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Downy Mildew - Looks like it's here to stay. Learn about it.
May 16, 2013

Welcome to the Flying Butterfly!

Not much was known about downy mildew when it first came onto the scene a few years back. Upset gardeners would go to their nurseries with a sickly impatiens plant wanting to know why it was dying. Admittingly, the nurseries were not completely certain of the problem themselves at the time. They did know it had to do with the soil.

In 2012 I saw nurseries recommending that gardeners rotate planting locations of Impatiens just as a farmer would rotate planting crops to revitalize the soil. After witnessing this for the second time I called the Master Gardener extension of The Ohio State University agriculture department. They did know what it was – Downy Mildew.

With the 2013 gardening season getting underway I am seeing many articles pop up regarding this problem and it appears for good reason. Downy mildew is now spreading across the United States and Canada
and is reportedly here to stay.

Already an epidemic in Europe, downy mildew’s early signs went undetected when it hit our shores in 2007. With various plant leaves turning color, commercial growers originally thought it was nutritional deficiency. It is now coming to our home gardens by way of Impatiens plants.

Here’s what has been learned since then:

Downy mildew is a generic term. There are different microbe parasitic organisms that effect different plants. In the case of our Impatiens plants it is a fungus-like organism called Plasmopara obducens.

Plasmopara obducens shows up as a white powder on the underside of a yellow or green Impatiens leaf. Depending on the Impatiens walleriana variety there may be subtle gray markings on the upper leaf surface.

Symptoms typically start with a few leaves turning yellow to white with possible stippling. Over time leaves become completely yellow, then brown killing them. Eventually leafless stems become soft and the plant collapses.

The organism carries into the soil where oospores, or fertilized eggs, survive for years killing any healthy Impatiens plants that are planted in the same area. But..

...not only does the organism live in the soil, it is carried by wind and water. Cooler temperatures, especially at night, accelerates the spread along with areas with continuous moisture. It is highly likely organism will carry to any planted or potted Impatiens in different locations also infecting them.

So what do we do if our Impatiens are looking sickly?

First, check under the leaves. Just because its color is yellow to brown does not mean downy mildew infection. The plant could only need water. If there is a white powder on the underside of leaf, that indicates downy mildew.

Upon detection of an infected plant remove it immediately and put it in a plastic bag to avoid spreading organisms. Don’t replant Impatiens in the same area for several years.

Does this mean we can’t plant any more Impatiens? That’s up to you. Currently it is only Impatiens walleriana that is affected. You can always opt for different types of Impatiens like New Guinea Impatiens or the hybrid Sun-patiens. So far these two varieties are showing a high resistance to the organism.

There are fungicides available but so far only to commercial growers and landscapers. To have affected areas of your garden treated you would have to contact them.

When buying Impatiens walleriana:

You may not see white powder under the leaves upon inspection. The organism could still be present, however.

Keep the telephone number handy of your local extension office. You'd be surprised how many questions they can answer for you! Until next time, Happy Gardening!

Elizabeth @ Easy Butterfly Garden

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