Well, gang, the unfortunate emergence of super bugs and super diseases marches on. You might want to work up a Plan B for that shade bed where you plant impatiens every summer.
Latest in the disease onslaught is Impatiens Downy Mildew (IDM). This fungus-like pathogen attacks our beloved bedding impatiens (Impatiens walleriana), which includes such popular strains as ‘Super Elfin’, ‘Accent’, ‘Dazzler’ and many more, plus the doubles, minis and ‘Butterfly’ types, along with old fashioned Balsam Impatiens and the wild orange jewelweed and yellow touch-me-not. Last summer (2012 season) IDM hit the Nashville area like a ton of bricks. If your impatiens were looking good and then started looking puny, quit blooming, defoliated quickly and were pretty much collapsed and gone in a few weeks, you had IDM. The disease showed up in early summer and spread like wildfire — I’d say 50% of my clients lost their impatiens plantings by mid-August last year. Hot, humid weather promotes the disease, meaning our area is just as perfect for growing Impatiens Downy Mildew as it is for growing impatiens. Aren’t we lucky.
So what is it and where did it come from?
Impatiens Downy Mildew has been around for a long time, but it hasn’t been a virulent plant-killer until just the last few years. Scientists don’t know yet whether the original disease has recently mutated into this much worse form, or whether this is a new closely related disease that jumped species from some other host into the impatiens family. IDM showed up first in Europe about three years ago, and over across the pond gardeners have cut back significantly on their impatiens plantings due to the widespread destructiveness of the disease.
In the US, the first reports of IPD were from Florida. It then spread north up the coast, west along the Gulf Coast and up to the Midwest. It’s even shown up in California; thirty states reported IDM outbreaks in 2012. Many of our bedding plants come out of Florida, particularly seedling plugs that ship to finishing growers all over the country, which is one possible way the disease was initially and inadvertently spread. Needless to say, professional growers recognized this new disease threat very quickly and jumped to find a rotation of fungicides in their greenhouses that would prevent infection. This rapid response has served to significantly clean up the plants coming out of the big growing outfits, so what arrives at garden centers like Moore & Moore is a good bet to be disease free. But unfortunately it doesn’t mean the plants won’t get a dose once they’re planted in the landscape. I’d say it’s actually likely plants will get infected after planting at this point.
Don’t worry about your other garden plants, though. Although Downy Mildew looks the same on lots of different types of plants, there are actually many different pathogens that produce a disease we lump under the name Downy Mildew, and each one is pretty host-specific. What this means is that Impatiens Downy Mildew only affects impatiens, and you don’t have to worry about it spreading around to other kinds of plants in your garden.
How does it spread?
In the garden, IDM spores are easily spread by splashing water, particularly at night (which is, of course, when irrigation systems are usually timed to kick on). But how did those spores reach the garden in the first place, assuming you put in clean, disease-free plants? Evidently they float in on the wind, folks. It’s unclear how far spores can travel, but they’re incredibly tiny – dust mites, if you will – and a tropical system could theoretically dump billions of IDM spores up the entire East Coast. It’s quite possible one did.
And more bad news: plant pathologists are pretty sure that the spores can also overwinter in the soil of the infected bed, so to avoid almost certain disaster DON’T plant impatiens in a bed where you had mysterious plant decline and death last summer.
What are the symptoms of Impatiens Downy Mildew?
I’ve nabbed some pictures off public domain horticultural websites to show you the progression of the disease (believe me, every hort school in the country, not to mention growers, breeders and chemical companies are all over this topic). As mentioned above, the entire progression takes maybe 3 to 4 weeks in typical summer weather.
The first shot shows a fairly early stage of infestation. You can see leaf yellowing, plus curling, wilting and tip death on newer growth.
Picture 2: If you flip over a yellowed leaf and look at the underside, a blanket of white will be found – these are the fruiting bodies that release the spores (the “downy mildew”).