Many readers of this column as well as many other gardeners have contacted me with their concerns about downy mildew in impatiens. Simply put, there will be no impatiens this year for you to purchase and if there are, don't buy them. Why not buy healthy looking impatiens plants? Healthy looking plants will suddenly collapse and die.
Let's take a look at downy mildew (By the way don't confuse it with powdery mildew. A good way to remember the difference... "D" is for "death.") Downy mildew of impatiens is not new but is presently rampant. It has been in the United States since the late 18oo's but became a problem in the greenhouse production of impatiens in 2004. It is a devastating disease outside North America as well especially in the Untied Kingdom.
What then is downy mildew? It consists of oomycetes, a cool word referring to a fungus-like microorganism. This organism is a filamentous, microscopic parasite that absorbs nutrients from plants. The oomycetes can swim and are produced in structures under the leaves of infected plants. They love cool (59 to 73 F.) moist conditions and are very long lived. It takes between five and fourteen days for a plant to show symptoms of the disease (which is why presently you shouldn't buy healthy looking impatiens.)
What does Downy Mildew of impatiens look like? In the early stages the leaves might look stippled or a paler green. The leaves often curl downward so people often think the plants need water which only makes matters worse. The early symptoms include large blocky yellow areas on the upper surface of the leaves. Soon these areas get bigger and turn brown. The underside of the leaves looks water-soaked. Soon one sees a purple brown mold. Usually the leaves and flowers of the impatiens are destroyed leaving the rest of the plant intact! Awful!
This fungus-like organism lives in the plant debris left at the end of the gardening season and survives winters down to 5 degrees Fahrenheit! The only good news is that downy mildew is not transmitted in seeds.
The Downy Mildew in impatiens is caused by Protoplasmic obducens. It is a relative of the fungus-like organism which can cause seedlings to suddenly collapse. It is also related to Phytophthora which causes late blight of tomatoes and potatoes. It only attacks impatiens . Other plants such as grapes, cucumbers, and coleus, each have their own species of Downy Mildew. This is important because at least it means that the downy mildew of one plant family cannot attack a different plant family!
This organism attacks all seed- or vegetative-propagated Impatiens walleriana, the common garden impatiens. This includes double impatiens and mini-impatiens. Any hybrids such as Fusion® are believed to be susceptible. It has been found on balsam impatiens as well as some native wild impatiens and jewel weed. New Guinea impatiens Impatiens hawkeri and certain inter-specific hybrids such as SunPatiens® appear to be immune to the disease.
What to do? This reminds me of a woman I met recently who said "What will I plant in shade if I can't plant impatiens?" I believe that this is a great opportunity for gardeners to try other plants. My suggestions: annuals such as coleus (hundreds of types!), caladiums, begonias, fuchsias, perillas, and then there are all the perennials for shade such as Columbine, Goat's Beard, Astilbe, Bergenia, Turtlehead, Bugbane, and Bleeding hearts to name just a few!
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