Acorn squash leaves are susceptible to some beetles and slugs, especially in the early growing periods. You also may see:
Squash vine borer
Striped cucumber beetles
Bacterial wilt (Erwinia tracheiphila)
Powdery mildew - Avoid wetting foliage if possible. Water early in the day so aboveground plant parts will dry as quickly as possible. Avoid crowding plants. Space apart and eliminate weeds around plants and garden area to improve air circulation. Use Actinovate to deal with the problem.