Pests. The female squash vine borer, a moth, lays her eggs in the stem of squash vines. Once the eggs hatch and develop into larvae, the larvae begin feeding on the plant, shutting down its vascular system and causing wilt (figure A). Water. Wilt can be caused by either too much or too little water. Always check the soil's moisture before you water a wilted plant.
Excessive heat. In the middle of summer, one of the most common causes of wilt is excessive heat, as gardeners in heat-prone areas know. While the condition may be unsightly, it's actually a good sign, because it means a plant's defense mechanisms are in good working order.
Throughout the morning hours, James' blackeyed peas look great. The leaves are open wide, capturing all the light they can by exposing as much surface area as possible to the sun's rays. By late afternoon, the leaves begin to wilt, and the plant looks as if it is sick. What's happening here is rather complex, botanically speaking, but basically the plant is wilting because it's losing water vapor through its exposed leaves faster than its roots can take water in. As a means of slowing the loss of water vapor, it tucks its leaves down to shield itself from the sun--or in other words, it wilts.
Your first reaction to this situation might be to water, but unless the soil is really dry, that won't help. Instead, just give the sun time to move westward, and the plant will perk up again. The wilting process can be stressful on plants, but it's the only survival mechanism they have.
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