Based on the symptoms, the wilting could be caused by two main problems: bacterial infection or Fusarium fungus. Since the leaves are not turning yellow, it is more likely to be bacterial wilt. To be more certain, you can cut the stem and place it in a glass of clear water, then observe. You may see white, thread-like strands coming out. Another possibility is damage from shoot and fruit borers, which can also cause wilting. In that case, only the upper part of the plant will wilt. If it is due to insects, you can split the affected branch and you may find tunnels or feeding paths made by the pest. These are quick ways to distinguish whether it is caused by pests or disease. Please check your plants carefully so you can decide what action to take.