Impatiens Downy Mildew Resources for Greenhouses

Update for 2017 growing season:

The Horticulture Program has had a few inquiries from growers regarding whether IDM is still an issue. It has been 5 years since the initial problem, so hopefully overwintering spores in previously infected beds are no longer viable.

There seems to be some confusion among growers and gardeners that the State of Maine "banned" impatiens from sale. For many reasons, the Department never imposed a ban or quarantine on growing or selling impatiens plants. In 2012, IDM spores were blown up from states to our south during a few major storms, and the weather conditions were just right for disease development. While there was only 1 confirmed report of the disease in Maine, unconfirmed reports of impatiens losses were common. A pest risk analysis was conducted and we found that natural movement or overwintering was just as likely to spread or continue the disease as human assisted movement in impatiens production. "The horse was out of the barn" so to speak. We did, however, strongly encourage growers to take into consideration that overwintering spores could survive in the soil for up to 5 years and that there was a potential for healthy impatiens produced in greenhouses to become infested once they were moved into a landscape setting. Our main position has always been that it is best for businesses to educate their customers on the risks and management of the disease. Some businesses chose to not sell impatiens all together, whereas others reduced the amount they produced and others grew just as many as they always did. The first few years we saw many informational signs and handouts at greenhouses about IDM, but as this disease has not developed into a persistent problem for growers and gardeners in Maine we have seen a decline in educational signage.

In 2013 there were only 4 reported cases of IDM, all in the landscape and occuring later in the growing season (August and September). Since 2013 we have not observed or heard of a single reported incidence of IDM in the landscape. No symptoms of IDM have ever been observed during inspections of impatiens at greenhouses and garden centers.

It has now been just about 5 years since this pest was a problem in Maine therefore any overwintering spores are, hopefully, no longer viable. There is always a chance that another perfect weather year could bring the disease back, so those who plant impatiens are still at possible risk. It really comes down to the ethics and customer service beliefs of each individual business as to whether or not they should inform customers of the continued risk of the disease.

Reports of IDM have not just decreased in Maine, but also in other states. Decreases in reports could be due to fewer impatiens grown, more preventative measures applied to the impatiens crop and unfavorable weather conditions for disease development in some parts of the country.

While the threat of this disease may have lessened over the past few years, it is still wise for greenhouses and gardeners alike to have a plan in place to prevent and manage impatiens downy mildew should symptoms be observed in future plantings.

Disease Facts

• Caused by: The water mold Plasmopara obducens,
• Hosts: All varieties of garden impatiens, Impatiens walleriana and balsam impatiens, Impatiens balsamina.  Wild jewelweeds are also susceptible.  
• Non-Hosts: New Guinea impatiens, Impatiens hawkeri, and other plants are NOT affected. 
• Symptoms: Yellowing of the upper leaf surface and downward curling foliage.  Early symptoms may resemble nutritional problems, but undersides of leaves are covered in a white fuzzy growth.  As the disease progresses leaves and flowers drop, leaving bare stems behind.  Eventually the entire plant collapses.
• Spread: Spores in water splashed from nearby infected plants, spores blown long distances by the wind or spores that over wintered in the garden soil.  Movement of infected plant material also facilitates the spread of the disease. 
• Environment: Cool-humid conditions. 
• Prevention: Avoid growing impatiens in environments where leaves stay wet for long periods of time such as in dense shade, or crowded plantings with poor air circulation.  Avoid overhead irrigation and water plants early in the day when foliage has plenty of time to dry before nightfall.  Inspect incoming shipments carefully and regularly scout the greenhouse, immediately remove any suspicious plants.
• Treatment: Infected plants will not recover.  Immediately remove plants with symptoms, including roots, bag and discard.  Do not compost plants with impatiens downy mildew.  Advise customers to not replant impatiens on sites where infected plants have been observed in the past.  Aggressive preventative fungicide programs can be employed in the greenhouse to prevent disease.
• Alternative Plants: Substitutes for impatiens include begonias, coleus and New Guinea impatiens.

*Photo Credits: Mary Ann Hansen, Virginia Polytechnic Institute and State University,
Don Ferrin, Louisiana State University Agricultural Center
www.bugwood.org