Powdery mildew on squash is often a sign of dry soil and poor air circulation and getting the foliage wet with overhead watering adds to the spread of this fungus among us. Removing the worst of the infected leaves is important to stop the spread but you must leave at least one third of the healthiest foliage for the plants to continue production. Rotate your crops, clean up in the fall and do not handle the plants when foliage is damp to prevent powdery mildew in the future. A copper fungicide spray can be used on some crops 7 to 10 days before harvest but you will need to read the label for each specific fungicide should you decide to spray. This late in the season you should still be able to harvest some squash even if you decide to dig and destroy the plants. Keep growing, Marianne Binetti