Expert: Long Island Gardener - 7/7/2007

Question
For several years, I have planted impatients in a window box type area (it is made of stone and surrounds the house). The impatients will die at the soil line.  When I pick up the wilted stalks the base is dry and brown, totally sealed off from the root.  Almost like they have been cauterized.   I replaced the soil several years ago, but it did not help.  I have placed slug bait around the plants thinking that slugs were eating them did not help and did not kill any slugs. I do not see anything on the stalks or inside the stalks that resemble critters, mites or anything.  Please help.

Answer
My first thought is that the temperature in that stone container is getting painfully high.  Pick up or borrow a max/min thermometer and see what the max temperature is there in the daytime.  Impatiens thrive in shade but they need civilized temperatures, too.  You did not describe the sun situation -- or where you are writing from.  I'm just wondering if this might be a possibility.

Your description of the 'cauterized' stems only reinforces my first thought. When the thermometer hits 95 degrees, Impatiens generally stop blooming and begin to fail.  Consider this first.

There is one other possibility.

A disease called 'Black Leg' caused by a Fungus called Pythium appears as darkened stem tissue near the base, down to the soil.  Infected stems 'wilt' and cannot transport water.  The plant wilts, the stalk collapses, and the plant usually dies.

There are a few easy ways to control this disease, if this is what your Impatiens have.  One is to be careful not to plant the Impatiens too deep.  Pythium starts in the roots, but does not always reach the numbers that would post a threat to your plant(s).  Another is to avoid over-watering, a mistake that is easy to make because Impatiens need damp soil and moisture to look their best.  Your stone containers must have drainage.  Do they?  Not just to keep the roots provided with Oxygen, but also to prevent Fertilizer buildup -- a condition which supports Pythium damage.

Pythium is always bad.  In high humidity, masses of fungal mycelium appear.  At high temperatures -- 80-95 degrees F -- it is devastating.
 
So with Pythium, the worst is yet to come.

There is another diseases, Bacterial Stem Rot, but the symptoms are slightly different from the ones you see on your Impatiens.  Impatiens with Stem Rot collapse with the main stem becoming mushy.  My money's on Heat Damage or Pythium.

If you want to go about wiping out the Pythium Fungi -- which I should tell you is one of the most common Fungi around -- I have to warn you NOT to go out and buy any old Fungicide.  There are only certain Fungicides that will work.  But there is one that was developed by Cornell University for Pythium on Grass.  The rights to make it are held by a company called BioShield:

http://www.bioworksinc.com/wheretobuy/hg.html

They sell it as 'RootShield' and it is intended for Grass treatments.  But you can try it on your Impatiens.  The species may be different, but it is possible that the Pythium will still succumb to the RootShield.

Alternately, you can give the old Cornmeal Killing Powder a try.  Pick up a cardboard container of Cornmeal -- the kind you make muffins from (NO BAKING SODA OR OTHER INGREDIENTS!) and sprinkle it on the soil, then plant some new Impatiens.  See what it does for you.

It's my bedtime so your answer will be blessedly short -- I will have to settle for dreaming about Pythium Fungus instead of going into all the good stuff there is to know about it.  That's for another day.  See what you think and let me know.  Any questions, I'll be here.  Thanks for writing.  And goodnight!