Be on the Lookout for Infected Impatiens

As you begin planting your impatiens this season, be aware of downy mildew that’s ravaging landscapes across the country.  Millions of flowers are prime targets this season for the disease and scientists don’t know where this latest outbreak started, or what’s causing it to spread so quickly.  One of the leading researchers focused on downy mildew, Colleen Warfield, says the disease, while devastating in the areas it impacts, can be managed by sharp-eyed contractors.

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The tell-tale white spores on the underside of foliage.

What it is:  Downy mildew is a fungus-like organism or water mold related to pythium and phytopthra that attacks Impatiens walleriana and interspecific hybrids with an I. walleriana parent as well as a few types of wild impatiens.  The disease has been identified in 32 states with outbreaks spotted through the Midwest last summer and early fall.

What it does:  Infected plants can look healthy – it takes up to 14 days for the velvety-white spores to grow big enough on the undersides of leaves to be seen with the naked eye. But by then, it’s too late. Once it arrives, it hits hard, turning once-lush mounds of pink and white flowers into a tangled mess of gnarled stems.

When it happens:  The pathogen seems to tolerate warm days, but when evening temperatures drop into the upper 50s or lower 60s, that’s an ideal time for the disease to develop, Warfield says. Combine that with “free moisture” – a rainstorm, or a few overcast days of drizzle or just a good layer of fog – and you’ll see the disease develop.  All it takes is four hours of leaf wetness for the spores to infect the plant.

What now?  If you have infected impatiens, remove the offending plant material as soon as possible, then bury or burn it.  Don’t replant the same species, and start thinking about alternative plantings.

For more information on downy mildew and what you can do click here.

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A bed of impatiens with downy mildew.

Quick Downy Mildew facts: If you install or maintain a property with bedding impatiens, here’s what you need to know and do to prevent downy mildew from devastating your accounts.

• Work with your grower to make sure she’s using preventative fungicides on her crops.  That can keep the disease from ever making it out of the gate.
• Rotate the beds you plant with impatiens, or mix up massive plantings with other varieties.  Monoculture can be dangerous.
• Consider alternative shade annuals like hypoestes, sedum or other species of impatiens, like New Guinea, which have shown high levels of resistance to downy mildew.
• Use preventative applications of fungicides.  They’re more effective than treatment after the disease is identified.
• Tear out and burn or bury any infected plant material.

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