Plasmopara obducens (impatiens downy mildew)

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Peronosporaceae
Genus	Plasmopara

Scientific Name

Plasmopara obducens

Common Name

downy mildew

Author: Mary Ann Hansen, Virginia Tech, Joyce Latimer, Virginia Tech, and Tom Creswell, Purdue University

Reviewed by: Margery Daughtrey, Cornell University; Colleen Warfield, Ball Horticultural Company

Pathogen

Plasmopara obducens is an oomycete in the order Peronosporales. It has coenocytic mycelium and produces ovoid sporangia (12 to 14 μm x 12 to 21 μm, as reported by Wilson; 19 to 22 μm × 13 to 17 μm, as reported by Wegulo, et al.) on monopodially branched sporangiophores. Branch points are at near-right angles. Sporangiophores are hyaline with slightly swollen bases and emerge from stomata. En masse, sporangia form a dense, white layer on the lower leaf surface of infected plants. The pathogen also produces thick-walled, light yellowish brown oospores (25-30 microns in diameter, as reported by Wilson), which can overwinter in plant debris. Oospores have been found in diseased stems, leafstalks and petals of flower buds (Toppe, et al.).

Symptoms and Signs

Symptoms and signs of impatiens downy mildew may include:

• downward leaf curl
• leaf yellowing or stippling
• white sporulation of the pathogen on lower leaf surface
• defoliation and flower drop
• stem rot

Symptoms of impatiens downy mildew vary, depending on when plants are infected. Plants infected early in development or those that are propagated from systemically infected cuttings exhibit stunting, a symptom that can easily be overlooked. Plants infected later in development typically exhibit late-season leaf yellowing and defoliation, which ultimately leave behind a cluster of leafless, flowerless stems: the so-called “green stick” syndrome. Prior to defoliation, leaves may curl downward or appear light green, stippled, or have gray markings. These symptoms can also be easily overlooked. During cool, moist weather the pathogen produces white sporangia on the lower leaf surface that are diagnostic for the disease; however, sporangia are not always present. In later stages of the disease, stems develop a watery rot and collapse.

Ecology and Spread

Impatiens downy mildew spreads mainly in two ways:

• wind-blown or water-splashed sporangia
• transplanting healthy plants into infested soil
• movement of infected plant material via trade

Plasmopara obducens grows systemically throughout the plant and reproduces by means of sporangia, which form on the lower leaf surface under cool, moist conditions. Sporangia are easily spread short-distance by air currents or splashing water and they may also travel long-distance on wind currents to new locations. Because sporangia may not be produced until 5 to 14 days (or even longer) after the plant is initially infected, asymptomatic plants may mistakenly be assumed to be disease-free.

The pathogen can also produce thick-walled resting structures called “oospores” in infected stems. Oospores can presumably survive for long periods in soil and can infect healthy impatiens planted into infested soil. It is not known how long the oospores of this particular species of downy mildew can survive, but oospores of other downy mildew species have been shown to survive for 8-10 years in infested soil.

Geographic Distribution

As of 2012, impatiens downy mildew was found in most states on the Eastern Seaboard, in the Midwest, and in states along the Pacific Coast of the United States. States with confirmed landscape or greenhouse reports include: AL, AR, CA, CO, CT, DE, FL, GA, IA, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, NC, NH, NJ, NY, OH, OK, OR, PA, RI, SC, TN, TX, UT, VA, VT, WA, WI, and WV. The disease has also been reported in Canada (British Columbia, Alberta, Ontario, and Quebec provinces), the United Kingdom, Norway, the Netherlands, Germany, Hungary, Serbia, Japan, Australia and South Africa.

Management

Prevention is the key to disease control in both the greenhouse and the landscape. Effective disease control in the commercial greenhouse involves both cultural and chemical means. Cultural control measures can also help to prevent or delay the onset of downy mildew in the landscape if home growers choose to plant common garden impatiens.

Cultural Control

Commercial Greenhouses

• Start with clean plant material by purchasing impatiens plugs, cuttings, or plants only from reputable sources.
• Do not carry over impatiens in the greenhouse from one growing season to the next.
• Minimize humidity and periods of leaf wetness, which favor this disease.
• Because sporangia of the downy mildew pathogen could blow into the greenhouse from infected plants outside the greenhouse, commercial growers should implement a preventive fungicide program whenever susceptible impatiens are grown.

Landscape

• If you choose to plant Impatiens walleriana, avoid overhead irrigation in the late afternoon or evening. Watering late in the day prolongs leaf wetness and favors disease development.
• Bag and remove infected plants and fallen leaves or flowers promptly after the disease is diagnosed to avoid introducing oospores to the bed. Oospores may overwinter in soil.
• Do not compost infected plant material.
• Planting I. walleriana into beds with a known history of impatiens downy mildew may increase the risk of infection; however, disease can also occur from wind-borne inoculum, regardless of the presence of soilborne inoculum.
• Plant species other than I. walleriana to avoid the disease altogether (see list of alternative plants below).

Chemical Control

Commercial Greenhouses

• Impatiens downy mildew is difficult to control once plants are infected, so fungicides should be applied preventatively.

• Apply preventative fungicide treatments at label rates and intervals, taking care to rotate among products with different modes of action (i.e. fungicides with different FRAC codes). Examples of recommended fungicide treatment programs can be found at the American Floral Endowment web site.

• In research trials conducted in 2012, an initial drench treatment with mefenoxam + fluopicolide (Subdue Maxx® + Adorn®), followed by regular foliar fungicide spray applications at 1 to 2-week intervals using other chemistries, was superior to using foliar spray applications alone for disease control.

• In research trials, plants treated in the greenhouse as described above (initial drench treatment, followed by foliar sprays) and transplanted to landscape beds remained symptom-free longer than plants that had not received the initial drench treatment.

• In trials conducted at Ball Horticultural Company, drench applications of mefenoxam + fluopicolide (Subdue Maxx® + Adorn®) consistently resulted in the longest duration of fungicide activity in all trials. It is important to note, however, that rotating fungicides is important to prevent the development of fungicide resistance in the pathogen population. A mefenoxam-resistant population of Plasmopara obducens was found in Florida at a research trial location where repeated applications of mefenoxam had been made over the previous year.

Landscape

• An effective fungicide treatment program in the greenhouse will have a carryover effect in the landscape bed and can continue to protect plants from the disease for up to four weeks or more. However, if the disease does not appear until late in the growing season, the carryover effect may not be helpful.
• Fungicides for controlling impatiens downy mildew are not recommended for home growers at this time; however, certain products are available to landscape professionals.

Resistance

All cultivars of I. walleriana and hybrids that have an I. walleriana parent (e.g. Fusion®, Butterfly®, and Spellbound® impatiens) are susceptible to impatiens downy mildew. I. balsamina (balsam impatiens) and I. glandulifera (Himalayan balsam) are also susceptible, but the latter is rarely produced horticulturally. Some native and weedy impatiens, including pale touch-me-not (I. pallida) and jewelweed (I. capensis), are also susceptible. New Guinea impatiens (Impatiens hawkeri) is highly resistant to the disease.

Alternative Plants for Landscape Beds

Because impatiens downy mildew is so destructive to common garden impatiens and because control options are limited for home landscapes, home growers and landscapers should consider alternative species of annuals for landscape beds. Some alternative flowering annuals for shade include: begonia (Rex hybrids, Rieger and hiemalis hybrids), New Guinea impatiens, Sunpatiens hybrids, lobelia, and torenia. Foliage plants for shade include: Alternantha, caladium, coleus, and dichondra, and for sun to partial shade: polka dot plant (Hypoestes), sweet potato vine (Ipomoea), and Swedish ivy (Plectranthus).

Diagnostic procedures

Impatiens downy mildew can be diagnosed microscopically when sporangia are present. Polymerase Chain Reaction can be used to amplify and detect the nuclear large subunit ribosomal DNA (nLSU rDNA) of the pathogen with primers NL1 and NL4.

Additional Resources

• Kageyama, K., Komatsu, T., and Suga, H. 2003. Refined PCR protocol for detection of plant pathogens in soil. J Gen Plant Pathol 69:153-160.
• Satou, M., Sugawara, K., Nagashima, S., Tsukamoto, T., and Matsushita, Y. 2013. Downy mildew of busy lizzie caused by Plasmopara obducens in Japan. J Gen Plant Pathol 79:205-208.
• Toppe, B., Brurberg, M.B., Stensvand, A. and Herrer, M.L. First report of Plasmopara obducens (downy mildew) on Impatiens walleriana in Norway. New Disease Reports (2010) 20, 33. The British Society for Plant Pathology.
• Wegulo, S. N., Koike, S. T., Vilchez, M., and Santos, P. 2004. First report of downy mildew caused by Plasmopara obducens on impatiens in California. Plant Dis. 88:909.
• Wilson, G. W. 1907. Studies in North American Peronosporales-II. Phytophthoreae and Rhysotheceae. Bulletin of the Torrey Botanical Club. 34(8):387-416.

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)