When tomato leaves turn yellow, the condition is known as chlorosis. Chlorosis is a visual symptom, not a disease, signaling that the plant is struggling to produce chlorophyll, the pigment necessary for photosynthesis. This disruption in energy creation can be traced back to various underlying issues. Diagnosing the precise location and pattern of the yellowing is the first step in understanding and correcting the problem.
Yellowing Caused by Nutrient Deficiencies
The location of the yellowing often points directly to a specific nutrient deficiency, determined by whether the nutrient is mobile or immobile within the plant’s system. Mobile nutrients can be relocated from older leaves to newer, actively growing parts of the plant when soil levels are low. As a result, a lack of mobile nutrients first appears as chlorosis on the older, lower leaves.
Nitrogen (N) deficiency is the most common cause of yellowing that begins on the oldest, lowest foliage. Because nitrogen is highly mobile, the plant pulls it from mature leaves to support new growth, causing older leaves to turn pale green and then completely yellow. A magnesium (Mg) deficiency also begins on older leaves, but it presents a distinct pattern called interveinal chlorosis, where the tissue between the veins yellows while the veins themselves remain green.
Conversely, immobile nutrients cannot be moved once they are incorporated into the plant’s structure. Therefore, a deficiency in these nutrients will first manifest on the newest growth at the top of the plant. Iron (Fe) deficiency is a frequent culprit, displaying as interveinal chlorosis on the young, upper leaves, where the leaf tissue is pale yellow but the fine veins remain noticeably dark green. Manganese (Mn) deficiency also affects new growth, presenting a similar interveinal pattern to iron, but it is often characterized by a mottled or speckled appearance between the veins.
Water and Soil Management Issues
Improper watering practices and unbalanced soil chemistry are frequent causes of chlorosis, often by preventing the roots from absorbing nutrients even when they are present in the soil. Overwatering leads to saturated soil conditions that deprive the roots of oxygen. Oxygen deprivation impairs the root system’s ability to take up water and nutrients, resulting in pale, yellow leaves, often starting with lower foliage.
Underwatering, or drought stress, also causes yellowing, but it is usually preceded by the leaves wilting and becoming dry or crispy at the edges. Consistent watering is necessary to prevent stress that can lead to discoloration.
The soil’s pH level plays a major role in nutrient availability, acting as a chemical “lock” or “key” for certain elements. Tomato plants prefer a slightly acidic soil pH, ideally between 6.2 and 6.8. If the soil becomes too alkaline (high pH), nutrients like iron and manganese can become chemically bound to the soil particles, making them unavailable for the plant to absorb. This phenomenon, known as nutrient lockout, directly causes the yellowing symptoms of deficiencies, even without an actual shortage in the soil.
Identification of Pest and Disease-Related Yellowing
Biological agents, including fungi, bacteria, and pests, can induce yellowing through infection or physical damage. These symptoms are often accompanied by other distinct visual cues that aid in diagnosis.
Soil-borne fungal diseases, such as Fusarium or Verticillium wilt, cause yellowing and wilting that often affects only one side of the plant or a single branch. This one-sided presentation is a strong diagnostic indicator of a vascular disease that is clogging the plant’s water-conducting tissues.
Viral infections, like Tomato Mosaic Virus or Tomato Yellow Leaf Curl Virus, typically produce highly irregular yellowing patterns. This can appear as a blotchy mosaic of light and dark green, distinct mottling, or yellowing along the leaf margins, often combined with stunted growth and leaf curling. There is no cure for viral diseases, making plant removal the only viable control method.
Sap-sucking insects cause yellowing through direct physical damage. Pests such as spider mites and whiteflies puncture the leaf cells to feed, resulting in tiny, localized yellow or white speckles, known as stippling, on the leaves. These pests are often found congregated on the undersides of the leaves, and the damage usually starts on the lower parts of the plant before progressing upward.
Environmental Stressors Affecting Leaf Color
External, non-biological factors can cause temporary or permanent yellowing unrelated to deficiencies or pathogens. Temperature extremes are a common stressor; sudden cold snaps or chilly overnight temperatures can cause temporary yellowing or purplish discoloration. Intense heat above 85°F can also induce stress, manifesting as paleness or sunscald, particularly if plants are suddenly exposed to intense sunlight.
A lack of adequate sunlight, necessary for photosynthesis, causes leaves to turn yellow or pale green. Heavily shaded leaves cannot produce enough chlorophyll and will eventually yellow and drop. Ensuring that tomato plants receive at least six hours of direct sunlight is important to prevent this form of chlorosis.
Transplant shock is a temporary environmental stress that occurs immediately after moving a seedling into the garden. Root disturbance temporarily interrupts the plant’s ability to take up water and nutrients, leading to brief yellowing on the oldest leaves while the plant establishes new roots. This yellowing typically resolves itself once the plant has successfully adapted to its new environment.