@EddieF: I've just called Bonide, and the person who answered the call said this product was not recommended for mango trees. He said if a fruit tree is not listed on the label, don't apply the product to it.
Since this product can be used as dust or spray, straining the stuff will make sure the copper is properly mixed in the water, and yes to prevent the white residue.
For powdery mildew and anthracnose prevention for mango trees, the Bonide person recommended Captain Jack's Neem oil.