Answer: The white film on the leaves is a fungal disease called powdery mildew. Cucumber and squash plants are susceptible to powdery mildew, generally towards the end of the season when the nights are cold enough to stress the plants. However, the development of the disease can also develop in areas with humidity and poor air circulation around the plants, than cool night air. You might try removing the older leaves, especially when they begin to show signs of distress. Give your plants as much direct sunshine as possible and increase the space between the plants to provide excellent air circulation. There's not much you can do to save your plants once the disease takes hold - it's much easier to prevent than to cure. Wish I had more encouraging news for you!
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