Tuesday, September 11, 2012

What a sad summer for Impatiens.

Impatiens are one of the most common and colorful bedding plants in the United States. If you've seen your lush, full colorful impatiens fade and wither to scrawny stalks, you've seen Downy Mildew (IDM) up close. It is a plant disease (specifically a fungal-like pathogen) that is spreading. Downy mildew was spotted on impatiens on both coasts last year.

Powdery or Downy?
Since downy and powdery mildews are managed differently, it's really important to identify the issue correctly.
Downy mildew:
  • Appears very rapidly and is difficult to control.
  • Found on the underside of leaves
  • Causes the leaves and flower petals to drop off, and begins with leaf stippling, downward curling of leaves and leaf yellowing. After losing their petals and leaves, infected plants will die and appear like they had heavy frost damage.
  • Also affects basil, coleus, snapdragon, salvia, alyssum, pansy, rose, rosemary, and ornamental cabbage, and Perennials including aster, coreopsis, geranium, geum, lamium, potentilla, veronica and viola.
Powdery mildew:
  • Can occur on either the upper or lower surface.
  • Spreads slower than downy mildew.
  • Causes minor long-term damage, stunting growth.
  • Different species affect different plants/crops.
  • Powdery mildews are most severe when the weather is warm and dry, and they affect virtually all kinds of plants: cereals and grasses, vegetables, flowers, weeds, shrubs, fruit trees, and broad-leaved shade and forest trees. Many plants have been developed to be resistant to or tolerant of powdery mildew.
GOOD NEWS: New Guinea Impatiens (Impatiens hawkeri) are resistant. Next year, you can plant them in rotation with susceptible impatiens in fresh planting beds.

Largely considered a minor nuisance since the 1940s, DMI has been known from scattered and minor occurrences in the U.S. beginning in 2008 it began to spread in eastern parts of the U.S. It is a type of “water mold” that is weather dependent requiring humid, moist conditions and spreads by both airborne and water spores.
 

Without a susceptible host the pathogen will eventually die off in the planting bed.
 
To prevent the disease from living in your soil over the winter and returning next year: 
  1. Remove and dispose of infected plants (roots included) immediately.
  2. Don't compost the infected plant material.  
Speaking of next year, plan to plant your impatiens in different flowerbeds to avoid a re-occurrence of the disease.   It is safe to plant other flowering or foliage plants in affected beds next season.

Don't give up hope, with a few adjustments, common garden impatiens will continue to be a mainstay of our landscapes.

Sources:




More Resources:
Click here for the Ball Horticulture fact sheet.
Click here for the Syngenta fact sheet.

Click here for more information and photos.

Information compiled by Renee C. Brannigan
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