Ask Mr. Smarty Plants

One of the most common problems of Malvaviscus arboreus var. drummondii (Turk's cap or turkscap) in Texas is powdery mildew.  This fungus disease often shows up on leaves near the top of the plant, causing the leaves to be somewhat shriveled and spotted with areas of powdery white fungus.  The white areas are often more extensive on the lower side of Turk's cap leaves.  It is not too serious a disease, but the affected leaves should be removed and the plant sprayed with fungicide.  The Safer company makes a sulfur-containing, biodegradable, combination fungicide/insecticide that might be appropriate for the situation, although in my experience fungicides seem to slow down but not eliminate the disease completely.

Powdery mildew does look somewhat like POWDERED sugar but not granulated sugar or salt.  So we should consider that your problem may be of another sort.  If the white material looks something like cotton you may be having an infestation of mealy bugs.  These individual bugs have a cottony appearance, and you can find some easy control methods in this eHow site How to Control Mealybugs.  The Safer product mentioned above should be effective for mealy bugs as well as any other insect pest.

If your problem is powdery mildew, it can spread by spores wafted through the air and settling on leaves in a moist or humid setting.  It would be good if you can position your Turk's cap plants where they can get some early morning sun or good air circulation to dry off dew or humid air as quickly as possible each day.  This would lessen the chances for continued serious outbreaks of the disease.