Why does my mountain ash have white spots and dropping leaves?

The condition you describe is most likely powdery mildew, which is caused by fungus. A wet winter provides ideal conditions for the development of the disease. It can also be caused or promoted by a lack of sun on the plant or insufficient circulation (air) through the crown. Check with your local nursery for natural treatments to suppress the disease and/or have the tree pruned to open up the canopy for better air circulation. You can also prune back other plants to give the ash more light.