This is powdery mildew, a fungal disease that attacks squash and kills the plants. Some varieties tolerate this disease better than others. Powdery mildew is more problematic in the fall than in spring. Use a benomyl containing fungicide for the most effective control of this disease. Your county agent can recommend the most current control method for this disease.
Mildew occurs most often on old foliage of declining plants.
Succession planting of squash will provide vigorous, productive plants and allow removal of older, more susceptible plants. It is difficult to control squash bugs especially when the insects have reached maturity. For satisfactory control, apply insecticides early in the season while the insects are small. Removing and destroying egg masses on the bottom of leaves aids in control. A. Summer squash varieties will cross with one another, with acorn squash and with some pumpkins.