Chlorosis is the term for the shift from green to yellow on tomato plant leaves, caused by insufficient chlorophyll production. This discoloration is a symptom, not a cause, indicating an underlying problem interfering with the plant’s physiological processes. Pinpointing the exact reason is necessary because treatment varies significantly depending on the source of the stress. The location and pattern of the yellowing offer clues for diagnosis.
Non-Nutritional Cultural Stressors
Improper watering is the most common non-nutritional cause of leaf yellowing. Overwatering saturates the soil, displacing oxygen needed by root cells to absorb nutrients. This suffocation damages roots, impairing water and mineral uptake, which causes leaves to yellow, often starting with lower foliage. Conversely, severe underwatering stresses the plant, leading to dehydration and wilting that also triggers yellowing.
Extreme or sudden temperature changes can induce chlorosis. Cold stress, such as nighttime temperatures below 55°F, inhibits chlorophyll production and nutrient movement, causing general paling. Intense heat or prolonged high temperatures can shock the plant, resulting in scorched or yellowed foliage, especially on exposed leaves.
The soil’s pH level dictates nutrient availability, even if nutrients are physically present. Tomato plants prefer a slightly acidic pH (6.0 to 6.8). If the soil becomes too alkaline (high pH), micronutrients like iron and manganese become chemically bound and unavailable for absorption, directly causing chlorosis.
Physical impediments, such as soil compaction or damage during cultivation, hinder the roots’ ability to function properly. Compacted soil limits oxygen and restricts root growth, negatively affecting the plant’s capacity to absorb water and nutrients. This physical restriction can mimic nutritional deficiencies, leading to systemic yellowing.
Essential Nutrient Deficiencies
Deficiencies in mobile nutrients cause chlorosis to appear first on older, lower leaves, as the plant moves limited resources to support new growth. Nitrogen deficiency causes uniform yellowing across the entire leaf, starting at the base. Since nitrogen is a primary component of chlorophyll, its depletion results in fading green color as the plant prioritizes the upper canopy.
Magnesium deficiency, another mobile nutrient issue, presents as interveinal chlorosis on older leaves. The tissue between the leaf veins turns yellow while the veins remain distinctly green. This “green vein” pattern is a classic diagnostic marker for magnesium shortage, necessary for the chlorophyll molecule’s structure.
Potassium deficiency is identified by yellowing or a scorched, brown appearance along the edges and tips of older leaves, known as marginal chlorosis. Potassium is crucial for water regulation and enzyme activation, and its lack impairs the plant’s vigor, starting with older tissues before progressing inward.
Deficiencies of immobile nutrients manifest on the newest leaves first because the plant cannot relocate them from older tissue. Iron deficiency is a common example, causing interveinal chlorosis on the youngest, uppermost leaves. These leaves appear pale yellow with fine green veins, differentiating this pattern from magnesium deficiency.
Pests and Common Pathogens
Biological threats, including pests and diseases, induce chlorosis through tissue damage or internal obstruction. Fungal and bacterial wilts (Fusarium and Verticillium) are soil-borne pathogens that invade the plant’s vascular system (xylem). The infection restricts water and nutrient flow to the foliage, leading to wilting and sectorial yellowing, often affecting one side or branch.
Fungal leaf spot diseases, like Early Blight (Alternaria solani), cause yellowing following the appearance of distinct lesions. Early Blight starts on lower leaves with dark, target-like spots surrounded by a yellow halo. As spots grow, the entire leaf turns yellow and drops prematurely, spreading the infection upward.
Sucking pests directly cause yellowing by draining chlorophyll-containing cells from the leaves. Spider mites, whiteflies, and aphids feed on the underside of the foliage, resulting in stippling—tiny pale dots—that coalesce into widespread chlorosis. This damage reduces the leaf’s photosynthetic capacity.
Viral infections often result in a mottled or mosaic pattern of yellow and green on the leaves. Viruses like Tomato Infectious Chlorosis Virus (TICV) can cause distinct interveinal yellowing and a brittle texture on older leaves. Viral diseases are typically incurable and require the removal of the infected plant to prevent transmission.
Systematic Diagnosis and Recovery
Systematic observation helps narrow the cause, starting with the location. If yellowing is uniform and starts on the oldest, lowest leaves, a mobile nutrient deficiency (likely nitrogen) is the primary suspect. If yellowing is limited to the newest leaves, an immobile nutrient issue (such as iron deficiency) is more probable. Examining the pattern is the next step: interveinal chlorosis on older leaves suggests magnesium, while a scorched edge points toward potassium. If the yellowing includes spots, lesions, or wilting, a pathogen or pest is the likely culprit.
Correction should follow a hierarchy, starting with easily adjustable cultural factors. Check soil moisture and perform a soil pH test before amending the soil with targeted fertilizers. Only after ruling out water, pH, and major nutrient issues should the focus shift to specific pest or disease management, such as applying fungicides or insecticidal soaps. Establishing a consistent watering routine, ensuring adequate airflow, and mulching are practical preventative measures.