Impatiens Downy Mildew: Tips for Retailers

Ellen C. Wells

If you’ve seen the lead story on the GrowerTalks’ side of this magazine, you know there’s a disease in the U.S. that could impact sales and your customer’s future success of growing impatiens in the landscape this season. Impatiens downy mildew is caused by Plasmopara obducens, and if there is a silver lining in a disease, it’s that this particular pathogen effectively impacts only Impatiens walleriana. New Guinea impatiens is highly tolerant of the disease.

For the specific details on impatiens downy mildew, turn to page GT48 and GT78. The topic we are addressing here is this: How can you, as a retailer, prepare for a spring that could look dim for one of America’s top-demanded floriculture crops?

Educate Your Staff
It’s important to know that impatiens downy mildew has limited confirmed reportings throughout the U.S. The disease has been verified in commercial landscape beds in coastal southern California; northeast Illinois; northern Indiana; the Twin Cities region of Minnesota; Cape Cod, Massachusetts; and Long Island and upstate New York in 2011. If you’re in those regions, keep an eye out for the disease.

No matter your location, familiarizing yourself and the entire garden center staff with the symptoms of impatiens downy mildew will help you spot infected plants that might have snuck onto your retail benches. The garden center’s landscape crew should also be familiar with symptoms in the landscape in case they appear in landscape beds.

Symptoms include:
Early Symptoms (more likely to be observed in the greenhouse):
• Lightly chlorotic or stippled leaves. Subtle gray markings may also appear on the upper leaf surface.
• Leaves may turn downward from the leaf margins.
• A white, downy growth may be present on the underside of affected leaves.
Advanced Symptoms (more likely to be observed in the landscape):
• Plants that become infected early in their development may be short, have small leaves and produce fewer flowers.
• Yellowing of leaves.
• Premature leaf and flower drop resulting in bare, leafless stems.
• Stems may eventually become soft and the plant will collapse, similar to the effects of frost damage.

If Spotted
If impatiens downy mildew is spotted on your retail benches or in beds, immediately place infected plants into sealed heavy-duty plastic bags (to avoid spreading spores around) and discard with the trash. If impatiens downy mildew is spotted in the landscape, remove the entire plant including roots and fallen debris. DO NOT COMPOST!

Inform Customers
The phrase “What they don’t know can’t hurt ’em” is not applicable here. In fact, it could end up hurting you. Customers who find impatiens downy mildew on their plants after purchase—whether it’s in baskets or beds—will likely assume your business is at fault. “What, me go back to Joe’s Greenhouses? They sold me those bum impatiens.” Not what you want happening.

An educated customer is a grateful customer. Inform them of:
• the disease’s existence, especially if the disease has been verified in or near your region.
• the symptoms (leaf yellowing and premature leaf and flower drop) to watch for.
• that although plants may be healthy when they leave the garden center, they could become infected once planted into the landscape, with the risk of susceptibility being higher in beds with a previous history of the disease.
• its higher incidence of occurring in locations that are heavily shaded, densely planted and stay moist for long times.
• planting measures that could lower the likelihood of the impatiens downy mildew—and diseases, in general.

These points can be communicated to customers with in-store signage, announcements via your store’s e-newsletter, website notices, and Facebook and Twitter updates.

Provide Alternatives
Consumers in most parts of the country can continue to plant and enjoy their beds and baskets of impatiens. For those customers in affected geographic areas, there are a handful of shade-loving, colorful annual varieties as alternatives.

Thankfully, New Guinea impatiens can safely be planted in beds that were previously infected with impatiens downy mildew. Coleus, while susceptible to a downy mildew, are not susceptible to the species that infects impatiens. Other shade-loving annuals include begonia, torenia, browallia, alternanthera, dichondra, lamium and plectranthus, among others. GP