The plants die rapidly. 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 of a problem in the fall than in the spring. The most effective fungicide control is benomyl or Benlate.
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. Q. My fruit blooms and sets young fruit, yet quickly becomes covered with a black, whiskery fungal growth. This is Chaonephora fruit rot, a soilborne disease which rots the young fruit and is particularly damaging during extended wet periods. It can be controlled with a combination of treatments which reduce relative humidity using raised beds and open foliage. This allows air movement to dry the soil and the foliage.