If they are white, fuzzy spots appearing on top of the leaves, but not damaging all the way through the leaf (to the underside) then it is PM (powdery mildew). A very common issue for many growers.
There are several things that can be done.
There are the foliar applications such as the one mentioned by ronny. You can also mix 3% diluted H2O2 (hydrogen peroxide from any drug store) in a 50/50 peroxide/water solution in a spray bottle to make quick work of the visible signs if it is minimal or you have few plants.
I use a sulfur burner for preventative and maintenance. This changes the pH level of the leaf surfaces and makes them inhospitable to PM.
Then there are the environmental factors ronny also mentioned, which will help preventatively as well for future grows.
Best of luck!