Potassium deficiency
Potassium plays an important role in encouraging photosynthesis (the process of plants harnessing energy from the sun). Young tomatoes cannot absorb potassium as easily from cold soil as they can from warm soil. That’s why, when planted in cold soil, their leaves may turn purple or yellow for a time. As the soil warms, leaves might turn to their natural green. But plants will likely be stunted for the rest of the growing season, producing few to no flowers and as a result, few to no fruit.
Treatment
To remedy potassium deficiency, feed tomato plants with fertilizer that is high in potassium. Choose fertilizers such as sulphate of potash, tomato feed, or certain organic potassium sources derived from sugar beet processing.
Phosphorous deficiency
Tomato plants may also struggle to absorb phosphorous from cool and wet soil. While tomato leaves deficient in potassium may turn purple or yellow, phosphorus deficiency causes leaves to turn either purple or dark green. Tomato plants deficient in phosphorous may also experience stunted growth, necrotic spots on the leaves, and leaf cupping. All these symptoms are more likely to arise on older leaves. This issue can cause delayed blooming and delayed maturity.
Treatment
Feed phosphorous deficient plants with a fertilizer such as superphosphate or bone meal.
Burnt by light
Depending on the chosen grow light and the variety of tomatoes being grown from seed, leaves might turn red or purple, become pale and sunburned, or start to look curly and crispy. This can be due to seedlings getting close to the light or just getting too much light in general.
Treatment
Adjust the light accordingly and, if needed, use a spray-on fertilizer specialized for seedlings. Follow the instructions on the bottle. Be careful not to spray too much, as too much fertilizer can also cause seedlings to burn.
Viral infections
Some viral infections cause tomato leaves to roll, too. Whiteflies carry tomato yellow leaf curl virus that causes leaves to become cupped or pale green in color. Leaves may also have yellow edges or purplish veins on the underside. As a result, flower and fruit production can become stunted. Curly top virus is transmitted by beet leafhoppers, which migrate from southern areas and pose a greater threat there in hot, dry springs with predominantly southwest winds. Plants that are infected by this virus turn yellow and stop growing. Upper leaflets roll and turn purplish along the veins. Leaves and stems become stiff. Fruit ripens early. Another potential problem is tomato spotted wilt/impatiens necrotic spot tospoviruses, carried by western flower thrips. While these viruses at first only occurred in commercial tomato production, now they have become an issue in some home gardens. Symptoms include dark brown to purple spots on the leaves, dark areas spreading to the stems and forming cankers. Leaf tissue becomes stiff, not limp. Yellow rings or spots on the fruit may appear. Fruit may also look distorted.
Treatment
Use row covers to protect tomato plants. In the event of tomato spotted wilt, remove and destroy the affected plants. The western flower thrips that transmit this disease are almost impossible to control even with soaps, oils, and sulfur dust.
Insect infestation
Tiny insects known as Garden springtail (Bourletiella hortensis) may injure small tomato seedlings. Garden springtails are extremely small, dark purple with yellow spots, wingless but using forked, tail-like appendages to project themselves into the air. They eat small holes in the leaves of small plants near the surface of the soil. It’s usually not necessary to control them or treat tomato plants in the event of their brief visit.
How to Prevent Tomato Leaves from Turning Purple
Planting tomatoes in containers can prevent a lot of pest and disease problems. Avoid planting tomatoes too early when they can be killed by a late spring freeze or struggle to absorb nutrients from cold soil. Potassium can wash away more easily in soils that are light, sandy, or chalky, while heavier clay soils hold potassium naturally.
