Issue 15, August 27, 2012

Downy Mildew of Impatiens Is Back in 2012

This week, the U of I Plant Clinic has confirmed downy mildew of impatiens (Impatiens walleriana) in Cook County. It is apparent that the recent rains and cooler night temperatures provided the perfect environment for disease infection. Earlier this year, in HYG issue #3 I warned readers to watch out for this unbelievably destructive disease.

This disease has sporadically been reported in the US since 2004 in greenhouses. However, many regional outbreaks of impatiens downy mildew occurred for the first time in landscape beds and container plantings in 2011. In 2012 (as of July 31st), there have been confirmed reports of impatiens downy mildew in most of the states in the eastern half of the United States as well as Texas and Oregon. Sadly, it appears that this disease may be here to stay.

Scout your impatiens and look for leaves curling downward on newer growth. Soon, white to light-gray fuzz may show on leaf undersides. New leaves may appear as stunted or discolored (yellow or pale green). Unfortunately, this disease can infect very quickly and cause complete leaf defoliation or plant collapse to occur.

Impatiens infected with impatiens downy mildew, picture taken by Stephanie Porter.

Impatiens leaf defoliation caused by infection of impatiens downy mildew, picture taken by Stephanie Porter

What can you do to protect your impatiens?

  1. When purchasing all garden impatiens (including double impatiens and mini impatiens), balsam impatiens, garden balsam, or rose balsam (New Guinea impatiens are resistant), be sure that they are in good health and inspect them for disease. Native wild impatiens (jewelweed) is also susceptible to impatiens downy mildew.
  2. Don't plant impatiens too closely together or in heavy shade.
  3. Do not water impatiens via overhead sprinklers (especially at night) and avoid any other conditions that may promote leaf wetness.
  4. Scout for this disease often, especially when the temperatures become cooler (spring or fall). Sporulation and infection will not occur during hot or dry conditions. EARLY DETECTION IS THE KEY!
  5. Remove all diseased plants AS SOON AS POSSIBLE! All infected plants, fallen debris, and roots should be removed and destroyed! DO NOT PLACE DISEASED PLANTS IN THE COMPOST PILE!
  6. Once plants are infected with this disease, there is NO chance of saving them!
  7. There are some fungicides available; however they are used for protection only! They will not “cure” this disease. In addition, the fungicides need to be applied often, so they usually are not an economical or a feasible choice for homeowners. Remember to rotate chemistry to avoid fungicide resistance.

For additional information on downy mildew of impatiens, you can check out the following websites: (Adobe PDF)

(Stephanie Porter)

Stephanie Porter

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