Virginia Tech College of Agriculture and Life Sciences
Department of Plant Pathology, Physiology and Weed Science
101-G Price Hall (0331) Blacksburg, Virginia 24061
Phone: 540-231-8020 Fax: 540-231-7477 Email: [email protected]
26 June 2012
A serious disease of impatiens, Impatiens Downy Mildew, caused by the pathogen Plasmopara obducens, was confirmed this week in Fairfax Virginia by VCE agent Adria Bordas and by the nearby Plant Diagnostic Lab at the University of Maryland. The disease has also been reported in Chesterfield County. This disease has been identified in greenhouse production in the U.S. since 2004, but in the latter half of 2011 it was diagnosed for the first time as a widespread problem in landscapes in California, the Midwest, the Northeast and Florida. Previously this month it was detected in locations in North Carolina and Maryland, so it is not surprising to see it show up in Virginia now.
All Impatiens walleriana (common garden impatiens) and I. walleriana interspecific hybrids are susceptible to the disease. New Guinea impatiens (I. hawken) and interspecific hybrids, such as SunPatiens®, are tolerant to the disease. Other bedding plants are not hosts to the impatiens downy mildew pathogen, although they may be susceptible to other downy mildew pathogens.
Downy mildews spread long distance by wind currents or they can be transported to new locations on infected transplants. Development of downy mildew is favored by cool, moist environmental conditions. The cool night temperatures still present in some areas of the mid-Atlantic are likely related to the continued spread and outbreaks of this disease. However, as temperatures increase, as is expected later in the week, disease progress and spread will likely be halted.
Early symptoms of Impatiens Downy Mildew are very subtle, since the upper leaf surface may remain green, exhibit stippling or turn only slightly yellow. Leaf edges may also curl downward and plants may become stunted. Under conditions of cool temperatures and high humidity, leaf undersides typically show a dense, white sporulation of the pathogen, but if conditions are hot and dry, sporulation will likely not be present. Later symptoms of the disease include stunting, defoliation, and flower drop, often with only bare stems remaining. This is the stage at which growers typically notice a problem.
If Impatiens Downy Mildew is identified in the landscape, growers should remove the plants, place them in a sealed garbage bag and dispose of them in the landfill. Since most commercial nurseries in Virginia are currently finished with impatiens production, commercial growers should not have to worry about this disease this year. However, for future reference, a link to detailed management guidelines from Ball® is listed below. Again, with high temperatures anticipated in the near future in Virginia, we anticipate that the disease will not become a widespread problem in landscapes in Virginia this summer. However, outbreaks may occur when temperatures cool down in the fall and the disease will likely be a problem for growers next year. This pathogen may be able to overwinter in soil, so removal of diseased plants is strongly recommended to minimize overwintering. Fungicide treatments are not recommended for home landscapes at this time.
A list of some useful online resources, which include more detailed disease control information, is provided below:
• The American Floral Endowment has a broad range of resources listed on its website (www.endowment.org), which can be accessed through its search engine or directly at http://www.endowment.org/afe-news/press-releases/221-controlling-downy-mildew-on-impatiens.html
• Landscaper Frequently Asked Questions Impatiens Downy Mildew, Ball®: http://www.ballhort.com/Landscapers/tips.aspx?articleid=16#article
• Impatiens Downy Mildew: Guidelines for Growers, Ball® http://www.ballhort.com/pdf/ImpatiensDownyMildewGrowerGuidelines.pdf
If you suspect Impatiens Downy Mildew, double-bag whole plants in sealed plastic bags and ship to the Plant Disease Clinic through your local Extension office (http://www.ext.vt.edu/offices/index.html). Package the sample to avoid damage to the delicate leaf tissue (e.g. use peanuts or newspaper to cushion). Mail samples early in the week to avoid delays in the post office over the weekend. If samples are collected late in the week, hold in the refrigerator in a sealed, double plastic-bag until shipment.
Elizabeth Bush & Mary Ann Hansen
Plant Disease Clinic