A shriveling donkey tail plant is usually a sign of underwatering or overwatering. If the plant is underwatered, the leaves will start to shrivel and become dry. If the plant is overwatered, the leaves will become mushy and may fall off. Other causes of a shriveling plant include pests, diseases, and nutrient deficiencies. It is important to identify the cause of the problem and take appropriate action to save the plant.