Husker Hort

A Nebraska View of Horticulture

Impatiens Downy Mildew

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Impatiens with downy mildew

Impatiens with downy mildew

Did your once gorgeous flowering impatiens turn into bare looking sticks by the end of the summer? If so you are not alone. Impatiens downy mildew is a fungal-like infection that has been reported across the state this year. Find out more about this disease, how it spreads, and what you can do in the future to prevent another infection.

The symptoms of the infections aren’t as easy to spot as you might think. The first symptoms are leaves starting to appear light yellow or stippled yellow and green. The leaf edges begin to curl downward and appear to be wilted. The tell-tale sign is the fluffy white growth on the underside of the leaves. Often these symptoms are over-looked until the plant begins to lose its blooms and leaves, leaving bare stems. Eventually, the entire stems will begin to collapse and turn soft and mushy. All varieties and hybrids of Impatiens walleriana, common bedding plant impatiens, are susceptible to impatiens downy mildew. New Guinea impatiens, Impatiens hawkerii, are highly resistant as are other bedding plants in different genera that aren’t related to impatiens.

Underside of the leaf. Photo courtesy msue.anr.msu.edu.

Underside of the leaf. Photo courtesy msue.anr.msu.edu.

The development of impatiens downy mildew is highly influenced by the weather. Wet foliage, cool night temperatures, and moist air all contribute to ideal conditions for this disease development. Plants in heavily shaded locations where the leaves stay wet for extended periods of time will tend to have higher incidence and severity of this disease because moisture promotes infection. Impatiens downy mildew tends to be worse in locations where leaves stay wet for extended periods of time, beds that are densely planted, or in beds that receive overhead sprinkler irrigation, due to the leaves not being able to dry quickly.

It takes perfect environmental conditions for this disease to take hold. The pathogen can be introduced into a garden on infected transplants that aren’t yet showing the symptoms. Additionally, impatiens that are planted into beds that were infected the previous year may also become infected. A close cousin to the pathogen that causes impatiens downy mildew has been found to survive for 8-10 years in the soil.

Impatiens downy mildew can spread throughout the landscape. The pathogen produces spore-like structures called sporangia on the lower surface of the leaves of infected plants. These structures can be splashed short distances or become airborne and travel long distances on moist air currents and spread the infection. It only takes about four hours of a wet leaf surface for sporangia to form. Under hot, dry conditions, infected plants may not show symptoms of disease and produce sporangia on the lower leaf surface.

There are several management approaches for the landscape beds. Avoid planting Impatiens walleriana in those beds that had plants infected this past year. Consider alternative plants in those areas like coleus, caladium, begonia, or New Guinea impatiens. In beds with no history of impatiens downy mildew, I. walleriana can be planted, but take care to inspect and select disease free plants. Once a plant is infected, there isn’t a cure and it should be removed from the landscape. Also pick up any fallen leaves or blooms and consider removing neighboring plants. At the end of the growing season, completely remove all plant material to prevent the pathogen from overwintering.

Changing cultural practices can also help prevent this disease. If possible, reduce the amount of moisture and humidity that the plants are getting. Aim to water the plants in the early morning and provide deep infrequent irrigation to reduce the amount of time the leaves are wet. Space plants out so that there is adequate air circulation between the plants.

Fungicide applications are another option. Most fungicides can help prevent infection, but there aren’t any that will cure the disease once the infection has occurred.

Don’t let impatiens downy mildew stop you from enjoying these shade-loving annuals. Take precautions to ensure this disease doesn’t take hold of your landscape. But if it does, be prepared to try other interesting annuals.

For more information contact Elizabeth Killinger at elizabeth.killinger@unl.edu, 308-385-5088, on Facebook, Twitter, her blog at https://huskerhort.wordpress.com/, or visit the University of Nebraska-Lincoln Extension website: hall.unl.edu.

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Author: Elizabeth Killinger

A Nebraska Extension Educator out of Hall County with a focus in horticulture and sustainable landscapes.

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