The most likely cause of damage that looks like this on many plants is spider mites. They are sucking insects and cause this white mottled stippling on foliage when a plant is typically stressed. They are very hard to see, but the underside of infested foliage often looks dusty, sticky, and vaguely web-like and dirty. You can read more about spider mites and how to control them here: http://bit.ly/1pJ46WX You can also go to your local garden center for confirmation of its identity and ask for an organic or other recommended control for spider mites that is appropriate for your plant and then use the product according to directions. Do not use an insecticide as this would not be effective. You will need a product formulated with a miticide if it is mites.