Is the reddish dust on the underside of the leaves? If so it could be spider mites. Otherwise some pictures would be helpful to figure out what's going on. The way I treat spider mites is to blast the undersides of the leaves with the hose to wash off as many as possible, then I repeat that every couple days for a while until I'm sure they're under control. You can also use an insecticide, but it's critical to hit the undersides of the leaves since that's where they live.