Growing the perfect tomato plant is not always easy. Tomato plants are notorious for suffering from a wide range of diseases and pests. Often one of the first indications of a problem with the tomato plant is the yellowing of the plant's leaves. Once the leaves yellow, the plant often begins to display other symptoms. Yellowing leaves may indicate several common problems.
The fungal infections, Fusarium and Verticillium, were once quite common in tomato plants until varieties known as "VFN hybrids" were developed with immunity to the fungal root infections. The fungal infections live within the soil. Tomato plants with no immunity are highly susceptible when planted in infected soil. There is no cure for the infections. The first symptoms is a yellowing of the tomato plants leaves, wilt follows the yellowing leaves and soon the entire plant perishes.
Early blight is an airborne fungal infection. The first symptoms are tiny brown spots on the tomato plant's leaves. The leaf surrounding the spot begins to turn yellow, and then the leaf drops from the plant. The infection can spread to the fruit and stems of the plant. The fungal infection enjoys growing in warm, damp conditions. Control the fungus using a fungicide.
Mosaic virus is known as "tobacco mosaic." The virus lives on tobacco plants and spreads easily when a smoker handles cigarettes and then touches the foliage of the tomato plant. Leaves take on a yellow, mottled appearance. The leaves of the plant begin to curl, dry up and fall from the plant. The color mottling can occur on the fruit also. Smokers should not handle tomato plants without first washing their hands. There is no cure for the tobacco mosaic virus.
Whiteflies cause yellowing of the tomato plant's leaves as they suck the plant's sap with their mouths. The adult whitefly lays its eggs on the underside of the tomato plant's leaves. The larvae emerge and morph through several stages while feeding on the tomato. The flies produce multiple generations on the the tomato plant throughout the summer. The insects' nonstop feeding causes deformed fruit and yellowing foliage. Fungal infections often attack the plant in its weakened state.
The lower leaves of the tomato plant often begin to yellow, which is a sign of a nitrogen deficiency. Supplying the tomato plant with adequate fertilizer helps to prevent the deficiency from occurring.