Never heard of a scrub bush... could you add a photo to your question please.

We do need to know what plant you mean - scrub bush is a general term, doesn't seem to be a common name for any particular plant. We also need to know if the white covering is dusty or fluffy looking, on top of the bark or leaves, or within the tissues, or dotted about only on the bark or only on the leaves - so in fact, its easier to post a photo.