You can use a herbicide that contains dicamba, but only do so as a last resort and then use caution to avoid damaging desired plants. The undersides of my Confederate rose plants are covered with a white layer of fungus. It eventually kills the leaves. Your plant may have a fungal problem, but look closely to determine whether the white pest is not mealybugs, sap-feeding insects that form white, powdery colonies on leaf undersides and stems. Scale, a mealybug relative, could be the problem, too.
Scale can look more like a disease than an insect.
There are several scale species in gray, yellow, white or reddish brown. All weaken or kill a plant by sucking sap. Young scales, which can resemble tiny mealybugs, move about, but when mature they attach themselves to a stem or leaf and then don't move. While an individual scale is small, the insects will cluster on leaves, stems and trunks. You can remove scale by carefully scrapping it off with your thumbnail or with a plastic scouring pad.