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There are two major diseases of impatiens.

 One is the impatiens Necrotic Spot Virus.  This is a viral infection that is spread by an insect called thrips.  This disease usually infects young plants in nurseries and wholesale producer’s facilities, and is best controlled by producers using insecticides on the young plants.   If your plants had this virus there is no need to treat soil as the disease can only be spread from infected living plants to healthy plants by the larva of thrips.   You can avoid this disease by carefully selecting healthy bedding plants.

A second disease, and one that has been more difficult to control, is downy mildew of impatiens This is a fungal disease that first showed up in 2011 throughout the northeast.  The fungus infects healthy plants when its spores land on the plants, or exist in the soil.  If you had impatiens downy mildew on plants last year or the year before it is quite likely that your soil is infested with the spores, and could infect new impatiens plants.  There is no practical way to eliminate these spores from the soil.    It is generally recommended that gardeners find alternative plants for their landscape.  The article link at the bottom of this answer includes suggested alternate plants for impatiens.

You can find impatiens is local nurseries.  However you should not plant impatiens where you have had diseased plants in the past.  You might have better luck if you planted impatiens in new potting soil in containers off the ground. However if the bedding plants you choose are infected, new soil will not make any difference.  Unfortunately, there have been reports of infected plants being sold this season, and spores are around in soil anywhere infected plants were previously grown.  

Gary W. Moorman, Prof. of Plant Pathology, The Pennsylvania State University, reported the following on May 28, 2014.  “I have been informed by the PA Dept. of Ag. impatiens downy mildew (Plasmopara obducens) has been observed on plants in retail establishments in the northeastern, southwestern, and southeastern regions of PA.

This is the plant pathogen that caused major losses of impatiens in 2012. A good site for information is:

http://msue.anr.msu.edu/news/how_to_manage_impatiens_downy_mildew_in_the_landscape/

If you have additional questions you can contact your local Penn State Extension office.

Montgomery County Penn State Extension

Address: 1015 Bridge Road, Suite H, Collegeville, PA 19426-1179

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Office Hours:  Monday-Friday 8:30 a.m. - 4:30 p.m.

Montgomery County Penn State Master Gardener’s have a horticultural hotline.  You can get specific information and recommendations for your area from them.  You will find information on the hotline at.

http://extension.psu.edu/plants/master-gardener/counties/montgomery/hotline