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It's the Old Fungus Among Us

6/17/2013 10:59:00 AM

Tags: impations, powdery mildew, Mary Moss-sprague

impatiensSome of you may know me from my articles on food preservation. Well, right now, I’m so busy growing the stuff slated for preservation that I’m switching gears over to gardening topics, at least for a while.

Speaking of gardening, hurrah! The growing season is upon us, and like me, you’re probably hopeful about your garden plants being healthy and prolific. Will it ever warm up? Will the rain slow down? Some areas have certainly suffered more than their share of wind, moisture, low temperatures, and other impediments to happy plants.

The worst thing is, there’s more than just bad weather to contend with. We can’t ignore some nasty pathogens already lurking in the air and soil, and some of them are aided and abetted by consistently adverse weather conditions. Folks who’re trying to get their color-spot annuals in gear are finding a nasty surprise awaits them, in the form of plant disease.

A perfect example is a strain of downy mildew, a fungus-like pathogen bearing the scientific tongue twister, plasmopara obducens. (Nothing good can come from a name like that!) This season, it is manifesting itself in Impatiens walleriana, the most beloved of this popular bedding plant species. Growers and gardeners are now having to rethink their flower beds, planters, and hanging basket color spots in which garden impatiens usually enjoy a prominent place. This downy mildew is also showing up in the double-flowered mini, and Fusion and Butterfly impatiens.

This isn’t just some nuisance fluke, it’s a serious issue in nurseries nation-wide. The usual garden Impatiens plants are a huge nursery item, with yearly wholesale values numbering upwards of $10 million dollars as of 2003 and probably a lot more now.

What does this creeping crud look like? Well, it starts with a bit of leaf yellowing, followed by leaves curling or flagging downward. This may give the plant the appearance of just needing watering, but be aware and don’t be fooled. From there, the fungus continues to spread, and a white coating will appear, especially under leaves that have yellowed. Soon, the disease will become quite evident. Leaves will eventually drop off and the stems will even collapse. Not a pretty picture!

If you think that applying a good fungicide labeled for downy mildew management will curb the problem, you can try it, but  usually such a preparation must be reapplied every seven days and may still not be effective in protecting the plants.

Maddeningly, also, this pathogen lives in the soil and will insidiously infect new plants set in there. It will also overwinter in all its nasty glory, which means that garden impatiens shouldn’t be planted in flowerbeds or containers where the disease has previously been found. (This is similar to the way Verticillium wilt infects soil for years and destroys members of the nightshade family, including tomatoes.) Moist or humid conditions are perfect for the doggone stuff, too, offering it an open-ended invitation to move in.

Impatiens plants can become infected from other sources, including spores spread from nearby infected plants. This occurs when water is splashed (short distances) or spread by wind in longer stretches.

Before you become too discouraged, take a deep breath: fortunately, the also-popular-but-lesser-known New Guinea Impatiens is not affected, and I. balsamina is only slightly susceptible with damage usually limited to yellow leaf spots. Also lucky for us, this pathogen won’t spread to sunflowers or roses, since it is a fairly host-specific disease. So, you won’t have to completely write off the idea of using Impatiens, just rethink which type you’ll plant instead.

What can be done if you find your favorite plants succumbing to the downy mildew and can’t find the New Guinea impatiens? Think about alternative plants, especially if you live in a high-risk (moist) area. Good choices might include begonias, coleus, torenia, or others. You can do a computer search for more shade-lovers, too.

On this encouraging note, I will say good-bye, but will return very soon with more discussions of plant pathogens, how to recognize them, and, best of all, how to work around them. Until then, work your fingers in that soil, and enjoy the Spring!

Photo by Fotolia.

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