Tomato leaves curl because of transplant shock, disease, too much nitrogen, or extreme amounts of moisture or drought. Curling leaves are evident on vine tomatoes more often than on bush tomatoes.
Lower leaves tend to curl upward, while leaves further along the stem tend to curl inward toward the vein when shock, disease or climate conditions are at fault, according to the Clemson University Cooperative Extension. In addition to curling leaves, the tomato mosaic virus causes a blotchy color on the leaves and brown spots on the inside of the fruit. The yellow leaf curl virus stunts tomato plants and causes purple veins on the leaves.