UMass Extension Greenhouse Crops and Floriculture Program

Impatiens Downy Mildew

Impatiens Downy Mildew in landscape
Impatiens Downy Mildew spores
Impatiens Downy Mildew - stick stage
Impatiens Downy Mildew - chlorotic foliage

Downy mildew of Impatiens, Plasmopara obduscens, has been reported sporadically since 2004 but it wasn’t until 2011 that widespread outbreaks were observed in landscapes in Massachusetts.

Symptoms on  Impatiens walleriana  typically start with a few leaves that appear slightly chlorotic or stippled, and become completely yellow over time. Some varieties will have subtle gray markings on the upper leaf surface. A white, downy-like growth (airborne spores) may be present on the underside of primarily yellow leaves, but can also be found on the underside of green leaves. As the disease progresses, premature leaf drop results in bare, leafless stems.

Research has shown no evidence of seed-borne transmission. Young plants, seedling cotyledons, and immature plant tissues are most susceptible to infection; leaf symptoms are often first observed on the younger growth. Plants infected at an early stage of development may show marked reductions in growth and leaf expansion.

Two types of spores are produced that can initiate disease infection.

  • Short-lived (dispersal) spores produced in the white downy-like growth on the undersides of infected leaves. These spores will not overwinter but will spread the disease by blowing in wind currents and by water splash. It is not know how far these spores can spread, however, it is thought that the spores can spread many miles and a minimum of hundreds of yards.
  • Resting (survival) spores produced inside infected stems and leaf petioles (part that attaches to the stem). These resting spores, called oospores, release into the soil from infected plants debris where they can survive and potentially initiate new infections on Impatiens walleriana planted into the same garden beds for many years. It is not know how long oospores of Plasmopara obducens will survive in the soil. Oospores of other species of Plasmopara are known to be viable for 5-10 years.

Susceptible Plants: This disease only affects garden impatiens Impatiens walleriana. All varieties and intraspecific hybrids of Impatiens walleriana are susceptible to impatiens downy mildew, including both vegetative- and seed-produced I. walleriana.  There are no significant differences in susceptibility among varieties.

A few species of wild impatiens are also hosts of this disease, but there are no other known hosts. New Guinea impatiens (Impatiens hawkerii) is highly tolerant to this downy mildew. See FAQs for Landscapers in the references below.

Favorable Conditions: Development and expression of impatiens downy mildew is highly influenced by the weather.
Wet foliage, cool temperatures (especially at night), and moist air are ideal conditions for disease development.
The disease was noticed late in the season in several locations when night temperatures began to drop into the 50s in early September. Downy mildew is a water mold. As the name implies, it likes and requires moisture to sporulate and cause new infections. Plants in heavily shaded locations where the leaves stay wet for extended periods of time will generally have a higher incidence and severity of disease because moisture promotes infection and disease expression.
Disease tends to be worse in:

  • Locations where leaves stay wet for extended periods of time.
  • Very dense beds.
  • Beds receiving overhead sprinkler irrigation, because the foliage does not dry quickly.

How it Spreads: Impatiens downy mildew can be spread short distances by water splashing from infected plants and greater distances by windborne spores from infected plants.

Diagnosis: Growers should be on the lookout for impatiens downy mildew and advise landscape customers to do the same.  Send samples for confirmation to your state diagnostic lab.

Massachusetts Growers and Landscapers can contact the UMass Disease Diagnostic Lab.
The UMass Disease Diagnostic Lab can be reached by phone or email.
M. Bess Dicklow, (413) 545-3209, mbdicklo@umext.umass.edu

Landscapes
Infected plants should be pulled, roots, entire plants and leaf debris, and the area should not be replanted with susceptible garden impatiens species. Infected plants will not recover. Fungicide treatments will not work on infected plants. All infected impatiens should be pulled from the site, bagged and destroyed.  Do not place in compost piles. Allowing infected plants to remain in the landscape may allow the pathogen to overwinter as resting survival spores (called oospores), which may infect impatiens planted next year or longer.  New Guinea impatiens types, coleus, begonia, or other available bedding plants are safe to reset in the affected area.

Impatiens DM can occur in beds with no history of the disease on healthy plants if wind-dispersed spores blow in from other locations. Impatiens DM is not known to be seed borne, but healthy seed propagated plants can become infected from these wind-blown spores. It can take from 5 days to 2 weeks after inoculation before plants show symptoms, depending on environmental conditions.

Greenhouses
Remove and dispose of any infected Impatiens walleriana plants (standard impatiens, double impatiens) as well as all nearby impatiens plants. When discarding the plants, bag and remove the entire plant including the soil and any plant or soil  or leaf debris; do not compost. Evidence so far indicates that New Guinea impatiens Impatiens hawkeri are not affected by DM.

It is important for growers to remove infected plants early before oospores develop.

Impatiens DM is not known to be seed borne, so if possible, keep seed and vegetative impatiens in separate greenhouses in the future to prevent cross contamination. However, the young leaves (cotyledons) are very susceptible to infection.

Keep wild impatiens (Jewelweed, I. pallida and I. capensis) removed from around greenhouses and production yards and do not plant garden impatiens (Impatiens walleriana) in landscapes around greenhouses.

Being a new disease to our area, there is not a lot known about overwintering, re-infecting plants the next year, or persistence in the soil. Therefore, growers and landscapers are advised to practice prevention. Remember the disease triangle - susceptible host, pathogen and right conditions are needed for the disease to develop.

Fungicides - Greenhouse Production

There are fungicide programs that have been developed for greenhouse production. Below is a link to one developed by Colleen Warfield, Ball Horticultural Co. and others developed by M. Hausbeck (Michigan State University) and M. Daughtrey (Cornell University). While fungicides can protect healthy plants during the production stage, once plants are planted outdoors in landscapes, Impatiens downy mildew can infect plants from wind-dispersed spores blown in from other locations or from overwintering spores if planted in infected beds.

According to trials conducted by Colleen Warfield, Plant Pathologist, Ball Horticultural Co., fungicides can provide very good to excellent control of impatiens downy mildew when applied as foliar sprays to healthy plants only, prior to inoculation in repeated trials. However, fungicides were much less effective applied only 3 days after inoculation.
See fact sheet: Impatiens Downy Mildew Guidelines for Growers (Sept. 2012)

Impatien Downy Mildew Prevention and Management*

A. PREVENTIVE PROGRAM — Use when downy mildew has not been seen this year on your premises or in the landscape nearby, and your supplier has not experienced a disease outbreak.

1. First and last application:
SubdueMAXX (1.0 oz/100 gal) + Adorn (2.0 oz/100 gal) drench
Treat soon after plants received unless propagator has treated just before shipment.

2. Two weeks later:
A strobilurin (Compass O or Disarm or Fenstop or Heritage or Insignia or Pageant) spray, using high label rate + mancozeb (e.g. Protect DF at 1-2 lb/100 gal) as a tank mix

3. Two weeks later:
Segway (2.1 fl oz/100 gal) spray

4. Two weeks later:
Stature SC (6.12 fl oz/100 gal) spray

5. Repeat # 2, 3, 4 at two-week intervals, as needed.  Add mancozeb (Protect etc) to any treatment if desired for Alternaria leaf spot control

Last application, shortly before shipment:  SubdueMAXX + Adorn drench, as in #1

B. MANAGEMENT PROGRAM — Use when downy mildew has been found on impatiens on your premises or nearby, or your supplier has had a disease outbreak.

1. First and last application:
SubdueMAXX (1.0 oz/100 gal) + Adorn (2.0 oz/100 gal) drench
Treat soon after plants received unless propagator has treated just before shipment.

2. One week later:
A strobilurin (Compass O or Disarm or Fenstop or Heritage or Insignia or Pageant) spray, using high label rate + mancozeb (e.g. Protect DF at 1-2 lb/100 gal) as a tank mix

3. One week later:
Segway (3.5 fl oz/100 gal) spray + mancozeb (as above)

4. One week later:
Stature SC (12.25 fl oz/100 gal) + mancozeb spray (as above)

5. Repeat # 2, 3, 4 as needed.

Last application, shortly before shipment:  SubdueMAXX + Adorn as in #1

* Program developed by M. Hausbeck (Michigan State University) and M. Daughtrey (Cornell University) based on experimental data of M. Hausbeck (cucurbit and coleus downy mildew); M. Daughtrey (coleus and impatiens downy mildew); and C. Warfield, Ball Horticultural Company (impatiens downy mildew). Follow all label instructions and note warnings; local restrictions may apply.  Product names are given for information purposes only and are not an endorsement, nor is any criticism implied of products not mentioned.   3/7/12

Planning for Future and Alternatives to Garden Impatiens
Looking forward, we do not know the amount of inoculum that will be available in the industry and in garden beds and spring weather — these are all unknowns. Therefore, landscapers are advised to practice prevention if they plan to use garden impatiens in the future. Remember the disease triangle - susceptible host, pathogen and right conditions are needed for the disease to develop.

As many greenhouse growers of impatiens are discussing cutting back on garden impatiens production, talk with your plant supplier and discuss plans for next season. Greenhouse growers place orders in the fall for spring planting.

Landscapers are advised to consider including alternatives to garden impatiens into their landscape plans. New Guinea impatiens (Impatiens hawkeri) and hybrids such as SunPatiens are resistant to Impatiens DM  and can be used in place of Impatiens walleriana. While New Guinea impatiens are generally vegetatively propagated, the Devine series (new 2011) is seed propagated which may be less expensive. When planning, take into consideration that many of these more expensive plants require fewer plants per square foot than garden impatiens. Other suggested seed varieties of annual plants for shade include winged begonias (DragonWing, BabyWing), bronze-leaf and green-leaf fibrous-rooted begonia, tuberous-rooted begonia, lobelia (upright and trailing) and torenia. Shade annual plants with colorful foliage include, caladium, begonias, coleus, hypoestes and iresine. Nora Catlin, Cornell University compiled a good chart on alternatives to garden impatiens that includes crop time, garden height and width. It is listed under resources.

Resources

Fact sheets for Garden Retailers to print and distribute to customers:
Impatiens Downy Mildew in Home Gardens
(UMass Extension)

Annuals for Shade (UMass Extension)

Great Annuals for Shade (Cornell University)

Poster for Garden Retailers:
Impatiens Downy Mildew in Home Gardens & Landscapes
(pdf: file size 2.52 mb) (measures 11in x 17in)
Impatiens downy mildew Plasmopara obducens  is a new disease for home gardeners and landscapers in Massachusetts. Now there is a poster to share important information about this disease with customers. This is a link to a high resolution digital image (pdf) of the 11in x 17in poster that you can have printed at your local print shop.

References

Catlin, Nora. Downy Mildew of Impatiens, Some (But Not All) Questions Answered. e-Grow Alert. Vol. 1(8) March 2012.
e-Grow website link

Catlin, Nora and M. Daughtrey. Impatiens Downy Mildew in the Landscape (Updated March2013)

Catlin, Nora. Alternatives to Garden Impatiens Chart. Cornell University (Oct. 2012)

Warfield, Colleen, Ball Horticulture: Impatiens Downy Mildew Guidelines for Growers (Oct. 2012)

Webinar: Looking ahead to 2013 (Colleen Warfield, Ball Horticulture & Nancy Rechcigl, Syngenta

Compiled by Tina Smith
UMass Extension
Greenhouse Crops and Floriculture Program

1/2013