Question
Help!  We have a backyard area (approx. 1ft wide x 25 ft long) on either side of the pool that is mulched.  In the front yard the red mulch is also covering our landscaping areas consisiting of palm trees and a yellow & green bush.  
I am sick of WEEDS and now I am seeing this white powdery fungi looking stuff that is gross.  It seems to have taken the entire mulching area over.  It is under the top layer of mulch.  It destroyed my petunias.

Answer
Hey, Lisa.

That sounds like powdery mildew but without a picture I can't be certain. It also sounds like you are using large mulch because that's always when I have a problem with powdery mildew. The area being too wet is another problem since petunias like it relatively dry.

You will probably have better success if you cut down on the watering and switch to a smaller mulch. I use mulch that is one quarter to one half inch. The small mulch allows for greater air circulation, thereby keeping the mulch drier.

If you have at lot of rain in your area, like many areas of Florida do, it might be better to switch to plants that can take the rain and put your petunias in a pot so that the rainwater will drain from them.