Contact fungicides remain on the outside of the plant and form a protective barrier against diseases that penetrate the tissue. Contacts will not have any effect on diseases that enter through the root system. Systemic fungicides enter inside the plant and travel throughout the plant's vascular system. They are both protective and curative before and after the disease has started. Any disease that enters from any point on the plant will be subject to the systemic fungicide, this includes any new plant growth. Both systemic and contact fungicides are often alternated to get the benefits of both types of fungicides.
Most likely, the plant has powdery mildew. Contact your local lawn & garden center, nursery or cooperative extension for diagnoses. Spray with an appropriate fungicide that lists on the label the disease and the type of plant. Check out the following product labels for recommendations; Copper Fungicide, Liquid Infuse, Neem Oil, All Seasons Oil or Sulfur Plant Fungicide.
Please consider using products that are already formulated for both insect and disease control like All Seasons Oil, Bon-Neem, Citrus Fruit & Nut Orchard Spray, Dragoon Dust, Fruit Tree & Plant Guard, Fruit Tree Spray, Garden Dust, Neem Oil, Tomato & Vegetable 3 in 1, Rose Rx 3 in 1, Rose Shield or Rose Rx Systemic Drench, instead of mixing.
Contact your local lawn & garden center, nursery or cooperative extension for help in properly diagnosing the type of blight you have. Next, select a fungicide that lists that type of blight with the plant variety you are applying the product to.