From Sophia: Powdery mildew is a white powder-like fungus that particularly grows on cucurbits such as cucumbers and squash, French tarragon, bee balm, and other plants under adverse conditions. Powdery mildew does not generally kill the plants but will reduce their productivity and vigor. You can help prevent the disease by giving plants plenty of air circulation, only watering in the morning when the plants have time to dry, not planting cucurbits in the same place two years in a row, and destroying insect pests when you find them.
Once you have powdery mildew, you can manage it by removing the worst affected leaves with pruners. DO NOT put the leaves in your compost pile! Either burn them or throw them in the trash. Wash hands and tools with soapy water or dilute bleach solution after handling affected foliage and before touching other plants. Plants can be sprayed with a copper- or sulphur-based organic fungicide, or with a mixture of 1 teaspoon baking soda to 1 quart of water, once per week on the upper and undersides of the leaves until 2 weeks before harvest. This should eliminate the disease.