Friday, February 22, 2013

Impatiens Alternatives for 2013

Horticulture Educator, Steve Bogash, guest blogs on alternatives to impatiens, which were devastated last year with a new disease, Impatiens Downy Mildew, (Plasmopara obducens).

Photo Courtsey IFAS Palm Beach Extension
Last season, we at Penn State Extension started getting calls about Impatiens losing their leaves and collapsing long before frost. The best calls were requests for bunny rabbit control as gardeners thought that rabbits had eaten all of their Impatiens’ leaves.

It turns out that we have a new disease, Impatiens Downy mildew, that specifically hits what most of us know as the “Common Garden Impatiens”, Impatiens walleriana. (Busy Lizzy) This disease does not infect other plants as of this time. There are other species of Downy mildew that infect most every plant if the pathogen is present and the conditions are right. However, it is very important to note that this disease does not infect New Guinea or Sunpatiens or any other flowers or herbs.

Downy Mildew on Impatiens
This disease was first reported in the United States in 1942. Since then, there have been sporadic outbreaks, but things started getting serious in 2004. Then widespread regional outbreaks were reported in 2011. The map showing where it was found in 2012 does not leave much of the U.S. disease-free.

Pennsylvania, like many states is a hotspot for this disease. If you had infected plants in your flower beds last year, then it is extremely likely that things will get worse this year. This pathogen overwinters quite well and can persist for many years.

How do you know if your Impatiens had it last year?
  • Yellowish or pale-green foliage
  • Downward curling of the leaves
  • Distorted leaves
  • White to light-gray fuzz on the undersides of the leaves. There are excellent images on the web if you search for “Impatiens Downy Mildew.”
  • Emerging, new leaves that are smaller than normal and discolored.
  • Flower buds that either fail to form or abort before opening.
  • Stunted plants
If you have relied on Impatiens for color in your shadier areas in the past, things are going to change at least until plant breeders come up with resistant varieties. For now, your best options are to use other plants that still perform well in the shade, and if you are fortunate enough to have avoided the disease so far, cull infected plants quickly if you see symptoms. Also, healthy-appearing plants adjacent to the diseased plants should be culled. Do not compost these plants, dispose of them.

Landscapers and those holding Pesticide Applicators Licenses have an additional option with an array of fungicides which if applied properly (timing and rate are very important) can control this disease. Home gardeners have a single active ingredient, phosphonic acid, marketed as Agri Fos, from Monterey Products and Exel LG from Organic Laboratories that should provide good control if used according to their labels.

The best option for many of us is to use alternative plants that perform well in the shade. Here are some to consider:

Begonia 'Black Fancy' - UGA 2012 Trials
Bigleaf begonias: The Penn State Flower Trials at the Southeast Research and Extension Center (Manheim area) identified several of these as superior performers. Look for the Whopper series.

Picture Courtesy of Cornell University

Wax begonias: These are a mainstay in the shade for many gardeners as they are compact, have good leaf color and texture and are available in market packs for larger plantings.

Lobelia erinus - Picture Courtesy of Cornell University
Lobelia: While they will perform best in the sun, they will tolerate partial shade.

Impatiens hybrid 'SunPatiens Compact Electric Orange'
SunPatiens: the plant-growing industry is currently looking at these as the go-to replacement as they are resistant to this disease even under heavy pressure and there are both upright and spreading types.

Impatiens hawkerii 'Tamarinda Orange Orchid'
New Guinea Impatiens: As with SunPatiens, New Guineas are resistant to this disease. In addition, there are some really interesting variegated types.

Torenia fournieri - University of Illinois (Wishbone Flower)
Torenia: This underused plant is worth a look as it performs well from partial to full shade.

Solenostemon 'ColorBlaze Marooned'  Coleus
Coleus: look for those that are labeled for shade.

Picture Courtesy of Cornell University
Polka Dot Plant: Interesting spotted foliage plant now in many hybrid forms. It is shade tolerant, but not truly dark areas.

Alternanthera 'Purple Knight' (Joseph's Coat)
Joseph’s Coat: Another interesting foliage plant that does well in partial, but not heavy shade.

Our shady areas will probably look a bit different for the next several years, but there are some good options to Impatiens. Go to the SunPatiens’ website for advice on spacing, learn what this disease looks like and plan for another great gardening season.

Update: March 9, 2013.  Master Gardener Tina Clinefelter (Clinton County) of Gardening in the Keystone State, adds her alternative selections to the list.

Update: March 20, 2013.  Illinois Extension has more on the disease, with links to more alternatives from Michigan State University.

Update: March 23, 2013.  The University of Maryland's Home and Garden Information Center describes the disease and offers alternatives here.

Update: April 9, 2013.  Steve's counterpart for Buck's County, Educator Scott Guiser has a post on the subject which you can read here.  He also links to the Michigan State Q&A and another alternatives list from Ball Seed Company.

Update: April 19, 2013.  Washington Post horticulture writer Adrian Higgins covers the topic here.

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