Yellow leaves on a tomato plant may indicate a serious problem. Although under watering the plant and a lack of fertilizer may be the issue, more serious issues often lie at the heart of yellow leaves. Choosing disease-resistant cultivars, as well as practicing good cultivation, may prevent some tomato plant disease.
Phosphorous deficiency is a lack of the macronutrient phosphorous in the soil. Phosphorous is abundant in soil, says Colorado State University, but may not be available to a tomato plant when the soil is too cool. A deficiency will cause yellowing of the lower leaves, as well as wilting. Avoid planting tomato plants too early in the season and cover the soil with mulch to keep it warm. Tomato plants side dressed with a phosphorous heavy fertilizer will not exhibit this issue.
Septoria Leaf Spot
Septoria leaf spot is the most common foliar disease of the tomato plant, says Iowa State University Extension. Septoria is caused by the fungus Septoria lycopersici. Appearing first as small, water-soaked spots that become 1/8 inch in diameter, leaves that are heavily infected become yellow, wither and die.
Early blight is another plant disease caused by fungus, but is from the Alternaria solani spore. The lower leaves of the tomato plant often fall off before brown to black spots appear, merging together to form larger blotches. Leaves eventually turn yellow and drop off. Early blight also causes black, concentric rings to form at the stem of the fruit.
Fusarium wilt, caused by the fungi Fusarium oxysporum f. sp. lycopersici, attacks only certain tomato plant cultivars. According to the Alabama Cooperative Extension, the virus initially causes a yellowing and falling off of the lower leaves of the plant. The yellowing and wilting spread up the plant as the fungus moves through the vascular system. Eventually, the yellow leaves fall off and the plant will die.
Verticillium wilt attacks over 200 different plant species, including tomato. Yellow blotches appear first on the lower leaves, progressing upward through the plant. Verticillium wilt, unlike fusarium, appears on both sides of the leaves.