In October, 2011, there were several reports of downy mildew on garden impatiens
(Impatiens walleriana). This disease was first reported in the U.S. in 1942, but recently several sources say this disease has been confirmed on impatiens in coastal southern California, northeastern Illinois, northern Indiana, Cape Cod in Massachusetts, Long Island and upstate New York. If you think that you experienced this disease in your garden, there are no worries of it infecting other plants. However, it is very, important that you remove and destroy infected plants. There is still a chance that this disease pathogen could overwinter and infect impatiens the following year. I have read that New Guinea impatiens have resistance to this disease and can be a considered a choice that can be planted in an infected area. End of the season fungicides are not recommended. Watch for this disease next year! Keep a careful watch on impatiens and catch this disease in the early stages and remove infected plants immediately! Fungicides can provide some protection, but will not protect impatiens for the entire season.
Several weeks ago, we received an email from from Diane Anderson saying that this downy mildew had infected most of the impatiens at the University of Illinois Trial Gardens. In the next several pictures you can see just how devastating downy mildew can be, if impatiens have been infected.
Symptoms of downy mildew of impatiens have been described as: "yellowish or pale-green foliage, downward curled leaves, leaf distortion, white to light-gray fuzz on leaf undersides, new leaves that are small or discolored (yellow or pale green), flowers buds that fail to form and stunted growth".
The plant pathogen that causes downy mildew is an oomycete and is spreads by oospores as seen below.
For more information on Down mildew of impatiens, you can check out the following websites:
Here on Cape Cod we referred to it as Impatiens Plague last summer. At the garden center we're thinking about how it will influence our buying of annuals for next year...area residents have traditionally loved their Impatiens. As a garden communicator, I'm taking the view that this offers us a chance to shake things up. Those who have routinely planted flats of Impatiens for years are being pulled out of their rut. Those breeding new Impatiens have a challenge, and we all have the opportunity to change, like it or not.
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