Downy Mildew Keeping Diagnostic Clinic Busy

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Closeup shot of downy mildew on an impatiens leaf. Downy mildew has been decimating landscapes full of impatiens in South Florida this year. Photo courtesy of Aaron J. Palmateer, assistant professor and extension specialist at the University of Florida. Photo courtesy of Aaron J. Palmateer, assistant professor and extension specialist at the University of Florida.

Downy mildew has moved into Florida and is currently devastating impatiens in the landscape. I first encountered the disease on January 13, when a landscape sample from Palm Beach County was submitted to the Florida Extension Plant Diagnostic Clinic. Over the next week, samples started coming in from all over Palm Beach County, where it seemed impatiens were dying everywhere.

As time progressed, diagnostic samples began to arrive from landscapes further south in Fort Lauderdale, Doral, Miami and eventually showed up in a large planting of impatiens right across the street from the University of Florida’s Tropical Research and Education Center in Homestead.
It’s been a very mild winter in South Florida with recent daytime temperatures reaching the upper 80s. It’s definitely a little warm for downy mildew, but the cooler nighttime temperatures combined with high relative humidity created the perfect recipe for disease. In fact, there’s an abundance of inoculum for a plant pathologist to conduct fungicide efficacy trials right now.

We currently have several trials underway with hopes to identify cost-effective management options.   There is some very effective chemistry for controlling this pathogen, but the bigger issue is going to be cost of management, especially in the landscape.

Aaron J. Palmateer is an assistant professor and extension specialist in ornamental plant pathology at the University of Florida, IFAS (Institute of Food and Agricultural Sciences). Palmateer can be reached at ajp@ufl.edu.
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