Seeing tomato seedlings suddenly turn yellow is known as chlorosis. This means the plant is not producing enough chlorophyll, the pigment responsible for the green color and for capturing light energy. Chlorosis is a symptom, not a specific disease, indicating that something in the plant’s environment or internal chemistry is out of balance. Observing where the yellowing begins and the pattern it follows helps diagnose the problem and restore the plants to health.
Environmental Stressors
The most frequent causes of yellowing relate to environmental conditions that stress the roots or inhibit plant function. Overwatering is often the primary culprit, especially for seedlings grown indoors or in containers. Saturated soil displaces oxygen, suffocating the roots and preventing them from absorbing nutrients and water, which results in a uniform yellowing of the foliage.
Underwatering can also cause yellowing, but this usually follows noticeable wilting and leaf crisping. Temperature is another major factor, as tomato plants are highly sensitive to cold, especially below 50°F. Cold stress inhibits the roots’ metabolic processes, making it difficult for the plant to take up and transport nutrients.
Inadequate light can cause seedlings to become pale or yellow, a process associated with etiolation. Seedlings require intense, direct light for at least 14 to 16 hours a day to develop healthy color. When light is insufficient, the plant conserves energy by reducing chlorophyll production, leading to overall paleness.
Identifying Nutrient Deficiencies by Leaf Location
If environmental factors are ruled out, yellowing often points to a nutrient deficiency, diagnosed by observing which leaves are affected first. Nutrients are classified as mobile or immobile, and their mobility determines where the deficiency symptoms first appear. Mobile nutrients, such as Nitrogen (N), Phosphorus (P), Potassium (K), and Magnesium (Mg), can be relocated by the plant from older leaves to newer, actively growing tissues.
A deficiency in a mobile nutrient causes chlorosis to appear first on the older, lower leaves as the plant sacrifices them to support new growth. Nitrogen deficiency, the most common, presents as a generalized pale yellowing that starts on the lowest leaves and progresses upward. Magnesium deficiency also affects older leaves first, but appears as interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain green.
Immobile nutrients like Iron (Fe), Calcium (Ca), and Sulfur (S) cannot be moved once incorporated into a leaf structure. A deficiency in an immobile nutrient will manifest in the newer, upper leaves first. Iron deficiency is a classic example, causing distinct interveinal chlorosis on the youngest leaves, where the entire leaf may eventually turn very pale while the veins stay green.
The soil’s pH level significantly affects nutrient availability, even if the elements are technically present. Tomato plants prefer a slightly acidic pH range between 6.2 and 6.8. If the soil is too alkaline, elements like iron may become chemically locked up and unavailable for root uptake, mimicking a deficiency.
Biological Threats
Yellowing can also be a symptom of attack by living organisms, including pests and diseases. Pests feed by sucking sap directly from the foliage, resulting in spotty or generalized chlorosis. Common culprits include aphids and spider mites, which congregate on the undersides of leaves. Aphids cause leaves to become distorted and yellow, while mites leave behind fine stippling where they have extracted chlorophyll.
Fungal diseases are also a concern, often starting on the lower leaves, similar to mobile nutrient deficiencies. Early Blight causes dark brown spots with a surrounding yellow halo, beginning on the lowest leaves and moving upward. Fusarium Wilt is a soil-borne fungus that causes yellowing and wilting, often on only one side of the plant, by blocking the flow of water and nutrients. Identifying physical damage, mold, or distinct lesions helps differentiate a biological issue from an environmental one.
Immediate Steps for Recovery and Future Prevention
Correcting identified environmental stressors, especially watering issues, is the primary step in recovery. If the soil is saturated, allow the top inch or two to dry out completely before watering again. If the plant is root-bound or in overly dense soil, repotting into a slightly larger container with a well-aerated potting mix can help restore oxygen to the roots. For cold-stressed seedlings, raising the ambient temperature above 65°F allows root metabolism to recover.
If a nutrient deficiency is suspected, apply a diluted liquid fertilizer cautiously, as young seedlings are easily burned by high salt concentrations. A balanced, water-soluble fertilizer, diluted to about one-quarter strength, is appropriate for initial feeding. If nitrogen deficiency is the likely cause, a fertilizer with a higher nitrogen ratio, such as fish emulsion, can provide a quick boost to the plant’s foliage.
To prevent future problems, ensure seedlings receive adequate air circulation, which helps to strengthen stems and discourage fungal growth. Before transplanting outdoors, the process of hardening off is necessary to acclimate the plants to wind, direct sun, and temperature swings. This gradual exposure over a week or two reduces the shock that can cause post-transplant yellowing and wilting.