Should our Garden Club Sell Impatiens? What is the risk for Impatiens Downy Mildew in Illinois?

These are good questions. This is because many people don't understand downy mildew of impatiens (Plasmopara obducens) .  When someone buys a plant and it gets sick, they often want to blame the person/business from where it was purchased.  However, in reality, it may not be the fault of that person/business.

I think you are on the right track if you have a "reputable supplier".  This means you are confident that the pathogen that causes downy mildew on impatiens is not going to be on the plants that you are selling.  When this disease first became a problem in the U.S., it was found  in a few nurseries and not in the landscape. Since 2011, it has become a problem in landscapes!  In Illinois, we are receiving reports that this disease is a problem in the Chicago area.  Some may want to know how they can be sure that an impatiens supplier is reliable.

 Plant or finish growers of impatiens may be more at risk for downy mildew infection if:

• Impatiens production is at the same time of impatiens growing in the landscape. -This is not likely in Illinois because plant production occurs during March or April.
• Impatiens plugs or liners are from an area where infected impatiens are currently growing or impatiens downy mildew has been reported in the landscape. - This could be likely if plugs are grown in Southern States and shipped here for finishing.
• Impatiens are growing in an area where impatiens downy mildew was confirmed in the landscape in 2011 or 2012. - This could be a problem for the finish grower or end user is they planted in a landscape bed where downy mildew existed previously.

All cultivars of impatiens walleriana are susceptible to downy mildew; however New Guinea impatiens have a high resistance to this disease.

For more information:   http://www.ballhort.com/Growers/_default.aspx 
                                    http://hyg.ipm.illinois.edu/article.php?id=424
                                    http://hyg.ipm.illinois.edu/article.php?id=354

All plant diseases are best explained by referring to the plant disease triangle.   

Picture taken from apsnet.org

In order for any plant disease to occur, you need to have a susceptible host (impatiens walleriana),disease pathogen present (downy mildew), and favorable environment (impatiens downy mildew thrives in moist conditions and cool temperatures -which can occur at night). 

If you are selling impatiens from a reputable grower and the impatiens that they have provided are thought to be "disease-free", they are not likely to be spreading the plant pathogen that causes impatiens downy mildew to customers.  Based on observations of the last several years, I believe that this disease is a problem because it has become established in the landscape.  Many landscapers/growers do not understand that if they have impatiens infected with impatiens downy mildew, they need to dispose of the infected material!  If they do not, they will continue to have further infection in the same area or at a later date.  If the removal of diseased material does not take place, this disease pathogen will be lurking in yards and landscapes, waiting to infect your and your neighbor's healthy impatiens. Many landscapers and gardeners often plant impatiens in the same area, every year. Impatiens growers are quickly finding that if they have a disease outbreak of downy mildew, they will have it the following year. 

We advise that you remove all of the diseased plant material to avoid further infection of impatiens; however, it may be difficult to completely rid the planting area of this disease and this disease pathogen is known to overwinter in the soil.  I have recently been asked the question: "If I had downy mildew of my impatiens the last two years, should I plant something else?"  My answer is "Yes". 

But, as we all know, we can have a plant disease pathogen present in our landscape, but may not have a problem, because we may not have the favorable conditions required for this plant disease to develop.  We do know that we have had favorable conditions for the infection of downy mildew to occur in Illinois in 2011 and 2012.  Unfortunately, we cannot control the weather, but we can make some predictions based on previous reports of impatiens downy mildew infection in Illinois.  We know that in 2011, we started to receive reports of this disease in the landscape in September; however, this disease was new and many may not have reported earlier disease outbreaks.  In 2012, this disease was reported in July and this is when people really started to become aware of the true devastation of impatiens downy mildew in Illinois.  Most of the 2011 and 2012 reports of downy mildew infection in Illinois were from the Chicago area.

Bed of impatiens infected with impatiens downy mildew

Based on the reports of impatiens downy mildew infection and weather conditions in the Chicago area during the last two years, I believe that there is a strong possibility that we will continue to see outbreaks of impatiens downy mildew in the future in the Chicago area. However, it is much harder to make predictions about this disease in downstate Illinois; but it is possible.  

If one decides to sell impatiens, I think that I would provide a caution statement with the impatiens explaining that your hope is to provide customers disease free material (not a total guarantee), but there is a strong possibility that impatiens could be infected by impatiens downy mildew inoculum, which could be present on infected impatiens debris/soil, lurking in landscapes.  

"Growers are responding to the gardeners requests for alternative shade tolerant plants, so do not hesitant to ask what else you could offer in leiu of impatiens." - Richard Hentschel, University of Illinois Extension Horticulture Educator

Please refer to this website for alternative to impatiens:  http://flor.hrt.msu.edu/IDM/index.htm

Impatiens leaf with sporulation of downy mildew on the underside of the leaf

(Stephanie Porter)