The sudden appearance of yellowing leaves on a tomato plant, technically known as chlorosis, is the most common visual sign of distress. Chlorosis results from the plant’s inability to produce sufficient chlorophyll, the pigment necessary for photosynthesis. Because yellowing can stem from numerous causes, ranging from soil chemistry to infectious pathogens, accurate diagnosis requires careful observation of the yellowing pattern and location. Determining the precise location and appearance of the discoloration is the first step toward resolving the problem.
Yellowing Due to Nutrient Imbalances
The location where chlorosis first appears on a tomato plant provides a significant clue about which nutrient is lacking. Nutrients are categorized by their mobility within the plant, which dictates whether symptoms appear on older, lower leaves or newer, upper leaves. Mobile nutrients, such as nitrogen and magnesium, are readily moved by the plant to support new growth when supplies are low, causing the old growth to yellow first.
Nitrogen deficiency typically causes older, lower leaves to turn uniformly pale yellow or greenish-yellow. Since nitrogen is a primary component of proteins and chlorophyll, a lack of it causes a generalized paling that spreads upward as the plant cannibalizes older tissues to sustain new growth. Applying a fast-acting liquid feed, like fish emulsion, can quickly deliver nitrogen to the plant.
Magnesium deficiency also affects older leaves first, but the yellowing pattern is distinctly interveinal; the tissue between the leaf veins turns yellow while the veins remain green. Magnesium is a central atom in the chlorophyll molecule, and its depletion leads to this characteristic “green skeleton” pattern on the lower foliage. This deficiency is often resolved by a foliar spray of Epsom salts (magnesium sulfate).
Iron is an immobile nutrient, meaning the plant cannot relocate it once incorporated into tissues. Consequently, iron deficiency first manifests as bright yellowing on the newest terminal growth. Similar to magnesium deficiency, the leaf veins often remain dark green, creating an interveinal chlorosis pattern specifically on the youngest leaves.
Soil pH plays a powerful role in nutrient availability, often causing deficiencies even when the nutrient is present in the soil. Tomatoes prefer a slightly acidic soil pH, typically between 6.0 and 6.8, to absorb nutrients efficiently. If the soil becomes too alkaline, a high pH can chemically bind elements like iron, rendering them insoluble and unavailable for root uptake, a condition known as nutrient lockout.
Yellowing Due to Water and Environmental Stress
Improper watering is perhaps the most frequent cause of chlorosis in tomato plants, relating directly to root health. Overwatering saturates the soil, displacing the oxygen roots need to respire and absorb nutrients. This lack of oxygen leads to root damage or root rot, preventing nutrient uptake and resulting in general yellowing and wilting of the entire plant.
Conversely, underwatering can also cause yellowing, usually starting with the lower leaves that turn yellow and eventually drop off. This condition is often preceded by visible wilting, as the lack of moisture prevents the plant from maintaining turgor pressure and transporting nutrients. The plant sheds older leaves to reduce surface area and conserve its limited water supply.
Temperature extremes can trigger temporary yellowing, particularly on the most exposed foliage. Cold snaps can temporarily disrupt nutrient uptake, causing new growth to appear pale or yellow, sometimes accompanied by a purplish tint from phosphorus issues. Excessive heat can stress the plant, causing rapid transpiration that outpaces the roots’ ability to supply water, leading to heat-related stress and chlorosis.
Inadequate light exposure causes the plant to produce insufficient chlorophyll, resulting in pale, weak, or yellowing growth. When tomato plants are crowded or grown in shaded conditions, they become elongated as they stretch to find light, a condition called etiolation. The resulting pale appearance is a form of chlorosis, indicating the plant lacks the energy to maintain dense, dark green foliage.
Yellowing Due to Pests and Diseases
Yellowing confined to specific spots or areas often indicates a biological agent, such as a fungus, bacterium, or pest. Fungal diseases, like Early Blight, start by causing yellow splotches on lower leaves that develop into dark brown spots with a distinct yellow halo. Fusarium Wilt, another fungal pathogen, often causes yellowing and wilting on only one side of a plant or a single branch, a key diagnostic feature of vascular diseases.
Bacterial infections and wilts also cause yellowing accompanied by rapid decline or specific lesions. If the yellowing is accompanied by a mottled, mosaic, or streaked pattern, it may signal a viral disease, such as Tomato Chlorosis Virus. These systemic infections typically cause general yellowing and curling of leaves, often mimicking a nutrient deficiency but without the clear pattern of mobile or immobile nutrient issues.
Certain sap-sucking pests cause chlorosis by physically damaging the leaf tissue and extracting the chlorophyll-rich contents. Spider mites and aphids feed on the underside of leaves, causing tiny yellow or white spots called stippling to appear on the upper surface. This localized feeding damage results in yellow patches that can cause the entire leaf to turn yellow and drop if the infestation is severe.
The diagnostic process requires a methodical approach, beginning with observing the pattern of yellowing—is it uniform or interveinal, old growth or new growth? Next, assessing the growing environment, including soil moisture and temperature, helps rule out cultural problems. Finally, inspecting for spots, lesions, or visible insects helps determine if the cause is a disease or a pest.