Downy Mildew on Impatiens - Forecast for 201310/19/2012

Downy Mildew on Impatiens - Forecast of 2013 


Courtesy of: Rick Yates, Technical Services Manager, Griffin Greenhouse and Nursery Supplies, Inc. 9/24/12



Throughout the fall and winter of 2011-2012 concern was brewing that the most popular bedding plant annual in the Unite States may be in for a rough ride for the 2012 growing season.  The crop is Impatiens walleriana and the culprit is a virulent strain of downy mildew.  Concern mounted as reports from parts of Europe and South Africa confirmed massive losses of impatiens in the landscape for two years in a row, to the point where few if any impatiens are grown in those regions now.  In the frost free areas of Florida impatiens are grown for winter color because planted mid-fall they thrive in the southern Florida winter climate  Unfortunately, downy mildew exploded there in the fall of 2011, thriving on the combination of moderate temperatures and frequent showers. 


Commercial growers were alerted that going forward bedding plant impatiens would require preventative measures to protect them starting soon after seed germination throughout the greenhouse production cycle until they left the retail garden center   The growers did a good job of producing disease free impatiens for the landscape in the spring of 2012.  Unfortunately, soon after the plants leave the greenhouse and are planted outside the protection applied by the growers starts to dissipate leaving the impatiens vulnerable to infection.  Wind-blown spores allow this disease to spread at an amazing rate when weather conditions are favorable, and spread it did  (While not a total remedy, quickly digging up infected plants, bagging them and disposing in a landfill reduced the amount of disease spores left behind.)  By June reports of downy mildew infected impatiens started streaming in and by late summer there were more infected than healthy impatiens in landscape.  Plant pathologist tell us that the ideal weather conditions for infection are temperatures in 60's to low 70's with frequent foliage wetting from rain or overhead irrigation  What is so sobering is that we saw downy mildew spreading quickly through impatiens even during the hot dry start to the summer of 2012.  This reinforced the stiff challenge this disease creates for impatiens lovers.



Downy mildew of impatiens produces a resistant spore in the stems of declining impatiens plants that persist in the environment for 1-5 years, even without a plant host.  Combine large amounts of disease inoculum persisting in the landscape to cause reinfection and a wind-blown spore stage or rapid dispersal and it adds up to a dark cloud hanging over the impatiens crop for 013.   Most experts agree that impatiens are likely to fail in the landscape in 2013 and that alternatives should be sought.  All types of Impatiens walleriana, including vegetatively produced double impatiens, balsam and jewelweed are known to be susceptible.  New Guinea Impatiens and Sunpatiens are believed to be resistant to downy mildew.  There are many other plants that can serve as suitable replacements for impatiens while the plant breeders pursue more resistant types of impatiens.