We don't usually recommend chemical controls for powdery mildew in the home garden. Powdery mildew's prevalence is very season-dependent. It may be bad one year and not be so bad the next. In addition, good control can be achieved through simple cultural practices. Clean up garden debris thoroughly this fall, and don't plant squash or cucumbers in the same place next year. Provide adequate space for the plants. If possible, use trellises or fences to keep the plants elevated so that more air can circulate around them. Don't use overhead irrigation, and don't water at night. Mulch under and around plants, and plant resistant varieties. Finally, powdery mildew usually occurs near the end of the season and attacks plants that are declining anyway. Chemical treatments are usually fungicide sprays, applied at the very first sign of disease and every seven to 10 days thereafter. You may not want to do this in your home garden.