Impatiens Downy Mildew - Fungicide Program

January 23, 2013

Are you growing garden impatiens this year? This is a frequent topic of conversation among growers as everyone struggles with this decision.  Some are not growing garden impatiens, many are reducing their numbers (by 30% - 70%), and some are continuing as in years past.  There are many unknowns for the upcoming growing season in terms of disease pressure and the amount and sources of disease inoculum – both in the greenhouse and in garden beds.  However, if we are growing garden impatiens, we are growing a Downy mildew susceptible crop in a high risk area. Remember the disease triangle - susceptible host, virulent pathogen and favorable conditions are needed for disease to occur.  Therefore, growers and landscapers are advised to practice prevention. Now is the time to educate yourself and your customers.

Impatiens Downy Mildew is not known to be seed borne, however, the young leaves (cotyledons) are very susceptible to infection, especially in the humid greenhouse environment.

If you are growing plugs, talk to your supplier, to determine the fungicides applied to the plugs and whether they were grown in a high risk area.

Fungicides - Greenhouse Production

Preventive fungicide programs have been developed for greenhouse production. Below are sample programs developed by Colleen Warfield, Corporate Plant Pathologist, at Ball Horticultural Company and Extension Pathologists including 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  by wind-dispersed spores from other locations   and overwintering spores  present in infected beds.

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

The fungicide program below was 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).

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

1. First and last application:
Subdue MAXX (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 e.g. (Protect DF) to any treatment if desired for Alternaria leaf spot control

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

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

1. First and last application:
Subdue MAXX (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 at 7 day intervals, as needed.

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

Follow all label instructions and note warnings.  Product names are given for information purposes only and are not an endorsement, nor is any criticism implied of products not mentioned.

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 Sept. 2012)

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

Some Resources To Educate Your Customers

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

UConn: Impatiens Downy Mildew http://www.ladybug.uconn.edu/factsheets/documents/impatiens.html

Poster for Garden Retailers:
Impatiens Downy Mildew in Home Gardens & Landscapes (pdf: file size 2.52 mb) (measures 11in x 17in)
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.

Tina Smith, UMass Extension & Leanne Pundt, UConn Extension
with input from Bess Dicklow, UMass Extension Plant Pathologist

Topics: