Choose grape plants that are bred to be resistant to powdery mildew. Allow adequate space between plants for ventilation, because mildew thrives in cramped quarters. Water plants in the morning so that they have the remainder of the day to dry off. If a plant is infected, prune off infected branches or remove the entire plant and destroy it to prevent further spread of the disease.
There are many natural ways to destroy and prevent powdery mildew. Sulfur is especially effective at preventing and destroying powdery mildew. Use a spray made of crushed garlic cloves and water once a week to mist the leaves and stems of the grape plants. Baking soda makes the surface of the leaves unsuitable for powdery mildew growth, and can also be made into a spray when combined with water.
There are some organic treatments available on the market, if you don't wish to make your own concoction. For those who do not require organic treatments, look for liquid fungicides that specifically treat powdery mildew. You can visit your local garden center or shop online through gardening websites, or by doing a search for "powdery mildew fungicide".