The world of horticulture is abuzz with news of the "Downy Mildew" fungal disease that's killing the ever-popular shade annual Impatiens walleriana - that's the one shown above, the most popular type of all. (New Guinea Impatiens are not affected by the disease.) So I asked Homestead's education coordinator Gene Sumi about it and he referred me to the University of Delaware website for the closest scientific advice, there being nothing about it yet on the University of Maryland's website.
So here's what I found out - that Downy Mildew can be prevented, but not cured, so prevention is key. Read on to learn about the gardener options: preventing the disease, or substituting other plants for the Impatiens until the disease is under control (we hope). And there's lots of great alternatives - time to get creative!
The earliest symptom on affected plants is yellowing on the top leaf surface, often accompanied by a downward curling of leaves. Yellowing is followed by leaf drop, stunting and overall poor condition of plants. Production of sporangia of the downy mildew pathogen occurs on the undersides of the leaves in abundance, can be seen as white patches on the lower leaf surface of what is normally a smooth surface (not hairy). The disease is spread by air currents, by water splash, and on infected leaf material and seeds.
Downy mildew on impatiens is an aggressive disease and has been confirmed in most Eastern states, as well as Texas, California, Oregon and Washington. The pathogen is favored by wet, humid conditions, typical of the coastal climate found in Delaware. There is no harmful effect to humans from contact with the downy mildew pathogen.
Affected plants should be removed from garden beds or pots and be discarded in the trash, not composted nearby. The pathogen may overwinter in Delaware (and Maryland), especially in a greenhouse or sheltered location.
Seedlings can be drenched with fungicide before planting. Control on plants in home gardens may be accomplished by using phosphorus-based fungicides preventatively. Gene recommends spraying with Agra, the type of low-toxicity fungicide recommended by Delaware, applying every 14-21 days. It's a systemic, so just spray it on the leaves.
For Our Customers
Homestead is taking two measures in response to this widespread disease. First, we're carefully inspecting all Impatiens from the grower for signs of disease. And we're taking steps to educate our customers about the risk of Downy Mildew, what to do about it, and also what alternatives they might consider to using Impatiens this year, or until the Downy Mildew epidemic is over. Informational signs will be posted in close proximity to the Impatiens we'll be offering, and the recommended fungicide for preventing the disease will be there, as well.
Now let's get to alternatives, shall we? First, my own favorite for season-long color in the shade is the increasingly popular annual Coleus, which now comes in an amazing assortment of colors, shapes and patterns, as well as for sun or shade. Coleus assortments are a mainstay in both my containers and borders. The first two photos below show some of the prominent Coleus displays on my (former) front porch.
Next, a professionally designed container arrangement of Coleus at Chanticleer Garden in Pennsylvania.
Next, see how great they look massed under trees.
Finally, Coleus also combine well with shade-loving perennials like ferns and Japanese Forest Grass.
Stay tuned for more options; we've got plenty.