White spots can be due to a couple of things. First, white areas can develop if the plant is getting too much sunlight. They like bright light but not intense sun. Powdery mildew will look like the leaves have been dusted with talcum powder. Not many fungicides may be labeled for indoor use. Potassium bicarbonate sold under various trade names would work. Be sure to read the label. You would need to take the plant outside to spray it and make sure it is completely dry before bringing it inside. Cultural practices are almost as effective. Be sure there is good air circulation around the plant. Remove the affected leaves, keep water off the foliage, keep the plant away from cold air drafts, don't water late in the day,and avoid drought stress.
