The Purdue Plant and Pest Diagnostic Laboratory

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April 17, 2013

Impatiens Downy Mildew Greenhouse Alert

Gail Ruhl, Tom Creswell and Janna Beckerman, Department of Botany & Plant Pathology, Purdue University

Impatiens downy mildew has been confirmed in greenhouses in Indiana and several other Midwest states, including Ohio. The pathogen that causes the disease, Plasmopara obducens, is not a true fungus but is a member of the Oomycota, often referred to as water molds.  Impatiens downy mildew is extremely aggressive on bedding plant impatiens (Impatiens walleriana and hybrids of this species). New Guinea Impatiens (Impatiens hawkeri and related hybrids) have high resistance to impatiens downy mildew. Bedding plants of different genera, such as coleus, caladium and begonia are not susceptible to impatiens downy mildew. If you have chosen to grow impatiens this year you should be scouting for this disease daily and utilizing a preventive fungicide schedule as recommended by the floriculture industry (see links below). For those of you who are not already on a treatment schedule or those who adopted a wait and see approach, it may be too late to implement preventative treatment!

Symptom description: The earliest symptoms, a slight curling or yellow/green stippling of the foliage, may be hard to spot (Figure 1). Under high humidity the pathogen will become visible on the UNDERSIDE of the leaves as sporangia are produced, resulting in a heavy coating of white sporulation (Figure 2). The downy mildew fungus sporulates prolifically and sporangia can become airborne and move from one plant to another when infected leaves are disturbed (Figure 3).

How to scout: At an early stage, sporulation might not be present and under hot, dry conditions infected plants may show no symptoms of disease and produce no sporangia on the lower leaf surface. Look for a slight chlorosis (light green-yellowing) and downward curling of the leaves (Figure 1). Periodically turn leaves over to look for white sporulation, especially under conditions of high humidity (Figure 2). Send suspect symptomatic plants to the Purdue Plant and Pest Diagnostic Lab for confirmation of the presence of downy mildew.

Preventive treatment: Management of this disease is through prevention – there are NO CURATIVE sprays. Plasmopara obducens thrives in cool (63-73F) moist conditions. Minimize humidity and leaf wetness, aiming for leaf wetness periods of less than 4 hours. Space plants so that air moves easily between plants and leaves dry quickly. Avoid condensation on the leaves by confining irrigation to early morning watering so leaves will dry more quickly. Use drip irrigation, if possible to keep foliage dry. If infection is found, immediately bag and remove the infected plants, any fallen leaves, blossoms and the nearby Impatiens walleriana. DO NOT compost infected plant material since oospores formed by Plasmopara obducens allow the pathogen to survive from one season to the next. A list of available fungicides for preventive treatments, not intended to cure already diseased plants, can be found at the following links:

Disease Prevention: Downy mildew and other diseases of impatiens (pdf file) - Syngenta Flowers

Impatiens Downy Mildew Prevention and Management (pdf file)

Impatiens Downy Mildew - A Review (pdf file) - Nora Catlin, Cornell Cooperative Extension of Suffolk County, Cornell University

Impatiens Downy Mildew - Picture of the Week, August 27, 2012

Click image to enlarge

Figure 1. Early symptoms (slight leaf curl and mottle) of downy mildew. Photo by Janna Beckerman

Figure 2. Plasmopara obducens sporulating on underside of leaf. Photo by Tom Creswell.

Figure 3. Plasmopara obducens sporangia. Photo by Gail Ruhl

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service