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Impatiens Downy Mildew
Impatiens downy mildew is a destructive foliar disease of Impatiens walleriana that is capable of causing complete defoliation or plant collapse, especially in landscape plantings under moist conditions and cool nights.
While there have been sporadic reports of impatiens downy mildew in U.S. greenhouses since 2004, it was not until summer 2011 that regional outbreaks of this disease were seen for the first time in landscape beds and container plantings in North America. In early January 2012, outbreaks of impatiens downy mildew were observed in landscape beds and greenhouses in south Florida. It is unclear whether this was a continuation of the 2011 outbreaks or a new cycle of disease for 2012. The spread of this disease continued throughout the spring, and by October 2012 impatiens downy mildew had been confirmed in landscape beds and/or greenhouses in 32 states and Washington, D.C.
Thanks to Ball Seed Co. and Cornell University. for most of the information used in this article.
Host Plants for this Disease
All cultivars of Impatiens walleriana (common garden impatiens) and interspecific hybrids with an I. walleriana parent are susceptible including Fusion, Fiesta and Patchwork.
A few wild species of impatiens are also susceptible; however, there are no other bedding plant species that are known hosts.
New Guinea impatiens (Impatiens hawkeri) including Fanfare, Divine, Celebration, Celebrette, and Sunpatiens have high resistance to this disease.
At this point we are hearing that the Big Box stores are ordering impatiens the same as usual, essentially forcing growers to produce them even if they are scared to do
so. They are banking on the public not being aware of the disease, or not being afraid of it if they are. They are confident that there will be customer demand for
impatiens, since gardeners are so addicted to the historical dependability of this bedding plant
Plants Unlimited will sell Impatiens in 2013 but we have cut our orders dramatically. We will monitor Impatiens closely for this disease in our greenhouses and from our growers. Since we buy from several growers, each delivery will be separate of others. In addition, we offer many other shade tolerant annuals as possible substitutes,
We constantly look for the best growers and follow this strategy: "Buy the best and leave the rest"
|How does this Disease Spread?
Sporangia (sac-like structures filled with zoospores) produced on the underside of infected leaves are easily dislodged and can be spread short distances by water splash, and longer distances by air currents.
Two main routes for entry into a garden
1. infected plant material
2. Wind-dispersed, aerial spores from infected plants growing elsewhere (may travel on the order of hundreds of miles).
CAUTION: Infected plants not yet showing symptoms may result in the inadvertent movement of the pathogen.
-Light-green yellowing or stippling of leaves
- Subtle gray markings on upper leaf surface sometimes visible
- Downward curling of infected leaves
-White downy-like fungal growth on the undersides of leaves
- Stunting in both plant heightand leaf size when infected at an early stage of development
-Leaf and flower drop resulting in bare, leafless stems
-Infected stems become soft and plants collapse under continued wet and cool conditions
What can/should you do if you grow Impatiens?
This year, it came in late when we saw it in the landscape, which is why some people didn’t notice it,. In some cases in Europe, the disease hit them early and hard, causing massive destruction in the landscape. European weather conditions through the summer consist of cool, cloudy, windy and wet weather. That, coupled with a lot of impatiens used in landscape settings and easy movement of wind-blown spores over miles, doesn’t take long to have a big-time problem.
Logic would suggest that since this disease only affects one species of impatiens of commercial importance, it would be easier to pinpoint and eradicate. But it’s not that simple.
The trouble is, it’s one of the most important bedding plants out there that is used ubiquitously,. Try to look for symptoms and, if you have to down the line, do crop rotations at the landscape level within those particular beds if symptoms were seen the previous season. Try to lessen the impact of the disease by not putting Impatiens walleriana in those beds and rotate with another crop,
Some Alternate Plants in place of Impatiens walleriana
Although nothing beats the color range offered by Impatiens walleriana, there are other options—especially since the disease is host specific.
New Guinea impatiens, begonias and coleus are obvious choices to replace impatiens because of their shade tolerance. Growers in the UK are also trying out phlox and cosmos.
Here is a list of shade orpartial shade annuals
Begonias - fibrous
Begonias - tuberous
Begonias Dragon Wings
New Guinea Impatiens
FOLIAGE,FERNS & GRASSES
Begonia Million Kisses
Fuchsia - upright
New Guinea Impatiens
Zonal Geraniums (partial shade
Sedum - creeping
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PO Box 374 - 629 Commercial Street (Route1)
Rockport, Maine 04856
207-594-7754 fax 207-594-8510
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