Impatiens are among the most important bedding plants in the United States, a tried and true performer with spectacular color variety. The species
Impatiens walleriana, and its derived cultivar lines, exhibit perhaps the broadest color range in the genus. However, in recent years the greenhouse industry has been challenged by a fungal-like pathogen, previously known, but largely considered a minor nuisance. Impatiens downy mildew (
Plasmopara obducens) was initially described in the late 1800s and has been known from scattered and minor occurrences in the U.S. since the 1940s. It causes premature defoliation and flower drop and over the last few years has been impacting Europe, especially the United Kingdom, with devastating effect to the marketability of Impatiens there. Beginning in 2008 widespread findings of the pathogen were made in both landscapes and growing operations in eastern parts of the U.S. Fortunately, the industry has been quick to respond, with companies like Ball Horticultural and Syngenta leading the charge in recommending management strategies and chemical treatment options for growers (see link below).
Like most fungal and fungal-like pathogens, Impatiens downy mildew is very weather dependent requiring cool and moist conditions, which likely explain its success in many areas of the United Kingdom and significant impact in the U.S. in 2008 and 2011, which were particularly wet and relatively cool years. However, what would cause a pathogen known to be present in the U.S. for so long to suddenly show such aggressive and harmful tendencies? Could there have been some loss of disease resistance in recent lines of
Impatiens walleriana or could a new more aggressive strain of the disease have developed? Another possibility could be that we are looking at a new and previously undescribed species of the genus
Plasmopara that is similar to, but distinct from
Plasmopara obducens.
Many basic biological questions remain and through the efforts of industry, in collaboration with academic and government researchers, the resulting answers will help inform the necessary steps forward to protect the Impatiens market in the U.S. and its fundamental role in our gardens and landscapes. ANLA and OFA will do our part in helping our industry partners in answering these fundamental questions and identifying the path forward.
For more information regarding the pathogen and some of the research efforts to resolve the issue please visit:
Systematic Mycology and Microbiology Laboratory of the USDA
Ball Horticultural
