Question
I live in southwest FL & my holly bushes are very green & healthy looking but on the undersides of the leaves it looks like snow or a thick white powdery substance.  What is it and how do I get rid of it?

Answer
What you describe is going to be cottony scale insects. Wipe of the cottony material and you will see little cigar shaped bugs underneath.  These bugs will cause the plant to have yellow leaves that will fall off if not controlled.  Use Bayer Advanced Tree and Shrub Insecticide.  Mix according to label instructions for the size of the plant and pour it on the soil around the base of the plants.  It is taken up systemically and will kill the bugs within a week or so.  It also provides up to a year's protection from reinfestation.  I hope this helps solve the problem.  Thanks.