It can sometimes be hard to pin these things down to one specific cause. So lets just assume there are multiple problems and arrange some treatment.
White Mildew: Powdery mildew can be treated with regular applications of a fungicide such as neem oil, sulfur or triforine. Purchase a product and follow the treatment regiments on the product's labeling.
Leaf Curl: There are four major reasons for leaf curl in orange trees: pests, disease, water stress and weather. Sometimes it’s a combination of all four. Get a hold of some copper fungal spray and follow the treatment regiments on the product's labeling.
This is hard to say. It depends on: your climate, your tree's age, soil type and tree type. In general you should follow the regiment suggested to you by the person who sold you the tree and be observant, that being said:
Signs of Underwatering Trees:
Wilted or curling leaves that turn brown at the tips or edge. A sparse canopy of off-colored and undersized leaves. The leaves may be yellowing; tends to look like its in a transition period from summer to fall when its underwatered.
Signs of Overwatering Trees: (This could be you)
The ground around the base of the tree is constantly wet and new growth tends to wither. Leaves tend to seem fragile and will peal from the stock easily. As I said this could be you (mildew problems) slighting decrease your watering regime.