What is Powdery Mildew? Your plants will look like someone put baby powder on them. The mildew can be on top and on the bottom of the leaves.
The Wilts... This can happen because of fungi and bacteria. Your plants leaves will turn yellow and wilt. I think this happens because of over-watering.
Blight..Brown spots or patches on your plants leaves...you might even see them shrivel up and rot.
Rust... I get this on my Roses. It makes the leaves look like someone placed rust all over the leaves.