Tomatoes are the single most popular crop for home gardeners in the United States. From the dedicated gardener who grows bushels of tomatoes for canning, to the casual grower with a cherry tomato in a patio pot, millions of Americans enjoy this garden favourite. Unfortunately, tomatoes can be a delicate and difficult plant to grow. Many different problems can cause whitening or yellowing of the leaves. Most of these problems are identifiable, and most are preventable or treatable.
The most common reason for whitening of tomato leaves is sunburn. This may seem strange for a plant which grows best under the strong summer sun. However, most tomato plants are started from seed indoors or in a greenhouse, which decreases their exposure to direct sunlight. Tomato seedlings require a "hardening off" period. The plants are placed outside during the day for brief but gradually increasing periods for a week or two, acclimatising them to the increased sun intensity, stronger winds and temperature variations. Plants which are not given this "hardening off" period may become sunburned if planted directly in the garden. Sunburned plants may exhibit leaves which turn white, partially or entirely, including the veins. Some leaves may recover, but in extreme cases, leaves may die, and the plant may not survive.
Tomato plants require three main nutrients, like any green plant. Nitrogen, potassium and phosphorous provide the plant's basic needs. Plants suffering from nitrogen or phosphorous deficiency may exhibit whitening or yellowing of the leaves. Applying any all-purpose fertiliser for vegetables will help this problem.
Plants also require smaller amounts of other nutrients and minerals, such as manganese, iron, calcium and magnesium. In severe cases, deficiencies of calcium or magnesium can cause whitening or yellowing of leaves. These deficiencies are characterised by leaf veins which retain their green colour. A weak solution of 2 tablespoons of epsom salt to a gallon of water per plant will help a magnesium deficiency. Applying a small amount of common garden lime will help a calcium deficiency.
Certain diseases such as Alternaria, or early blight as it is sometimes called, can cause yellowing or whitening of leaves or portions of leaves. Septoria leaf spot can cause whitish or greyish spots on leaves. Treatments for these and other diseases can be found at your local garden centre.
Certain pests such as psyllids, whiteflies and aphids can cause leaf whitening or yellowing in cases of heavy infestation. Many controls exist for these and other pests. Consult your local garden centre for an appropriate treatment.