White Powdery Substance on the Underside of Leaves

I just got back from vacation and noticed that several of my adult plants have small patches of a pure white powdery substance on the underside of some leaves only.  It wipes off with a fingertip.  I'll get a pic up in a bit but...any ideas?
 
Most likely whitefly. Spray with water hose only hard enough to wash off the white, not kill the plant.
Might have to do this for a few days until the whitefly either die or leave.
 
Or you can chemical bomb it for a few days and hope the whitefly either die or leave.
 
Suncoast. Florida?

Is it just the underside? If so, probably whitefly.

Whitefly seems almost innocuous but don't be fooled. It's easy to say " oh, I'll get it tomorrow" but let it go to much and it festers and eventually you'll get a bad infection. It'll look like black soot on the bottom of your stalks.
 
Whitefly is rampant down here right now (so is every other kind of pest, spider mites and aphids) probably due all the debris the city still has yet to pick up from downed trees. As a precaution I have started on a 3 day treatment plan of neem oil. Spray Neem oil, leave it alone the next day, hose down throughly the following day and start over again for 2 weeks. So far no bugs.

Much easier to deal with when you catch it early enough.
 
possible suspects that come to mind for white spots that come off: powdery mildew, white flies, or those fuzzy white mealy bugs
 
Powdery mildew:
Powdery-Mildew-cornell.jpg

 
 
White flies:
maxresdefault.jpg

 
 
Fuzzy white mealy bugs:
IMG_3019.jpg

 
 
 
 
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