The whiteish dusty powdery stuff might be Powdery Mildew but the yellowing of the leaves, chlorosis, probably is from another source.
Sometimes just washing the plants leaves can help eliminate the Powdery Mildew while other times using something like a 50/50 mixture of fat free milk and water or mixing 1 teaspoon of Baking Soda in 1 quart of water and spraying that on the leaves will help.
The chlorosis is another problem and requires a close look at the whys to determine how to correct for it. There are many reasons a plants leaves loose the chlorophyll they need.

Here is a link that might be useful: plant chlorosis