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.
Some Great Summer Annuals
Spring of 2013 has turned out to be a long, cool, moist affair, and the pansies and violas we planted last fall sure love it. But our inevitable hot season isn’t far away, so it’s time to yank ‘em out and put summer color in beds and containers, while the plants are fresh and the pickins’ are good.
With new plants available courtesy of plant breeders, and old standbys like Impatiens walleriana are getting mowed down by disease, what to plant? Here are some of our favorites.
First, about impatiens. There’s a previous blog entry that goes into detail about the issue (which see), but in short, you’ll probably have more success in your shade beds with something else. Try SunPatiens® for somewhat similar showy stands of color, as it doesn’t get the new impatiens disease. SunPatiens plants get bigger and taller than the impatiens you’re used to, even the ‘Compact’ series, so you won’t need as many to fill a space. ‘Compact Deep Rose’ is a vibrant deep orangey-pink that’s a traffic stopper. ‘Kong’ Coleus is another eye catcher. This series grows to be about 20” tall and wide with really big leaves, and looks terrific under trees. ‘Kong Rose’ is my personal favorite – a hot chartreuse border surrounds a field of burgundy, which in turn surrounds a patch of fluorescent hot pink in the center of the leaf.
In shaded containers I’ve been using Rex and angel wing begonias, caladiums, and torenias, particularly the Moon series, which trail fetchingly. In fact, I’ve used Torenia ‘Blue Moon’ as a groundcover in a shady bed with very pleasing results. Rest assured –there are a surprisingly large number of plants besides impatiens that work in the shade.
Torenia ‘Blue Moon’
For sun to part shade, there’s a huge array of choices. Vinca (Catharanthus) has appreciated for years in hot, dry, sunny settings. Now vinca comes in new colors combined with improved disease resistance producing reliable garden plants that won’t rot out suddenly. Look for ‘Cora’ series, bred in both upright and cascading forms, and ‘Titan’ series, with the largest vinca blooms yet. I’m a sucker for the hot fuchsia and soft lavender blue hues now available.
A top favorite annual of mine is Angelonia. What a great plant! Its non-stop spires are terrific in containers as a vertical accent and in the garden as a taller backdrop to lower bedding plants like petunias. Breeders have been working enthusiastically with this genus, and you can now find lavender blue, purple, plum, burgundy, raspberry pink, lavender pink and white, as well as bicolors. Several strains are available. I’ve had fantastic results with the ‘Serena’ (18” tall),‘Angelface’ (24” tall); I’m trying ‘ArchAngel’ this year, too. The ‘AngelMist’ line, another great performer, comes in upright and spreading forms. The spreaders are FANTASTIC container plants. The lavender blue shade goes with everything. A new, shorter version of ‘Serena’ called ‘Serenita’ is available this year, reported to reach 12”-14” in height.
Another relatively new sun lover comes to us from Australia, where it’s used as a perennial groundcover: Scaveola. Available in blue-purple and white (and I once saw a very pale yellow variety), this rugged spreader is tough as nails, performing for us as an annual. Deep green shiny foliage is incessantly accompanied by clusters of distinctive fan-shaped blooms. This plant is happy in a bed or a pot, as long as it gets enough sun.
If you love baby’s breath, you’ll love the annual Euphorbias. Billowing clouds of tiny snow-white blooms are produced all summer, and serve as a gorgeous foil to whatever colorful neighbor you’ve put nearby. The two white varieties I’ve used are ‘Diamond Frost’ and ‘Breathless White’, both wonderful plants. In my experience ‘Diamond Frost’ in particular gets bigger over time than you might think, especially when used a bedding plant, so do give it adequate room. There are faint pink forms available, too, (‘Blush’) with darker, maroon-tinted foliage. These are very pleasing but haven’t bloomed as heavily for me as the white types. All of them are excellent performers in containers.
One plant I love is sold as an annual but is actually perennial: Dianthus x chinensis hybrids (garden pinks). These are reliable performers, blooming from the moment you plant them through the summer, and then beginning in April the next year, on and on. Look for the ‘Telstar’ and ‘Floral Lace’ strains, which come in a variety of classic pinks shades. And another pink that I adore is a taller variety called ‘Bouquet Purple’, bred originally for the cut flower trade but which has proved to be an admirable garden subject. I have one in a container on my patio that’s been blooming away there 5 years. Blooms are a hot fuchsia purple on 24” wiry stems.
Dianthus ‘Floral Lace Purple’
There are tons more types – the ever-expanding ‘Profusion’ zinnia line, Pentas strains like ‘Graffiti’ (short) and ‘Butterfly’ (tall), the Dragon Wing begonias, delicate-looking Dichondra ‘Silver Falls’ to spill over the sides of sunny pots, newer and more compact ornamental sweet potato varieties. There are more and better summer annuals available than ever before.