Yellowing on the leaves of a tomato plant, a condition known as chlorosis, is a frequent concern for home gardeners. This color change signals that the plant is struggling to produce enough chlorophyll, the green pigment necessary for photosynthesis. Because yellowing is a symptom rather than a specific diagnosis, its location on the plant and the pattern it forms on the leaf are the most important clues for identifying the root problem. The cause can range from simple nutrient imbalances in the soil to more complex issues like disease or environmental stress.
Yellowing Caused by Cultural and Nutritional Imbalances
Problems originating in the soil often manifest as distinct yellowing patterns based on how a plant moves nutrients. Nutrients classified as “mobile,” such as nitrogen and magnesium, can be relocated by the plant from older leaves to support new growth. This means a deficiency in a mobile nutrient will first appear on the older, lower foliage.
A nitrogen deficiency, the most common cause of general yellowing, causes the entire lower leaf to turn pale green or yellow. Magnesium is also a mobile nutrient, but its deficiency results in interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins remain dark green.
Iron, conversely, is considered an “immobile” nutrient, meaning the plant cannot move it from old tissues to new ones. An iron deficiency therefore causes chlorosis to appear first on the newest leaves at the top of the plant, with the tissue between the veins turning pale while the veins stay green. Addressing these issues usually involves soil testing to confirm the pH and nutrient levels. Correcting a nitrogen deficiency is done with a high-nitrogen fertilizer, while iron or magnesium deficiencies may require specific supplements or adjusting the soil pH to ensure the existing nutrients become more available for uptake.
Yellowing Due to Water Stress and Temperature Fluctuation
Yellowing can also be a direct result of improper water management, which stresses the root system. Overwatering is a common culprit, as saturated soil displaces oxygen. When roots cannot breathe, they become damaged and lose their ability to absorb necessary nutrients and water, leading to a generalized yellowing of the foliage.
Conversely, underwatering causes stress, initially leading to wilting before the leaves turn yellow and develop dry, crispy edges. This can be corrected by adjusting the irrigation schedule to provide deep, consistent watering.
A third environmental factor is temperature fluctuation, where sudden cold snaps below 50°F or prolonged heat waves above 90°F can shock the plant. This temperature stress can cause rapid, non-pathogenic yellowing or sun-scald that looks white or yellow-white, often affecting the exposed portions of the plant. Solutions for water stress involve ensuring the soil has good drainage and maintaining a consistent moisture level. Using mulch can help regulate soil temperature and prevent rapid moisture loss during heat waves.
When Yellow Leaves Signal Disease or Pests
When yellowing is accompanied by spots, lesions, or wilting, it is often a sign of a biological threat requiring urgent action. Fungal diseases are a primary concern, with Early Blight typically starting on the lower, older leaves. This condition creates dark brown spots with characteristic concentric rings, often surrounded by a yellow halo on the leaf tissue.
Late Blight presents as irregularly shaped, water-soaked lesions that turn brown or black, often with a white, downy growth on the underside in humid conditions. Unlike nutrient deficiencies, these blights are infectious and spread rapidly, necessitating the removal of affected foliage and the application of an appropriate fungicide. Vascular wilts, such as Fusarium and Verticillium wilt, are soil-borne diseases that cause one-sided yellowing of a leaf or wilting on only one side of the plant.
Pests also contribute to yellowing by feeding on the plant’s sap. Sap-sucking insects like Spider Mites and Whiteflies pierce the leaf tissue to feed, which results in a distinct stippled, speckled, or mottled yellowing pattern rather than uniform chlorosis. Whiteflies often concentrate on the undersides of leaves, causing the upper surface to turn yellow. Managing these pests often requires targeted treatments like insecticidal soaps or horticultural oils, alongside pruning of heavily infested areas.
Diagnostic Guide: How to Determine the Exact Cause
Determining the cause of yellow leaves involves a systematic approach using visual clues and environmental context. The initial step is to inspect the location of the damage, noting whether the yellowing begins on the older, lower leaves or the newer growth at the top.
The second step is to examine the pattern of the yellowing, looking for uniformity versus specific markings. A leaf that is uniformly pale yellow suggests a general deficiency like nitrogen or root suffocation from overwatering. Conversely, interveinal yellowing or the presence of dark spots and lesions signals a specific nutrient lock-out, disease, or pest infestation.
Finally, assess the immediate environment by checking the soil moisture and drainage. If the soil is soggy, overwatering is likely stressing the roots, whereas dry, compact soil indicates underwatering. Once the cause is confirmed, any severely affected foliage should be promptly removed and discarded away from the garden to prevent the spread of potential diseases.