Common reasons why tomato leaves turn purple include phosphorus and/or potassium deficiency and curly top virus.
Phosphorus/Potassium Deficiency in Tomato Plants
When tomatoes are deficient in phosphorus or potassium, the veins of their leaves turn purple and there may be a purplish tint throughout the leaves as well. These deficiencies are most likely to occur when tomatoes are planted too early in the season because even if there are plenty of nutrients in the soil, the plants can’t access phosphorus and potassium when the soil is too cold.
To prevent deficiencies, avoid planting tomatoes too early in the season. If the problem has already occurred, place plastic mulch over the soil to warm it. As temperatures rise over time, this problem will usually go away on its own and the leaves will turn green again. However, some tomato plants may be stunted throughout the growing season and produce little fruit if they suffer from a deficiency early on (my cherry tomato plants grew well and produced abundantly after recovering from a nutrient deficiency, but they were robust hybrid plants, which tend to be tougher than heirlooms).
Curly Top Virus in Tomato Plants
Another common cause of purple leaves on tomato plants is curly top virus. It should be easy to tell the difference between curly top virus and phosphorus or potassium deficiency because leaves roll upward with curly top virus, whereas they maintain a normal shape when the problem is a nutrient deficiency.
Curly top virus is spread by the beet leafhopper. Infected plants eventually turn yellow and their growth stops. The problem is more likely to occur when spring is hot and dry and there are southwest winds. There are no good chemical controls for the problem, so the only way to deal with it is prevention. Protect tomato plants with row covers to guard against leafhoppers.
Note: If your tomato plant leaves are curling but not turning purple or yellow and the plant is still growing well otherwise, it may just be a temporary condition resulting from weather stress or inconsistent watering.
- Missouri Botanical Garden, William T. Kemper Center for Home Gardening. (2012). “A Visual Guide: Tomato Foliage, Stem & Root Problems.” PDF Available Online.
- Newman, S., & Pottorff, L. (2013). “Recognizing Tomato Problems.” Colorado State University Extension.