Pest Alert: Impatiens Downy Mildew

Impatiens downy mildew detected in multiple landscape locations in North Carolina 

Kelly Ivors, NC State University 

Downy mildew of impatiens is caused by the ‘fungus-like’ organism Plasmopara obducens. The group of organisms that cause downy mildew diseases are not true fungi- they are more closely related to the well-known plant pathogens Phytophthora and Pythium than they are to true fungi. This is an important distinction to understand because many of the traditional fungicides used to control fungal diseases of plants do not have efficacy against the downy mildews. All types of propagated Impatiens walleriana, including double impatiens and mini-impatiens, and any I. walleriana interspecific hybrids, such as Fusion® impatiens, are susceptible to downy mildew; however, all New Guinea impatiens (I. hawkeri) and interspecific hybrids such as SunPatiens® are tolerant to downy mildew. No other bedding plants are known hosts of this particular downy mildew, although there are a few other downy mildew species that attack other floriculture plants like coleus and basil. 

What does downy mildew look like? 

A really good resource about identifying the disease, as well as disease control information, can be found here. 

In addition, a webinar presented by Dr. Colleen Warfield of Ball Horticultural Inc. can be found here. 

Downy Mildew (Photo by: Kelly Ivors)

Downy Mildew (Photo by Kelly Ivors)

Downy mildew likes cool, wet/humid environmental conditions. The current conditions we’ve experienced recently across the state of North Carolina are conducive for this disease. The important thing to remember is that downy mildew is spread by wind currents, water splash or by the movement of infected plants. We know that the disease is now in our area and that the spores of the pathogen have the ability to spread long distances in air currents. Be on the look-out for it! So far it has been confirmed in both the western and piedmont areas of North Carolina. 

Fungicide treatments are not recommended for plants in the landscape; instead, all infected impatiens should be pulled from the landscape and destroyed. Fungicides are not always 100% effective at eliminating the disease. Allowing infected plants to remain in the landscape may allow the pathogen to overwinter as resting structures (called oospores), which can start a new epidemic later in the year or in following years if impatiens are replanted in the area. New Guinea impatiens, coleus, begonia, or other available bedding plants are safe to reset in the affected area.