Downy Mildew On Impatiens vs. Boxwood Blight

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.

AS BASF’s Kathie Kalmowitz made her way across Florida in mid-February, she saw firsthand just how poorly impatiens were faring in landscapes. Every bed Kalmowitz saw was stripped of leaves, the result of a downy mildew pathogen gone wild.

Recently, Greenhouse Grower caught up with Kalmowitz, a technical expert who offered insight on how the downy mildew on impatiens threat ranks against other disease issues. Kalmowitz also tells us how the current threat to impatiens compares to the new boxwood blight scare.

GG: How does downy mildew on Impatiens walleriana compare to other disease issues you’ve encountered?

KK: It’s a perfect storm when the inoculum appears in the environment and the present climatic conditions allow the disease to move quickly toward devastating an entire planting.

Still, I feel this kind of falls into the more normal range of how a host-specific pathogen can spread so quickly in a species. The reason why there’s such heightened awareness is because it’s such a huge crop. It’s loved from the North to the South. It is a staple of landscape color; it’s always been a prolific flowering plant. It holds up really well and has great form. It has all these attributes, so people have a high regard for it.

GG: How does the new boxwood blight issue compare to downy mildew on impatiens?

KK: Boxwoods are a beloved woody ornamental, just like impatiens are a beloved bedding plant. So there is a heightened awareness of this disease. People are really nervous because they don’t want the disease to spread. If we can keep it to the areas where it was already found, it would be better. I don’t think we know enough about boxwood blight [as of late February]. We don’t know how it becomes established in a boxwood planting. Does it infect a mature boxwood as much as one in a production stage?

I know it does move across all boxwoods. Unlike impatiens and the walleriana species, this disease moves from Japanese to American to English [boxwoods]. And that means if you get boxwood blight in your nursery or in your landscape, then you have to switch out of all boxwoods.

GG: What do we know about existing products and their ability to combat boxwood blight?

KK: Our product Pageant fungicide is extremely broad. It has two active ingredients that work together in order to have activity on the target. We have cylindrocladium on our label. It was in the English trial and, thus, there is some precedence to using our product.

[North Carolina State University Associate Professor and Extension Specialist] Kelly Ivors has been asked to do a containment trial. We all look forward to her work this spring. She has been asked to do her trial in a more isolated area of the research station so boxwoods can be contained and not exposed to production areas that have a lot of woody plant production.

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One comment on “Downy Mildew On Impatiens vs. Boxwood Blight

  1. Paul Westervelt

    It's worth pointing out that downy is spread by wind and boxwood blight isn't, so a planting of Impatiens walleriana is more likely to be infected than a planting of boxwood as long as you're careful about where your box come from and take regular, common sense protection measures. Also, while it's true that boxwood blight is thought to infect all boxwood (it hasn't been tested on every cultivar yet), it's incorrect to assume that getting boxwood blight in your nursery or landscape means you "have to switch out of all boxwoods". Many European gardens have had boxwood blight on their grounds for years and still have healthy plantings. If conditions aren't right, there's no blight. We saw a hedge at Wisley that had a blighted plant removed and the adjacent plants are perfectly fine.