Inspect the leaves for a white substance known as powdery mildew. As with many plants, powdery mildew can attack curry plants, leading to distorted and discolored foliage. White spots on affected leaves are always present, and yellowish-brown spots may appear at the tips and throughout the entire body of leaves when the disease is severe. Applying sulfur and horticulture oil fungicides before powdery mildew is present helps prevent the disease. Fungicides that contain bacillus subtilis may kill the fungus after it affects the curry plant.
This is likely powdery mildew disease. It starts out looking like grey mold on the leaf and can then spread to cover the entire leaf surface. You can read more about it, along with options for treatment, here: http://www.clemson.edu/extension/hgic/pests/plant_pests/flowers/hgic2049.html If the various treatments described in the link above do not work and If you start having some wilt on the plant too then it might become difficult to save it.