Impatiens have long been the most popular shade-loving annual for the Kansas City market. But the development of a disease new to the area is causing us to rethink the use of this old-time favorite. The disease is called downy mildew. It’s been spreading across the country the last few years and finally was confirmed on impatiens in this area last summer. Now that it is here what should we do?
Downy mildew of impatiens survives in the soil for an indefinite period of time. Most recommendations are to not replant impatiens in areas where downy mildew has been detected. Downy mildew first appeared in local gardens following the cool, rainy and humid period last August. Symptoms include leaf yellowing followed by what appears to be a wilting effect. Plants infected while young will be stunted. Humid conditions will allow a white coating to appear on the underside of the leaves, which is caused by the spores of the fungus.
Infection can be passed by water splashing from nearby infected plants or spores that have overwintered in the soil. Infected plants will not recover and fungicide sprays are not reliable to protect the healthy plants. As stated, once it appears in an area it does not go way, as the spores remain the soil.
Prevention is the key. Impatiens downy mildew is encouraged by wet leaf surfaces, shade and crowded plantings. To help reduce the spread into your garden: purchase only healthy plants; reduce crowding by increasing spacing between plants; and avoid overhead watering, especially in the evenings and at night. Also, don’t be in such a hurry to plant in early May. This plant will do best, and establish better, once the soils have warmed. A mid-May planting time is best to help reduce the disease. Once you have a problem with downey mildew of impatiens the planting area is probably doomed for more impatiens
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The question then becomes what do I plant once downy mildew has been confirmed in my garden? That is a really good question, as we do not have a lot of annual choices that flower in the shade. Impatiens downy mildew only affects the common bedding plant
, which includes many of the double flowering types and popular series on the market. It doesnot
affect the New Guinea impatiens.
New Guinea impatiens are not always that adaptable to our local soil condition. They require good soils and moisture for best growth. Basically I am saying they are not a slam-dunk to use as a replacement for regular impatiens. If you select New Guineas as your replacement option be sure to heavily amend the soil with organic matter and provide good irrigation. If not this could lead to failure.
Other replacement annuals include begonias, which is probably the best option, along with coleus and, to a lesser degree, Torenia. I know this list is very short but many of the shade loving annuals will not tolerate our summer heat and clay soil conditions. Diversity is your friend.
So as you head out to the garden centers keep in mind the potential problem with downy mildew. If you had the problem last year you might want to look at other plant options. For those of you that have not seen the problem in your yard, go ahead and give impatiens a try. Only time will tell how widespread this problem will become in KC.
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for answers from Johnson County Extension Master Gardeners and other experts.