Impatiens Downy Mildew Update – Spring 2013

Impatiens Downy Mildew Update – Spring 2013

Since most of Cape Cod was hit by Impatiens Downy Mildew in 2012, those who love this annual are asking, “Now what?”

Here’s what we know about this problem:

  • We know that IDM is an oomycete, or water mold. The spores move and spread quickly in cool, wet conditions. All varieties and hybrids of Impatiens walleriana, the common impatiens, are susceptible.
  • You might have seen the following symptoms on your plants last year. Your Impatiens might have sulked, failing to grow. Leaves might have yellowed and curled under, eventually dropping off so you were left with groups of plants that look as if the leaves had been eaten. Some people noticed this early in the summer, and others didn’t see these symptoms until the fall. This presentation of the problem corresponds with periods of cool, damp weather; those who planted early when the conditions were constantly moist had early onset of the disease, while those who planted later saw the development in the fall once the temperatures fell and regular rain resumed.
  • Studies show that plants can be infected with the disease and not show many symptoms until damp, cool weather returns. We also know that impatiens which are nutrient-deficient show signs of the disease before those that are well fertilized.
  • Spores of IDM can not only reside in the soil but are also wind born from other infected plants, so replacing the soil in gardens, pots, or window boxes won’t necessarily prevent infections. Spores also overwinter in colder temperatures than we have on Cape Cod, so it’s likely that this problem will be with us for a while.
  • Those who want to try planting impatiens in spite of the risk of downy mildew should plan to delay planting until warmer weather. On Cape Cod the weather often warms and becomes drier around the second week of June, but this varies tremendously from year to year so let the current forecasts be your guide.
  • Proper irrigation techniques should also be practiced. Don’t water impatiens beds too frequently. Never water in the evenings or overnight. Automatic irrigation that comes on daily or every other day before daybreak is practically a prescription for downy mildew since it is keeping the plants damp at the coolest part of the day. Instead, set systems to water deeply every four or five days in the morning.
  • Fungicides can be applied to slow the progression of the disease but the products that are effective against IDM need to be applied by a professional who has proper certification. Homeowners can try using Actinovate (active ingredient Streptomyces lydicus) first as a soil drench and later applied to plants as directed. Combined with later planting, proper irrigation, and repeat applications according to directions this might suppress IDM for the summer. There is no “silver bullet” cure at this point, however.
  • The only way to be totally free of this disease is not to plant varieties of Impatiens walleriana. Other impatiens such as New Guinea and Sunpatiens are not susceptible and can still be planted. There are also many colorful alternatives that can be used in shade and part-shade gardens.
This is Coleus Kingswood Torch – colorful for sun or shade, and late to flower.
Whopper Begonia in Rose also is very adaptable and can be planted in sun, part-shade or shade. You’ll have more flowers if the plant is in some sunlight.

1 Comment

  1. […] read more about this disease, see this tip sheet from Ball Horticulture or in our 2013 Update. Tags: downy mildew, impatiens, sick plants | Category: Plants, Problem […]

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