This is powdery mildew. It is a fungal disease that attacks squash, killing the plants. Some varieties tolerate this disease better than others. Powdery mildew is more of a problem in the fall than in the spring. The material benomyl, or Benlate, is the most effective fungicide for the control of powdery mildew. Mildew occurs most often on old foliage of declining plants. Succession plantings of squash will provide vigorous, productive plants and allow removal of older, more susceptible plants.