White spots on roses are caused by fungal disease and insect activity. The white spots are usually visible on the surface of flower, leaves or stem, but sometimes they are not so apparent. That is why it is important to look on the underside of leaves. Sometimes, insects hide at the base of the flower. Gardeners should discard diseased or damaged plant material and maintain good air circulation, according to "American Horticultural Society Practical Guides: Roses" by Linden Hawthorne (see Resources).
White spots on roses are often caused by powdery mildew, a fungus disease that affects the surface of leaves. The white mildew is spread by the wind and develops during humid periods, according to the University of Illinois Extension website. Mildew is reduced with water, sanitation, fungicide spray and pruning.
Some forms of aphids take on a waxy cast that make them appear as white spots on roses. Integrated Pest Management at the University of California says aphids suck the sap from plants and are usually harmless. If too many aphids are present on the rose plants, they can cause the leaves to turn yellow and curl, so check plants often.
Other insects called scale can cause white spots on roses. Scale looks like oyster shell and appear white as eggs and nymphs. Pruning of damaged branches is recommended by Plant Answers.
The Leaf Hopper is a grey, green or yellow insect that measures 1/4 to 1/2 inch in length. Some forms of the bug feed on the underside of leaves that causes a white stippling pattern, according to the Texas Agriculture Extension Service.
White spots can appear on roses when the plants receive too much sunlight or heat. The Master Gardeners of The Southwest View News describe the problem as sun scorch, a form of sun bleaching that can kill leaves. Climbing roses are also susceptible to sun scorch when excess heat radiates off support walls and hits the back of plants.