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Impatiens Downy Mildew: The Sky Is Not Falling (Tech Tip )
By Syngenta Technical Services

The horticulture world has been abuzz with discussions of a 'new' disease of impatiens - Impatiens Downy Mildew. The disease is not actually new, having been in the US since at least 1897 when it was reported on wild species of impatiens. The disease has been more prevalent in the last ten years and was significant in some landscape plantings in the US during late summer 2011. However, prudent measures will allow this beautiful plant to continue to be a valuable part of our crop mix.

The causal agent of impatiens downy mildew affecting crops is Plasmopara obducens, a "water mold" or oomycete. It only infects some species within the genus Impatiens and cannot threaten other flower crops. Impatiens walleriana (common garden impatiens) are affected by impatiens downy mildew but New Guinea impatiens (I. hawkeri) are highly tolerant of this disease.

Symptoms

Impatiens infected with downy mildew can simply appear non-vigorous with yellowish or pale green foliage and mild, inconspicuous mottling. These subtle symptoms can be missed or mistaken for other problems. As the disease progresses, advanced symptoms may include stunting of plant growth, downward curling or distortion of the leaves, loss of leaves, wilting and plant collapse.

The most distinct sign of downy mildew of impatiens is white to light gray downy 'fuzz' on the undersides of the leaves, but because the fungal growth is only visible on the undersides of the leaves, it can be missed. Frequent scouting is essential. To confirm a potential diagnosis of impatiens downy mildew, submit samples to the Syngenta diagnostic lab. You can contact flowers.tech.help@syngenta.com for more information.

Prevention - Cultural Practices

Prevention of impatiens downy mildew is essential because control of an infection is extremely difficult. Start with propagation material free of the disease. Since the disease is not transmitted by seed, seedling plugs of I. walleriana that have been isolated from any source of the disease will be disease-free. Vegetatively propagated I. walleriana from reputable suppliers should be disease-free. However, since the disease may be initially symptomless, it is prudent to isolate vegetatively propagated I. walleriana from seed-propagated impatiens.

Maintain moderate humidity, provide air movement, irrigate early in the day and avoid foliage moisture for extended times.

Any impatiens plants left after the main crop has been shipped should be removed from the greenhouse immediately and discarded. No volunteer plants should be allowed to grow in or around the production area, even briefly.

Prevention - Plant Protection Products

A downy mildew prevention program should include a rotation of effective fungicides (see Table 1) with different modes of action.

Table 1. Examples of products suitable for use in a greenhouse control program for impatiens downy mildew.

MOA

(FRAC#)

Product

Active Ingredient

Activity

Application

REI

Company

4

Subdue Maxx® fungicide

Mefenoxam

Systemic

Spray

Drench

48 hr

REI exemptions (0 hr) for certain drench applications

Syngenta

11

Heritage®* fungicide

Azoxystrobin

Systemic

Spray

4 hr

Syngenta

11

Disarm® O fungicide

Fluoxastrobin

Systemic

Spray

12 hr

OHP

11

Fenstop™ fungicide

Fenamidone

Systemic

Spray

12 hr

OHP

11+7

Pageant™ fungicide

Pyraclostrobin + Boscalid

Translaminar (11) + Systemic (7)

Spray

12 hr

BASF

21

Segway™ fungicide

Cyazofamid

Translaminar

Spray

12 hr

FMC

33

Aliette® fungicide

Fosetyl-AL

Systemic

Spray

12 hr

OHP/Bayer

33

Alude™ fungicide

Potassium salts of Phosphorous Acid

Systemic

Spray

4 hr

Cleary Chemical Corp.

33

Vital® fungicide

Potassium phosphite

Systemic

Spray

4 hr

Phoenix

40

Micora™ fungicide

Mandipropamid

Translaminar

Spray

4 hr

Syngenta

40

Stature® SC fungicide

Dimethomorph

Translaminar

Spray

12 hr

BASF

43

Adorn® fungicide

Fluopicolide

Systemic

Spray

Drench

12 hr

Valent

M

Protect DF fungicide

Mancozeb

Contact

Spray

24 hr

Cleary Chemical Corp.

NOTES:

• Always test products on a small area before using on an entire crop.

• For brevity some products with the same mode of action have been omitted.

*Syngenta supports a FIFRA Section 2(ee) recommendation for Heritage to control Impatiens Downy Mildew on bedding plants in AL, AR, AZ, CA, CO, CT, DE, FL, GA, HI, IA, ID, IL, IN, KS, KY, LA, MA, ME, MI, MN, MO, MD, MS, MT, NE, NC, ND, NH, NM, NJ, NV, OK, OH, PA, RI, SC, SD, TN, UT, VA, VT, WA, WI, WV and WY.

Young plant production

Preventative measures in propagation are good safeguards. Since propagation time is short, one or two applications should be sufficient (Table 2).

Table 2. An example of a program for prevention of impatiens downy mildew as well as other diseases of impatiens during propagation.

Propagation Week

Plugs

(Seed or URC)

1

2

Drench - 21 or 4

3

4

Spray - M or 11

Finished plant production

Finished plant production may be as long as 12 weeks from transplant to shipping, which provides more time for any impatiens downy mildew to spread. The example rotation in Table 3 provides options for multiple applications of protective fungicides for downy mildew as well as other common diseases of impatiens.

Growers can apply preventive products for early-season protection against downy mildew in the landscape. We specifically recommend a drench application to finished plants shortly before shipping. While no product will give season-long control of impatiens downy mildew, drench applications prior to shipping have been shown to provide early season protection against the disease and longer, successful performance in the landscape.

Table 3. An example of a program for prevention of impatiens downy mildew as well as other diseases of impatiens during production of finished plants.

Crop

Week (post-transplant)

Application

FRAC #

Fungicide / AI
(example)

Target Diseases

4 inch pot or smaller

1

Recommended treatment (drench)

4+43

Subdue Maxx + Adorn

Pythium, Phytophthora, Downy Mildew

2

Optional treatment

(spray)

M

Mancozeb

Downy Mildew, Leafspots, Botrytis

3

Recommended treatment (spray)

40+M

Micora Mancozeb (optional)

Downy Mildew

Leafspots, Botrytis

4

Optional treatment

(spray)

11+M

Heritage Mancozeb (optional)

Downy Mildew, Leafspots, Botrytis

5

Recommended treatment (spray or drench)

33

Potassium phosphite

Downy Mildew, *Pythium, Phytophthora

6

Optional treatment

(spray)

11

Heritage Mancozeb (optional)

Downy Mildew

Leafspots, Botrytis

6 inch pot

7

Recommended treatment (spray)

40

Micora Mancozeb (optional)

Downy Mildew

8

Optional treatment

(spray)

11

Heritage Mancozeb (optional)

Downy Mildew

Leafspots, Botrytis

9

(final week)

Recommended treatment (drench)

4+33 or 4+43

Subdue Maxx + Potassium phosphite or Subdue Maxx + Adorn

Downy Mildew, *Pythium, Phytophthora

Larger containers

9

Recommended treatment (spray)

40

Micora

Downy Mildew

10

Optional treatment

(spray)

11 or 21

Heritage or Segway

Downy Mildew, Leafspots, Botrytis

11

(final week)

Recommended treatment (drench)

4+33 or 4+43

Subdue Maxx + Potassium phosphite or Subdue Maxx + Adorn

Downy Mildew, *Pythium, Phytophthora

*Additional diseases controlled with a drench application

What to do if you find downy mildew

If impatiens downy mildew is found, all plants with symptoms and any fallen leaves must be immediately placed in sealed bags and removed from the greenhouse. Also remove and discard plants from a buffer area of three feet (one meter) radius or more around infected plants. Use an approved greenhouse disinfectant to disinfect greenhouse surfaces contacted by infected plants. Begin a preventive fungicide program (Table 3) on all remaining I. walleriana.

Alternatives to impatiens

While no plant is an exact replacement for impatiens, there are options if it is necessary to substitute for impatiens:

Begonia - Bada Bing®, Bada Boom®, Eureka™, Varsity™, Volumia™, Braveheart®

Euphorbia - Euphoric™ (avoid deep shade)

Geranium - Caliente®

Lobelia - Techno®

New Guinea Impatiens - Sonic® and Super Sonic®

Nicotiana - Saratoga™

Petunia - Picnic™,Sanguna®,Whispers™, Petunia Bravo™, Duvet™, Storm™, Ultra™, Plush®, Ramblin™ (avoid deep shade)

Salvia - Picante™, Salsa™

Torenia - Duchess™

Resources

©2012 Syngenta. Important: Always read and follow label instructions before buying or using Syngenta products. The instructions contain important conditions of sale, including limitations of warranty and remedy. All products may not be registered for sale or use in all states. Please check with your state or local Extension Service before buying or using Syngenta products. Syngenta supports a FIFRA Section 2(ee) recommendation for Heritage to control Impatiens Downy Mildew on bedding plants in AL, AR, AZ, CA, CO, CT, DE, FL, GA, HI, IA, ID, IL, IN, KS, KY, LA, MA, ME, MI, MN, MO, MD, MS, MT, NE, NC, ND, NH, NM, NJ, NV, OK, OH, PA, RI, SC, SD, TN, UT, VA, VT, WA, WI, WV and WY. Some or all of the varieties listed herein may be protected under one or more of the following: Plant Variety Protection, United States Plant Patents, Utility Patents, and/or Plant Breeders' Rights and may not be propagated or reproduced without authorization. Bada Bing®, Bada Boom®, Braveheart®, Caliente®, Duchess™, Duvet™, Euphoric™, Eureka™, Heritage®, Micora™, Petunia Bravo™, Picante™, Picnic™, Plush®, Ramblin™, Salsa™, Sanguna®, Saratoga™, Sonic®, Storm™, Super Sonic®, Subdue Maxx®,Techno®, Ultra™, Varsity™, Volumia™, Whispers™ and the Syngenta logo are trademarks of a Syngenta Group Company. Disarm ® and Fenstop™ are trademarks of OHP, Inc. Pageant™ and Stature® are trademarks of BASF. Segway™ is a trademark of Ishihana Sangyo Kaisha, Ltd. Aliette® is a registered trademark of Bayer CropScience. Alude™ and Protect™ are trademarks of the Cleary Chemical Corporation. Adorn® is a registered trademark of Valent U.S.A. Corporation. Vital® is a registered trademark of Phoenix Environmental Care, LLC.

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