Why Are My Tomato Leaves Turning Yellow?

Uniform yellowing low on the plant: think nitrogen or water

Start at the bottom of the plant. When the oldest, lowest leaves turn evenly yellow, including the areas between the veins, without spots or lesions, nitrogen deficiency is a leading suspect. Nitrogen is a mobile nutrient, so a short supply gets pulled from old leaves into new growth, meaning deficiency reliably starts low and works upward. A few yellow bottom leaves late in the season can also just be normal aging as the plant pours energy into fruit.

Watering problems mimic this. Both drought stress and soggy, poorly drained soil can yellow the lower leaves. Before adding fertilizer on a guess, check your watering and consider a soil test, which is the only way to confirm whether nitrogen or another nutrient is actually short rather than throwing product at the symptom.

Yellowing around spots: think disease

The diagnostic difference is spots. If the yellow tissue surrounds defined lesions, a fungal leaf disease is more likely than a nutrient problem. Early blight produces a few (often 5 to 10) circular brown spots up to about half an inch across with concentric rings forming a target or bull's-eye, ringed by a yellow halo, and it hits lower leaves first. Septoria leaf spot produces many smaller spots, roughly 1/16 to 1/8 inch, with gray or tan centers and dark brown margins, sometimes with tiny black pimple-like structures inside, again starting low.

Both spread when leaves stay wet, so overhead watering and crowding make them worse. Because early nutrient yellowing and early disease can look alike, do not assume: if you see spots with halos or dark margins, or the problem is climbing the plant, have it confirmed by your Extension office before choosing a treatment.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• University of Maryland Extension - Key to Common Problems of Tomatoes — Uniform yellowing of older lower leaves without spots points to nutrient (nitrogen) or watering issues, while distinct spotting on lower leaves indicates fungal diseases such as early blight and septoria; symptom location and pattern guide the diagnosis.
• Penn State Extension - Tomato Diseases and Disorders in the Home Garden — Early blight causes concentric-ringed target-like brown spots with yellowing surrounding tissue on lower leaves, and septoria causes numerous small spots with gray centers and dark margins, distinguishing disease from nutrient-related yellowing.
• West Virginia University Extension - Septoria Leaf Spot — Septoria leaf spot appears first on lower leaves as small circular spots with gray/tan centers and dark margins containing black fruiting bodies, and surrounding leaf tissue yellows; it spreads with wet foliage.

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