Impatiens Tale of 2013

If you depend upon impatiens for all-season color in your containers or garden, you may be disappointed this summer. If your impatiens die quickly, it probably isn’t the fault of the grower, the garden center or your own failing. The problem is a virulent strain of a water mold called downy mildew that has destroyed impatiens production in Europe and South Africa – and has now been confirmed in twenty states in America. Homeowners and businesses that expect masses of flowers, instead will see masses of dying, ugly plants.

Downy mildew infected impatiens

(Left) Downy mildew-infected impatiens showing leaf abscission. (Right) Landscape planting of impatiens following an epidemic of downy mildew. Photo credits: Mary Hausbeck, MSU

Not all impatiens are alike, and this disease only infects some of them – Impatiens walleriana is the victim, known to many people as the old-fashioned impatiens they buy in flats or as balsam or jewelweed. Newer cultivars such as double impatiens, Fusion and Spellwood are also susceptible. Fortunately, all the New Guinea type impatiens are not affected, including the Sonic and Supersonic series, Sunpatiens and the Divine series. 2013 may be the year of the New Guinea impatiens.

The Disease Story

Oval Spores

Oval spores produced on stalks that extend from the underside of the leaf. Photo credit: Mary Hausbeck, MSU

All diseases require three elements to succeed: 1) the pathogen, 2) the environment it requires, and 3) susceptible organisms. In the case of downy mildew, the organism is the impatiens crop. The pathogen consists of aerial spores that settle on plants. The environment for the spores to germinate is cool, moist conditions (temperatures from 59º to 73º Fahrenheit and either rain or overhead watering, especially late in the day). Here in Michigan, the rainy, cool spring was the perfect breeding ground for the downy mildew. Impatiens downy mildew also produces oospores that can survive through winter on plant debris above or below ground. So any soil where infected impatiens has grown is probably unsuitable for this crop for many years.

What You Saw

In greenhouses and garden centers, most impatiens looked healthy well into late spring. In those controlled situations, growers have products that manage disease outbreaks in early stages, and wind-blown spores aren’t likely to land on plants. When you bought and planted the impatiens flats or 4 inch pots at home – that’s when they were at risk. In two or three weeks, the plants showed pale green or yellowing leaves, some mottled foliage and eventually wilting, stunted growth, distorted leaves, severe leaf drop and total plant collapse. Light grey or white fuzz under some leaves is the definitive sign.

MSU greenhouse trial results

MSU greenhouse trial results. (Top left) Untreated control. (Top right) Adorn SC 2 fl oz drench. (Bottom left) Subdue MAXX EC 1 fl oz drench. (Bottom right) Heritage WG 4 oz + Capsil 4 fl oz spray. Photo credits: Mary Hausbeck, MSU

Prevent and Limit Diseases

For home gardeners or landscapers, some general principals can minimize the effect of this or another disease:

  • Avoid mass planting of anything. Mono-cultures incur the risk of a single species problem; diversity is good.
  • Water at the base of plants, if possible, rather than sprinkle all the foliage.
  • Water early in the day, if possible, so the garden can dry out before nightfall.
  • Thin out crowded plantings; lack of air circulation favors some diseases.
  • Do not compost diseased plants, but discard in garbage bags or bury them in a deep hole.

In the impatiens case, plants in brighter areas fared better than plants in deep shade.

Red Begonia Background (Andrew Schmidt | publicdomainpictures.net)

Red Begonia Background | Photo credit: Andrew Schmidt | publicdomainpictures.net

Greenhouse growers are not writing off impatiens completely. They suggest planting impatiens in containers instead of in the ground. Some growers will suggest planting New Guinea impatiens as mentioned earlier; others will suggest shade garden alternatives such as begonias.

As consumers, we should respect the risk that plant growers and farmers must take – weather, disease, insects, animals and an ever-changing marketplace.

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