Answer: Squash can be affected by blossom end rot, where the blossom end of the squash turns black and rots. It is more often noticed on tomatoes. The cause is lack of calcium in the soil and more commonly uneven soil moisture levels. Incorrect watering can also lead to root damage and can encourage mildew infections -- powdery material causes a white substance on the leaves and can cause defoliation much as you described.
Watering correctly should stop the blossom end rot and could help with the mildew, and if root rot was due to overwatering would also help with that. (Over watering and under watering can both cause drooping or wilting due to root damage.)
Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. There is no set schedule for watering, it depends on your soil type and on the weather. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
You may also want to check the soil pH. If the soil is too acidic, lime would be used to raise the pH; lime would also help supplement the calcium in the soil. YOur local county extension should be able to help you with the soil tests and interpreting the results.
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