Pest Alert: North Carolina Pest Alert for Spring 2013 Impatiens Downy Mildew

— Written By

Date: April 11, 2013

From: Kelly Ivors, Extension Plant Pathologist

Impatiens downy mildew was recently identified in a Virginia greenhouse in mid-January 2013 on impatiens cuttings taken from plants originally obtained from a North Carolina greenhouse in December 2012. Based on information from prior years, the 2013 impatiens downy mildew situation in our area is predicted to remain problematic. However, what time of year it shows up in the landscape and the level of downy mildew severity will depend on local weather conditions and how the ‘winds blow’. For a quick summary of the history, spread, symptoms and risks, check out Everything You Need To Know About Impatiens Downy Mildew. Downy mildew of impatiens is caused by the fungus-like organism Plasmopara obducens. The group of organisms that cause downy mildew diseases are more closely related to the well-known plant pathogens Phytophthora and Pythium than they are to true fungi. This is an important distinction 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 immune to impatiens downy mildew. No other bedding plants are known hosts of this particular downy mildew, although there are a few other downy mildew species that specifically attack other floriculture plants like coleus and basil. Given the high plant mortality associated with epidemics of impatiens downy mildew, everyone growing impatiens should be treating with fungicides preventatively, because once it starts it is very difficult, if not impossible, to control. However, 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. A nice list of planting alternatives to impatiens can be found here. A number of plant pathologists have been researching this disease to provide science-based information to greenhouse growers, plant retailers, landscapers and home gardeners. A list of resources can be found below: General information: Illustrations of symptoms in production and landscape Impatiens downy mildew Fact Sheet Impatiens downy mildew: a review Grower information: Impatiens Downy Mildew Grower Guidelines Preventive and responsive chemical control programs A list of fungicides labeled for downy mildew control Assessing Risk Landscaper / Gardener information: FAQ’s for landscapers Impatiens Downy Mildew in the Landscape A Practical Approach for Landscaper Gardeners

Written By

Photo of Carol HicksCarol HicksFormer Extension Coordinator cbhicks@ncsu.eduCenter for Integrated Pest Management - NC State University
Updated on Jul 23, 2014
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