Many plants are susceptible to downy mildew pathogens, which are usually not considered to be major disease threats. However, downy mildew of impatiens (Plasmopara obducens), a new disease to Illinois, is a major problem for all cultivars of impatiens walleriana, according to University of Illinois plant clinic diagnostic outreach specialist Stephanie Porter.
"When this disease first became a problem in the U.S., it was seen only in the nursery and had not yet established itself in the landscape," she said. "In 2011, we began receiving reports that it was a problem in landscapes in the Chicago area."
Based on the reports of impatiens downy mildew infection and weather conditions in Illinois the last two years, Porter believes that outbreaks are likely to continue in the Chicago area because many landscapers and gardeners plant impatiens in the same area every year. They often find that if they have an outbreak of downy mildew one year, they will have another the following year.
However, it is more difficult to make predictions about this disease in downstate Illinois. "We do know that we have had conditions favorable for downy mildew infections in Illinois in 2011 and 2012," Porter said.
Plant or finish growers of impatiens may have an increased risk for downy mildew infection if:
Â· Impatiens production is at the same time that impatiens is growing in the landscape. This is not likely in Illinois because plant production is during March or April.
Â· Impatiens plugs or liners are from an area where infected impatiens are currently growing or where impatiens downy mildew has been reported in the landscape. This could be a problem if plugs are grown in southern states and shipped here for finishing.
Â· Impatiens plants are growing in an area where impatiens downy mildew was confirmed in the landscape in 2011 or 2012. This could cause problems for the finish grower or end user.
Porter said that gardeners who find downy mildew should remove all of the diseased plant material to avoid further infection in the garden or in neighbors' gardens. However, it may be difficult to rid the planting area of this disease because the pathogen can remain in the soil.
There are some fungicides available for use against this disease; however, they are mostly used in commercial settings for plant protection. Fungicides will not "cure" this disease. Several protectant fungicide applications are required, so they usually are not an economical or a feasible choice for homeowners.
"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,'" Porter said.
She added that, if you are selling impatiens, be sure they are from a reputable and reliable grower.
"Growers are responding to the gardeners' requests for alternative shade-tolerant plants, so do not hesitate to ask what else you could offer in lieu of impatiens," advised Richard Hentschel, a U of I Extension horticulture educator.
Suggestions for alternative shade tolerant plants are available at: http://flor.hrt.msu.edu/IDM/index.htm.
For more information on downy mildew of impatiens, go to the following links: http://www.ballhort.com/pdf/ImpatiensDownyMildewGrowerGuidelines.pdf,
Source: Stephanie Porter, Visiting Plant Diagnostic Outreach Specialist - Crop Sciences, firstname.lastname@example.org
Pull date: May 1, 2013