Impatiens Downy Mildew: Identifying & Managing the Disease

Impatiens downy mildew is a pathogen that causes defoliation and plant death. Learn more about it and how to keep it out of your garden.

by Jessica Walliser
PHOTO: Jessica Walliser

If you enjoy planting shade-loving impatiens in your garden, you might find your plants suffering from a pathogen called impatiens downy mildew (Plasmopara obducens). Though this disease has been in the U.S. for at least a decade, it’s becoming more and more common.

What Is Impatiens Downy Mildew?

Impatiens downy mildew is a pathogen that affects every variety of Impatiens walleriana along with all of the hybrid impatiens that use this species as a parent.

At first, the leaves of infected plants turn a pale shade of green and curl in a downward direction. The second stage of infection causes white growths to appear on the lower leaf surfaces. Eventually all the leaves and flowers drop from the plant, leaving behind nothing but the bare stems which will eventually succumb to the disease, too, and droop to the ground.

How Does Impatiens Downy Mildew Spread?

Like many fungal organisms, the pathogen that causes impatiens downy mildew prefers wet weather and travels readily via soil, water and air. It spreads easily from plant to plant, especially during wet or humid growing season.

Because it can live for many years in the soil, if you’ve grown impatiens that showed signs of impatiens downy mildew in the past, you’d be wise to not plant them again for several years. You can try growing them in a different garden area, but depending on the weather and the prevalence of the disease, you could face a new infection.

How Do I Prevent Impatiens Downy Mildew?

Unfortunately, there’s little you can do to prevent infection once the sporangia are present. However, you should only purchase symptom-free plants from a reliable, local nursery. Often new infections arrive in a garden from transplants that were not yet showing symptoms. Since this pathogen is more prevalent in warmer climates, if you live in a northern gardening zone, avoid buying plants from nurseries that ship their plant stock in from southern greenhouses.

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To prevent the spread of this disease, water plants in the morning so they have plenty of time to dry before nightfall. And irrigate using drip irrigation or soaker hoses, if possible, to keep the foliage as dry as possible. Do not space plants too closely to ensure they have good air circulation as well.

Thankfully, this pathogen only harms impatiens. It will not infect other flowers or vegetables.

Are Some Impatiens Resistant to Downy Mildew?

There are a handful of other impatiens species that are resistant to impatiens downy mildew and they are a good option if you enjoy the look of impatiens but don’t want to risk losing your plants to an infection.

Try New Guinea impatiens (I. hawkerii) or SunPatiens, two darker-leaved impatiens varieties with prolific, brightly colored flowers. Both are resistant to this pathogen.

Another option, of course, is to skip the impatiens and plant a different shade-loving annual in their place. Thankfully, there are lots of beautiful options available.

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