Underside of Tomato Seedling Leaves Are Purple

Phosphorus Deficiency

• Phosphorus is the second-most important nutrient for plant growth. When phosphorus is deficient, tomato plants are weak, stunted and have deep purple coloring on the stems and underside of their leaves. Phosphorus deficiency can affect tomatoes at any stage. Most fine- and medium-textured soils in temperate regions have adequate quantities of phosphorus but sandy soil or soil in the tropics is likely to be deficient in phosphorus. Tomatoes grown in these soils may have purple leaves, and phosphate fertilizer should be provided.

Cool Soil

• When tomatoes are planted in cool soil, they may show symptoms of phosphorus deficiency even though the soil has adequate amounts of available phosphorus. The roots of tomatoes are unable to absorb phosphorus at cool temperatures. This is the most common cause of purple leaves in seedlings. As the soil warms up, these symptoms will disappear and the tomatoes will begin to grow normally. The same conditions can occur in a cool greenhouse. Slightly warmer night temperatures will eliminate the symptoms.

Curly Top Virus

• The upper leaves of a tomato plant with curly top virus will turn purple, especially along the veins. Plants turn yellow and stop growing. The leaves and stems will become stiff and the fruit will be very small and ripen prematurely. Young seedlings may die. The disease is most common in arid and semi-arid regions, and is spread by beet leaf hoppers. There are no chemical controls but it can be prevented if row covers are used before beet leaf hoppers appear. Diseased plants should be removed.

Other Problems

• Tomato spotted wilt virus causes dark brown to purple spots on the leaves and yellow rings on the fruit of tomato plants. As the disease progresses, the plant wilts. It is spread by thrips. In 2006 a new problem called tomato purple leaf disorder appeared in Florida. Research has not found a cause, although it appears it is an infectious disease possibly caused by a virus. Both of these diseases are more likely to be found in mature plants than in seedlings.