Downy Mildew on Impatiens

Published: January 1, 2013

An elderly country philosopher once remarked, "Don't worry, no matter how bad things seem to be, they can always get worse". Such is the case with many things in life, including our constant battle to manage plant pests in our production greenhouses. Every time we feel fairly confident in our management regimen, a new pest comes our way to challenge our ability as growers. For 2013 and beyond, that diseases for bedding plant growers might be impatiens downy mildew.

Garden impatiens (Impatiens walleriana) is one of our most important bedding plant species. In yearly polls conducted by growers' associations, impatiens ranks at or near the top of the list of most popular bedding plant based on sales volume. Its ability to produce vibrant displays of color in shady locations makes it unique to the plant world. Plant breeders have been successful in producing a plethora of new cultivars adding additional colors and growth habit to those already available. In short, impatiens are "money makers" for bedding plant producers.

Terminal leaves of an Impatiens plant infected with Plasmopara obducens. Note slightly chlorotic and curled appearance with white fuzz on the underside.
Photo courtesy of T. Shubert.

Impatiens downy mildew is a virulent, destructive foliar disease caused by the fungus Plasmopara obducens. The latter has the ability to infect Impatiens walleriana as well ashybrids having it as one of its parents. The disease is not a problem on New Guinea impatiens. While impatiens downy mildew has been a destructive disease for years in many parts of Europe, it was first identified in the United States in 2004. In 2012, cases of the disease were reported in nearly every state in the eastern one-half of the United States, including Missouri.

Symptoms of impatiens downy mildew usually are separated into two categories: early and advanced. Early symptoms include:

1. light-green yellowing or stippling of leaves;
2. subtle gray markings on undersides of leaves (sometimes visible);
3. downward curling of infected leaves; and
4. white, downy-like fungal growth occurring on the undersides of leaves.

Advanced symptoms include:

1. stunting in both plant height and leaf size;
2. foliage distortion (cupping);
3. leaf and flower drop resulting in bare leafless stems; and
4. infected stems become soft and plants collapse under continued wet, cool conditions.

Impatiens downy mildew is spread by structures it produces called zoospores. The latter are produced in saclike structures on the undersides of infected leaves. These zoospores are easily dislodged and spread over short distances via splashing water or over long distances through air currents. In most cases the disease organism's main entry into the bedding plant greenhouse is on infected plant material (e.g. plugs or cuttings) or by wind dispersal from infected plants growing outside the greenhouse. It should be noted that impatiens downy mildew is not spread via seed contamination.

Closer view of the white fuzz made up of sporangia and sporagiospores of P. obducens, the downy mildow pathogen.
Photo courtesy of T. Shubert.

For a disease organism to become virulent, in addition to a suitable host (impatiens in this case), there must be an environment surrounding the host conducive to infection. For impatiens downy mildew fungus this includes cool (59- 73°F), moist conditions which promote zoospore germination. Normally, the time between initial infection and the production of visible spores varies between 5 and 14 days. If the fungus is present, symptoms usually more subtle under warm, dry conditions.

As with any plant disease, prevention is the best control for impatiens downy mildew. Greenhouses should be disinfected prior to use and only disease-free plugs and cuttings should be used. Periods of high humidity causing foliage moisture should be avoided. This begins with watering early in the day to allow the foliage to dry quickly. Finally, the impatiens crop should be scouted frequently for early symptoms of the disease.

If symptoms (described above) are observed, prompt action is needed. Place all symptomatic plants and debris in closed bags and immediately remove them from the greenhouse. If sporulation is noticed, remove and discard plants in at least a three foot radius from the infected plant. Additionally, an aggressive preventive fungicide application program should be initiated.

The table lists an example of a fungicide application program for impatiens downy mildew. Please note that just as insecticides have different modes of action, so do fungicides. A good fungicide program rotates between chemicals with different FRAC numbers.

*Fungicide Resistance Action Committee, Source: Ball Seed Company

Given conditions conducive to infection, impatiens downy mildew has the potential of devastating one of our most popular, profitable bedding plant species. An IPM approach to the management of this new disease is advocated. This includes knowing the symptoms of the disease, starting with clean plants, scouting regularly for symptoms, destroying infected plants and initiating an aggressive fungicide application program.

The threat of impatiens downy mildew does not end in the greenhouse. The disease was identified in landscape plantings of impatiens in a number of states last year, particularly those in the north and northeastern regions of our country. Because the causal organism favors cool, moist conditions, spring and fall are the seasons when symptoms most likely would be observed in Missouri landscapes.

Management of the disease in outdoor settings starts with planting impatiens that are known to be free of the disease. This, however, might not totally eliminate the disease since it can spread from nearby, infected plants. If symptoms are observed on plants growing outdoors, they should be removed from the landscape with care and disposed of properly. Since the disease organism overwinters in the soil, growing a species other than impatiens in the same area the following year is recommended. For container plantings showing the disease, discard the growing medium and disinfect the container before planting it again.