Question
My black eyed susans have a white powdery substance on the stems.  I read that a remedy for this is better air circulation which is achieved by thinning out the plants.  Can I do this now (late June in Atlanta) or do I need to wait until the fall?  If I need to wait, what can I do about the powdery substance now?  Thank you.

Answer
Hi Roseanne,
Thanx for your question.  The condition you describe is powdery mildew and it is the result of overcrowding and from water contact in humid climates.  You can thin the plants as you already know and stop watering from above.  Only water at ground level.  Of course this doesn't help you when it rains does it?  

The best time to move perennials is in the fall or early spring.  You can thin them now but this may retard blooming.  It also puts additional stress on the plant that could in rare cases result in the plant's demise.  

The best thing to do now for control of powdery mildew is to avoid watering above ground level.

I hope this helps.
Tom