arrived about 250 miles southeast in Riverhead, N.Y.
Downy mildew on impatiens officially arrived late this summer, around mid-September, when leaf yellowing and leaf drop on impatiens occurred, as well as the appearance of white spores on the undersides of leaves. Three weeks after those symptoms, stems collapsed onto the ground while nearby flower beds were still flowering and looking healthy.
“They look very strange in this ‘stems-only’ stage,” says Daughtrey, senior extension associate at Cornell University. “We haven’t had any frost so this is purely a matter of downy mildew and conducive weather conditions.”
Daughtrey describes the Northeast’s weather late summer as cooler than usual and constantly rainy. Those conditions helped spread the disease.
“The disease is all over town,” Daughtrey says. “Replanting impatiens in these beds next year would be unwise, as there are many oospores in the stems that will probably survive in the soil to start new infections. New Guinea impatiens could be used, as they do not appear to be susceptible to this [downy mildew]. Or, petunias, marigolds, or anything else could work but [Impatiens] walleriana.”
Ball Horticultural Co. is responding to impatiens downy mildew with its own recommendations to landscape contractors. The company is advising landscape contractors to closely inspect beds of Impatiens walleriana, promptly remove infected plants and, as Daughtrey suggests, to plant alternative flowers and foliage next season in beds with a history of the disease.
“Scouting impatiens beds for this disease and promptly removing infected plants and leaf debris can help reduce overwintering spores in the soil that may initiate new infections next season,” says Colleen Warfield, plant pathologist for Ball.
According to Ball, all seed and vegetative species of Impatiens walleriana are susceptible to downy mildew. New Guinea impatiens, on the other hand, are highly tolerant of the disease.
To review Ball’s current impatiens downy mildew educational resources, alternative crop recommendations and answers to landscapers’ Frequently Asked Questions, visit BallLandscape.com.