Impatiens Downy Mildew Resources for Greenhouses

 

Impatiens Downy Mildew Infected Plant

In 2012 impatiens downy mildew (IDM) caused devastating losses to impatiens plantings in many states. In Maine, IDM was confirmed from one garden and unconfirmed reports of impatiens losses due to IDM were common.

For the 2013 growing season many Maine greenhouses greatly reduced the number of impatiens they produce or entirely eliminated impatiens from production. While no symptoms of IDM were observed during inspections of greenhouse and garden centers in 2013, there were 4 confirmed reports (Bath, Orono, Bangor, Portland) in landscape plantings in August and September.

Nationwide there were fewer reports of IDM in 2013, however, greenhouses and gardeners will likely continue to see losses from IDM in upcoming growing seasons. Decreases in reports of IDM could be due to fewer impatiens grown, more preventative measures applied to impatiens crop and unfavorable weather conditions for diesease development in some parts of the counrty.

Greenhouses and gardeners alike should have a plan in place to prevent and manage impatiens downy mildew should symptoms be observed in future plantings.

 

Disease Facts

  • Caused by: The water mold Plasmopara obducens,
  • Hosts: All varieties of garden impatiens, Impatiens walleriana and balsam impatiens, Impatiens balsamina.  Wild jewelweeds are also susceptible.  
  • Non-Hosts: New Guinea impatiens, Impatiens hawkeri, and other plants are NOT affected. 
  • Symptoms: Yellowing of the upper leaf surface and downward curling foliage.  Early symptoms may resemble nutritional problems, but undersides of leaves are covered in a white fuzzy growth.  As the disease progresses leaves and flowers drop, leaving bare stems behind.  Eventually the entire plant collapses.
  • Spread: Spores in water splashed from nearby infected plants, spores blown long distances by the wind or spores that over wintered in the garden soil.  Movement of infected plant material also facilitates the spread of the disease. 
  • Environment: Cool-humid conditions. 
  • Prevention: Avoid growing impatiens in environments where leaves stay wet for long periods of time such as in dense shade, or crowded plantings with poor air circulation.  Avoid overhead irrigation and water plants early in the day when foliage has plenty of time to dry before nightfall.  Inspect incoming shipments carefully and regularly scout the greenhouse, immediately remove any suspicious plants.
  • Treatment: Infected plants will not recover.  Immediately remove plants with symptoms, including roots, bag and discard.  Do not compost plants with impatiens downy mildew.  Advise customers to not replant impatiens on sites where infected plants have been observed in the past.  Aggressive preventative fungicide programs can be employed in the greenhouse to prevent disease.
  • Alternative Plants: Substitutes for impatiens include begonias, coleus and New Guinea impatiens.

*Photo Credit: Mary Ann Hansen, Virginia Polytechnic Institute and State University, www.bugwood.org