Question
Hi David, and thanks for the help.  I have 6 peony bushes in my garden along
a fence.  They are watered every other day by our sprinkler system.  They
have recently acquired a dusting of mildew, and I've done some research on it
and still don't know what is the best way to treat the problem.  I've read about
"Messenger", a harpin protein compound that is supposed to clear up the
mildew and make the plant stronger and healthier. I've read about "Serenade",
an organic fungicide for mildew control. Some say to cut the plant back,
almost to the ground, and it will be fine next year, although it may still
develop mildew again next spring.  May I ask what you would do?   Thanks so
much for your help.  I am a novice, but the peonies were doing so well land
I'd hate to loose them.  Marilyn

Answer
Sounds like you would like to treat these organically.  Powdery mildew will not totally kill the plants but will make them look awful and not flower.  I would try the Cornell fungicide that you make at home.  Mix one tablespoon baking soda and one tablespoon of light cooking oil in a gallon of water and shake to mix thoroughly.  Then spray the plant to runoff.  The solution has contact activity and will kill the mildew on the surface of the leaves.  You will have to spray every 4-5 days for as long as you want to keep the plants going.  At the end of the season, cut the old growth off and wait until next spring for new growth.  You may want to treat the new growth next spring with the mixture as a preventative.  Good luck.