MSU Extension

We’ve come a long way in preventing downy mildew on impatiens both in the greenhouse and the landscape. No longer does this disease need to be a limiting factor in producing this popular bedding plant if greenhouse producers follow fungicide recommendations.

Downy mildew disease is caused by Plasmopara obducens on impatiens and can quickly escalate into epidemics if not controlled, and that was the case several years ago. In fact, it seemed as though gardeners would not be able to successfully include this bedding plant without a risk of downy mildew disease.

However, multiple Michigan State University research studies conducted from 2013 to 2016 by Mary Hausbeck's Lab clearly shows that if impatiens are treated in the greenhouse with the fungicides listed in MSU Extension’s recommendations, the risk for a downy mildew outbreak in the landscape are significantly reduced or prevented entirely. This data has also shown that impatiens can be successfully grown in landscape beds even when the planting bed has a history of impatiens infected with downy mildew.

The 2016 recommendations include a newly registered fungicide that is especially strong against downy mildew. The critical factor to success is that impatiens plants must be treated with fungicides during the time they are being grown in the greenhouse with the strategy of having the most effective products used just prior to shipping and sale. The later greenhouse applications that are made prior to planting can ensure protection for an extended period once the plants are transplanted into the landscape. In 2015, test plots conducted in a downy mildew-infested garden site with impatiens treated only while they were grown in the greenhouse lasted until late September and even to early October.

View the recommendations below, or download a printable PDF at: 2016 Greenhouse Impatiens Downy Mildew

2016 Greenhouse Impatiens Downy Mildew (updated 3-16-2016)

1. First Application: (DAY 1)

• Subdue MAXX (1 fl oz/100 gal) drench.
• Treat soon after plants received unless propagator has treated just before shipment.

2. Second application: (DAY 7)

• Segovis* (3.2 fl oz/100 gal) drench

3. Third application: (DAY 21)

• Segway (2.1 fl oz/100 gal) spray

4. Fourth application: (DAY 35)

• Orvego (11-14 fl oz/100 gal)/Stature SC (6.12 fl oz/100 gal) spray or Micora (4-8 fl oz/100 gal) spray.

5. Repeat # 3 and # 4 at two-week intervals, as needed, until the final application prior to shipping. 

• Add mancozeb (Protect, etc.) to any treatment if desired for Alternaria leaf spot control.

6. Last application, shortly before shipment: 

1. Subdue MAXX (1 fl oz/100 gal) + Segovis* (3.2 fl oz/100 gal) drench.

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.

*Segovis is a new fungicide from Syngenta Professional Products. Do not use Segovis for more than 33 percent of the total fungicide applications per season per crop. Although we recommend using Segovis in your downy mildew control program, if the product is not available you can substitute Adorn** (1 fluid ounce per 100 gallons drench) in its place.

**The Adorn label states that the product must be tank-mixed with a product that is also effective against the target pathogen. Products that are labeled for drench applications would include Heritage and the phosphorus acid-based products (Jetphiter/Alude). Please note the reapplication interval on the label for all products used in your downy mildew control program. For example, if Heritage is used as the partner drench with Adorn, skip to treatment #4 in the above program for your following application.

Please contact your local MSU Extension floriculture educator for more information on this subject.

Hausbeck’s research at MSU on ornamentals is funded in part by Project GREEEN GR13-027, the Western Michigan Greenhouse Association, the Metropolitan Detroit Flower Growers Association, APHIS Award 14-8130-0254-CA and the Floriculture Nursery and Research Initiative of the Agricultural Research Service under Agreement #58-1907-0-096.