Tomatoes are among some of the most popular plants to grow in a home garden. Not only is the fruit highly versatile in cooking, but there are also a ton of varieties — at least 10,000, in fact — from which to choose. Some offer both purple fruit and purplish leaves, though these are arguably not as commonplace. Unless you have a specific variety of tomato plants, such as ‘Indigo Cherry Drops’ or ‘Indigo Rose’, it is not normal for your tomatoes to sport purple foliage. In most cases, the purple color is a cause for concern because it indicates either a phosphorus deficiency or a viral infection.

If the undersides of your tomato plant leaves look purple, you’re probably dealing with a phosphorus deficiency. In such cases, the plant stems can turn purple as well. These signs are most common in young or recently transplanted tomatoes, as their roots are still not absorbing enough phosphorus. In older, more established plants, the undersides of the leaves may look more reddish-purple instead. Severe phosphorus problems can eventually make the entire tomato plant turn purple. 

You can prevent phosphorus deficiency and keep the leaves from turning purple by not transplanting the tomatoes into the garden too early in the season. That’s because the plants cannot absorb phosphorus effectively when the growing medium is still cold. If you’ve planted them early, the purple color should eventually disappear as the soil warms. Otherwise, checking the health of your soil and adding fertilizer as needed can help remedy the problem.

Certain viral infections, such as the curly top virus, can also make tomato leaves turn purple. This disease is transmitted between plants by beet leafhoppers. As the name indicates, tomato plants that are infected with the curly top virus have upper leaflets that curl back(this illness is one of the several reasons your tomato plant may have curling leaves). You might also notice purplish coloring along the veins of the foliage. Other signs of curly top virus include stunted growth, or the entire plant turning yellow.

Curly top virus is difficult to control in tomatoes, but not impossible. The best way to prevent curly top virus is to protect the plants with row covers, preventing beet leafhopper access. You can also try growing tomatoes closer together, as beet leafhoppers are more likely to attack plants that have more space between them. It’s also important to know that the curly top virus can affect other common garden plants, such as cucurbits and peppers. Additionally, being aware of the most common pests that attack tomatoes in the garden can help you quickly spot and react to other signs of trouble.