Environmental stress from bouts of cool rainy weather, herbicide damage, severe pruning, sucking insects and viruses tend to cause tomato leaves to curl. Yo-yo watering and severe pruning may cause a tomato plant temporarily to develop thick, leathery leaves that curl upwards. In this case, the plant shows no effects on flowering, fruiting or growth. The plant should re-adjust within the week. <br /><br /><b>Herbicide damage</b>, particularly from <b>2,4-D</b>, may cause tomato leaves to curl first downwards and then upwards. In this case, the plant may show such effects as cat-faced fruit, stunted growth and twisted leaves.