The appearance of yellow leaves on a tomato plant is a clear sign of chlorosis, the botanical term for the loss of green coloration. Chlorosis occurs when the plant produces insufficient chlorophyll, the pigment essential for photosynthesis. When this process is inhibited, the plant cannot manufacture enough carbohydrates, signaling distress that requires immediate diagnosis and action. This guide focuses on identifying the specific cause of yellowing, from physical stressors to microscopic invaders, to help restore plant health.
Water and Root Stress
Improper soil moisture is one of the most frequent causes of yellowing leaves because it directly impacts the root system’s ability to function. Overwatering is especially damaging as it saturates the soil, displacing oxygen and causing the roots to suffocate and rot. When roots are compromised, they cannot absorb necessary nutrients and water, leading to a breakdown in chlorophyll production and the development of soft, yellow leaves.
Conversely, severe underwatering causes general plant stress, which also triggers leaf yellowing and eventually leads to dry, crispy leaf edges and wilting. To determine the cause, insert a finger two to three inches deep into the soil. If the soil is soggy, the plant is likely overwatered and needs improved drainage; if it is bone-dry, a deep, slow watering is required. Consistent, deep watering that allows the top layer of soil to dry slightly between applications is the best long-term strategy for preventing water stress.
Diagnosing Specific Nutrient Deficiencies
Yellowing patterns on the plant can often pinpoint a specific nutrient deficiency, as different nutrients have varying levels of mobility within the plant structure. Mobile nutrients, such as nitrogen and magnesium, are relocated to new growth when supplies are low, causing deficiency symptoms to appear first on older, lower leaves.
Nitrogen (N) deficiency, the most common nutritional issue, presents as a uniform yellowing of the entire leaf, starting on the oldest leaves and progressing upward. Since nitrogen is a structural component of chlorophyll, it is withdrawn from older tissues to support new leaf development. Corrective action involves applying a quick-release, nitrogen-rich fertilizer, such as a liquid fish emulsion or urea.
Magnesium (Mg) deficiency is characterized by interveinal chlorosis on older leaves: the tissue between the veins turns yellow while the veins remain green. Magnesium is a central atom in the chlorophyll molecule, and its lack creates this signature pattern as it is shuttled to new growth. This can be corrected by applying Epsom salts (magnesium sulfate) as a foliar spray (one tablespoon per gallon of water) to the undersides of the leaves.
Potassium (K) deficiency typically appears as yellowing or browning along the margins of the older leaves, often described as “scorching.” Potassium is essential for water and nutrient regulation, and its deficiency is made worse in sandy soils or where high levels of nitrogen are applied. Corrective action involves using a fertilizer high in potassium or applying a foliar spray of potassium sulfate.
In contrast, Iron (Fe) deficiency appears first on the newest, uppermost leaves because iron is immobile once incorporated into plant tissues. The young leaves show interveinal chlorosis, similar to magnesium, but the location is the key diagnostic difference. Correcting this involves applying a chelated iron product, especially if the soil pH is high, which prevents iron absorption.
Identifying Pests and Pathogens
Yellowing can also be a symptom of damage caused by living organisms, requiring pest control or disease management strategies. Certain fungal and bacterial wilt diseases cause yellowing that starts at the bottom of the plant and moves upward, often accompanied by wilting. Fusarium wilt, for example, is a soil-borne fungus that causes lower leaves to turn yellow and wilt, sometimes on only one side of the plant.
Viral diseases, such as Tomato Spotted Wilt Virus (TSWV), are spread by insects like thrips and cause irregular symptoms, including mottled yellowing, bronzing, or distinct spots on the foliage. Since there is no cure for viral infections, infected plants must be removed and destroyed immediately to prevent the virus from spreading. Managing the insect vector is the primary method of prevention.
Sap-sucking pests like spider mites and aphids also cause yellowing by physically damaging the leaf tissue and extracting chlorophyll. Spider mites create tiny yellow or white spots, known as stippling, and fine webbing, typically on the undersides of leaves. Aphids cause general yellowing and distortion on new growth, often leaving behind a sticky residue, and both can be managed with insecticidal soap or by introducing natural predators.
Environmental and Soil pH Stress
External, non-biological factors, particularly temperature and soil chemistry, can cause yellowing by interfering with the plant’s normal functions. Tomato plants prefer a slightly acidic soil pH, ideally between 6.2 and 6.8, because this range maximizes the availability of most nutrients. When the soil pH is too high (alkaline), micronutrients like iron can become chemically “locked up” and unavailable for absorption by the roots, inducing an iron deficiency.
Temperature extremes can also induce temporary chlorosis or permanent damage. A sudden cold snap can cause cold shock, resulting in yellowing, while intense sun during extreme heat can lead to sunscald, where foliage turns pale or yellow. Adjusting soil pH requires a soil test to confirm alkalinity before applying elemental sulfur to lower the pH, allowing the plant to absorb necessary micronutrients.