Downy mildew on impatiens causes gardeners to lose patience


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Q. I have been gardening for more than 20 years. My yard is quite shady, and I plant impatiens every year to provide color once the daylilies and astibles are finished blooming. I have never encountered a problem until this year. Every one of my impatiens has died. They started to bloom less, then the lower leaves would drop until there was nothing left but stems, then they would die, too. This has occurred regardless of where they are planted (beds, pots or raised planters), so I know it is not caused by the soil. My neighbor is having the same problem, as is our county courthouse whose impatiens are also dying. What is going on?

A. If it is any comfort, you are not alone in your disappointment with impatiens this year. Penn State Extension of Allegheny County has received numerous calls and emails from landscape professionals and home gardeners alike, wondering what is happening to the impatiens they rely on for season-long color in shady landscapes.

Downy mildew is a relatively new disease on impatiens, although it has long been a problem on cucurbit crops, roses and grapes. The strain of downy mildew that affects impatiens is not the same one that affects those crops, so they are not infecting each other. Despite the similarity in names, downy mildew has different symptoms than powdery mildew that many gardeners are familiar with on ornamentals such as lilacs, peonies and phlox.

The disease has been identified on impatiens in the United States since 2004, but the mild winter and extra-early spring had large greenhouse growers in the southern United States scrambling to control the disease on their crops much earlier than normal. Although commercial growers scout and treat crops preventatively, those fungicides only provide protection for so long. By the time impatiens are moved through retail outlets and planted in landscapes, little protection is left.

Downy mildew affects all types of standard impatiens (Impatiens walleriana), including doubles, minis and interspecific hybrids such as Fusion. It is not a problem on New Guinea impatiens (I. hawkeri). Traditionally, downy mildew does not overwinter well in colder climates. It overwinters in warmer areas of the southern United States and is blown north on the wind and with storm currents. Given the very mild winter of 2011-12, downy mildew may have overwintered farther north than usual and been in place to infect susceptible plants as soon as they were planted.

The first symptoms that most growers notice include yellowing and stunting of infected plants. Their leaves turn yellow and start to drop until all that is left are bare stems with a few pitiful leaves hanging on. The disease often starts with discreet yellow spots on the upper leaf surface that most people would barely notice. However, if you turn those leaves over and examine the underside, you would see the fuzzy white growth characteristic of downy mildew. Infected plants are reduced to barren stalks as they lose their leaves, and even those finally die. Cool, humid weather with temperatures in the 58- to 72-degree range favor development of downy mildew.

The disease may have come into your garden on infected transplants, or it may have blown into your garden on the wind. Even though researchers have not established how far the spores can travel, the related pathogen that causes downy mildew on cucurbit crops has been shown to travel over 600 miles in 48 hours. Also, if you had problems with downy mildew in 2011 and planted impatiens in the same place this year, the disease may have survived the winter on plant debris in the bed.

Unfortunately, fungicide applications are not as useful once impatiens have become infected in the landscape. Control focuses on sanitation and planting resistant species next year. Remove and bag infected plants as soon as they are noticed to reduce the spread to healthy plants. Allow adequate space between plants when planting to permit good air circulation, and avoid overhead irrigation, especially during cool weather.

If downy mildew has spoiled impatiens plantings this year, avoid trying to grow them in the same area next year. Shade-tolerant annuals that are resistant include New Guinea impatiens, begonia (Begonia spp.), caladium (Caladium bicolor) and coleus (Solenostemon spp.).

garden

Send questions to Sandy Feather by email at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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