Powdery mildew causes due to high humidity, crowded plantings or poor air circulation. To prevent from mildew improve air circulation by thining and pruning, don't fertilize and don't water plants from above.

Rotate your crops so you don't grow squash in the same area year after year. That way, any borer moths that do emerge won't have a place to lay eggs. You also can cover squash vines with soil at intervals to encourage secondary roots so plants can survive borer attacks. Plant varieties such as butternut or acorn squash that borer moths don't like.