Downy Mildew on Impatiens

Discolored upper leaf surface and downy mildew growth on lower leaf surface. <UF Laura Sanagorski>

updated 1/31/2012

Laura Sanagorski, Environmental Horticulture Extension Agent

Bill Schall, Commercial Horticulture Extension Agent

Downy mildew on impatiens is currently a concern in Palm Beach County.  High humidity paired with cool nights created the perfect conditions for disease development.  Downy mildews are caused by a variety of pathogens that tend to be specific to hosts; however Plasmopara obducens is the one that affects impatiens.  Some literature indicates that downy mildew favors about 50 – 72 degrees Fahrenheit nighttime temperatures.

HOSTS - Downy mildew affects all hybrids and varieties of Impatiens walleriana, also called Busy Lizzy.  New Guinea impatiens, Impatiens X hawkeri, is considered very tolerant.  Counties adjacent to Palm Beach County have reported less severe outbreaks in 2011 and early 2012.

Symptoms of downy mildew showing first on newer growth: leaf edges curling downward <UF Laura Sanagorski>

SYMPTOMS - Young plants and new growth are most susceptible and may show symptoms first.  Initially, leaves may look a little yellowish or speckled.  In fact, these symptoms look very similar to nutritional deficiencies.  You may see faint gray lines on the tops of leaves or notice leaf edges curling downward.  Sometimes the yellowing is not visible before leaf curling begins.

As the disease continues to progress, whitish downy looking growth will be visible on undersides of leaves. This whitish growth is spore-containing structures that have emerged from the lower leaf pores (stomata). Next, leaves and flowers will drop quickly, leaving mostly stems.

LIFE CYCLE of the DISEASE - The pathogen that causes downy mildew is a type of water mold and is classified as more closely related to algae than to fungi. Other similarly classified pathogens include Phytophthora spp. and Pythium spp.  Downy mildew can spread by two different types of spores.  One type, zoospores, moves through water.  They are also easily windborne when contained in a larger structure called a sporangium.  This explains  why this disease is spread by splashing overhead irrigation, rainfall and wind. The other type of spores, oospores, forms inside plant tissues where they can survive for years.  Downy mildew is very aggressive and can rapidly spread, so be sure to act quickly if you spot it.

Undersides of leaves show white downy mycelia growth. <UF Laura Sanagorski>

LANDSCAPE MANAGEMENT- This pathogen may erupt under high humidity, cool temperatures, and overcrowding.  Overcrowding reduces the wind movement between plants that helps dry leaves more quickly.  Prevention is the only effective management strategy.  Elimination of overhead and nighttime watering, excessive fertilization, overcrowded planting beds, and other stresses on the plants would all be very helpful in avoiding or reducing occurrence of this pathogen.  However, our tightly arranged mass planting practices in the landscape, irrigation systems, and periodic rainfall make this difficult to achieve.  Be sure to carefully inspect impatiens leaves and stems purchased for the landscape before they are installed to help ensure that affected plants are rejected.  Once in the landscape, the plants can be attacked by windborne spores, but it is always better to avoid bringing the pathogen in on plant materials.

Chemical treatments can be expensive and do not provide much curative function.  They are much more effective when applied preventively. Quickly remove and dispose of plants or branches that appear to be affected by early symptoms of downy mildew.  Follow quickly with recommended protective fungicide treatments.  Be sure to fully remove as much plant tissue as possible from these beds in the spring when impatiens are changed to other flower varieties to avoid oospores surviving until the next time impatiens are planted. For replacement in heavily infested beds, consider selecting a resistant flower species or another type of plant.

See Table 1 below for recommended professional fungicides to help manage downy mildew in the landscape.

NURSERY MANAGEMENT– It is critical that high levels of sanitation be maintained in the nursery.  Carefully inspect plugs or any other Impatiens walleriana plants brought into the nursery and reject any that appear to be affected by the pathogen.  Sanitize hard surfaces in the growing area between crops with quaternary ammonia compounds.  Remove all impatiens plant tissue between crops and do not compost on the nursery property.  Dispose of in dumpsters that are emptied offsite from the nursery property.

Scout frequently for this pathogen, especially as evening temperatures cool.  Our high relative humidity makes this variable extremely difficult to control.  Space plants to allow as much air movement between them as possible to hasten leaf drying after irrigation or rainfall.

If affected plants are detected, quickly remove them and any fallen leaves and flowers from growing areas and dispose offsite away from the nursery property.  Treat remaining plants with preventative fungicides as indicated in Table 1 below.  Wash hands thoroughly after handling problematic plants and before handling non-affected impatiens.

NON-COMMERCIAL HOMEOWNER RECOMMENDATIONS – follow landscape management recommendations as indicated above and utilize fungicides as indicated in Table 2 below.

Table 1 – Commercial Fungicides for Use in Managing Downy Mildew on Impatiens walleriana.

 

Commercial Landscape

 

Nursery

 

Greenhouse

Adorn

(fluopicolide)

Adorn

(fluopicolide)

Adorn

(fluopicolide)

Aliette

(fosetyl-Al)

Aliette

(fosetyl-Al)

Aliette

(fosetyl-Al)

Heritage

(azoxystrobin)

Heritage

(azoxystrobin)

Heritage

(azoxystrobin)

Pageant

(pyraclostrobin + boscalid)

Pageant

(pyraclostrobin + boscalid)

Fenstop

(fenamidone)

Protect

(manganese + zinc + ethylenebisdithiocarbamate)

Protect

(manganese + zinc + ethylenebisdithiocarbamate)

Pageant

(pyraclostrobin + boscalid)

Segway

(cyazofamid)

Segway

(cyazofamid)

Protect

(manganese + zinc + ethylenebisdithiocarbamate)

Vital

(potassium phosphite)

Stature

(dimethomorph)

Segway

(cyazofamid)

 

Stature

(dimethomorph)

Additional Notes:

Rhapsody (Bacillus subtilis strain QST 713) may also provide protection against some forms of downy mildew in Florida landscapes, nurseries and greenhouses when applied prior to infection.

Subdue Maxx (mefenoxam) may also provide protection against some forms of downy mildew in Florida landscapes, nurseries and greenhouses.

Soilguard (Gliocladium virens G1-21) may also provide protection against some forms of downy mildew in Florida nurseries and greenhouses when applied prior to infection periods.

Orvego (ametoctradin + dimethomorph) a BASF product should be available for Florida landscapes and nurseries by July 2012, according to the company.

Table 2 – Fungicides for Use in Managing Downy Mildew on Impatiens walleriana in home landscapes.

Non-CommercialHomeowner Products

Concern Copper Soap Fungicide(copper octanoate)
Ferti-lome Broad Spectrum Landscape and Garden Fungicide(chlorothalonil)
Monterey Agri-Fos(phosphorous acid)
Ortho Max Garden Disease Control(chlorothalonil)
Southern Ag Liquid Copper Fungicide(copper ammonium complex)
Southern Ag Triple Action Neem Oil(extract of neem oil)

Remember, the label is the law; be sure to use products only in a manner consistent with the manufacturer directions on the labels.  Please use pesticides safely.

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The use of trade names in this publication is solely for the purpose of providing information.  Palm Beach County Extension does not guarantee or warranty the products named, and reference to them in this publication does not signify our approval to the exclusion of other products of suitable composition.

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Please see Palm Beach County Cooperative Extension’s newest fact sheet - Downy Mildew on Impatiens.  

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11 responses to this post.

  1. Most comercial outlets “retail” don’t know any thing about this

    Reply

  2. [...] Downy Mildew has been a difficult to control pathogen on Impatiens walleriana in the past season.  Read all about it here. [...]

    Reply

  3. [...] See Identification and Management of Downy Mildew on Impatiens [...]

    Reply

  4. Posted by Richard C on July 4, 2012 at 1:20 PM

    All three of my pots of impatiens have gone bad after doing well. No more leaves or flowers, just bad looking green stems.

    Reply

  5. Posted by Frank on August 7, 2012 at 3:27 PM

    What other annuals can i plant that won’t be infected by downy mildew?

    Reply

    • Frank,
      New Guinea Impatiens are a nice replacement option. We also like Beach Sunflower, Blanketflower, Begonia, Coreopsis, and Bromeliads for low-growing color. Remember, diversity is the key to a healthy landscape!

      Reply

    • Posted by Traudel Machatzke on March 25, 2013 at 5:16 PM

      This information is very informative for members of my garden club and private gardeners.

      Reply

  6. Here’s a Youtube on how south Florida was affected with the impatiens downy mildew epidemic early in 2012.

    Reply

  7. Posted by sylvia baxter on October 27, 2012 at 10:55 AM

    do I need to dispose of soil?

    Reply

    • Dear Sylvia – no – you don’t need to replace the soil, as long as your are planting some other species in the place of your impatiens. The downy mildew that affects your Impatiens wallerani will not affect other species. You can even plant New Guinea Impatiens (Impatiens X hawkeri) instead.

      Reply

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