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 offers guidance to growers in need of getting downy mildew under control on America’s number one bedding plant.
GG: Why do you think the impatiens you saw in Florida earlier this year are struggling so mightily with downy mildew?
KK: I think the disease has simply been carried over in the soil from the previous year. When plants aren’t cleaned out and the residue is left to rot, the [downy mildew] spores are overwintering. So when the crop is replanted, the crop is coming into contact with those spores.
The key is for consumers not to plant impatiens where they had impatiens last year. Downies are very host-specific pathogens, meaning this particular downy goes to the walleriana impatiens. If you had a snapdragon with downy mildew or a rose with downy mildew, those pathogens would not move to impatiens.
GG: How fast is the disease spreading on impatiens in the landscape?
KK: Let’s say a tray has gone through the broker, a garden center and now it’s finally gone into the hands of a landscaper – so it’s probably been a couple weeks since it came out of the greenhouse. At that point, the last fungicide is starting to wear off. As beds are planted, you may all of a sudden see more typical symptoms of downy mildew – a little bit of a bronzing effect, or you turn the leaf over and see the fuzzy sporulation on the back of the leaf.
Still, I doubt if landscapers will be back in beds once they’re finished planting. By four weeks out of the greenhouse, the sporulation is taking hold and you see the plant start to defoliate. If you go back to a bed that was absolutely spectacular in color, all of a sudden you see sticks.
GG: What’s your impression of how growers are handling their own impatiens material in the greenhouse?
KK: I think the greenhouses are pretty clean. Growers are being cautious and putting out flats that are clean when they exit. But when impatiens are in a production greenhouse as plugs or as finished products, they’ve had at least two cover sprays. They’ve had a drench in the plug stage and at least one finished spray. So they’re clean going out, but impatiens probably also need a cover spray in the landscape.
GG: What advice can you offer growers looking for guidance as they try to manage their way through this particular pathogen?
KK: BASF has Stature SC fungicide for growers. This product, applied preventively, can stop sporulation of downy mildew, hence stopping the infection cycle. Stature is an excellent fungicide choice for growers as a last protective spray. We know garden centers are not going to apply fungicides. So it’s up to growers to provide at least the two-week window of greater protection once it gets out of their production greenhouses.
Additionally, there are things landscapers need to do. Landscapers should be responsible for putting a cover spray on, which they probably have not done in the past. They need to explain to their customers why because there’s going to be a cost built in.
If landscapers go back to a job and see that plants are already showing signs of infection (beginning to defoliate), they need to take those plants out immediately. They’re not going to save them. They need to remove them from the landscape and actually keep that residue isolated from any of their other beds.
Downy mildew is wind borne, so if you had an open landscape truck moving down the road, those spores could move off the material. Landscapers need to enclose the residue in a paper or plastic bag so they get the inoculum out of the environment.
GG: What can growers learn from the downy mildew on impatiens scene in Europe, where the issue has become an even bigger problem than here in the United States?
KK: Based on what I read from England, they thought the problem subsided. The truth is it’s an epidemic. I do believe it’s because England went through low years of not paying any attention that they’re struggling again.
For us, I think it’s going to take more than a one-year cycle in the U.S. to clean up our beds. The moral of the story is you have to watch how you plant impatiens. Even with a cover spray in the landscape, you could still get some disease in the newly planted beds. But you certainly can’t put new impatiens behind last year’s impatiens.
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.