Tomatoes are highly demanding and notoriously sensitive plants. One of the signs of a distressed tomato plant is yellowing tomato leaves.
There are many reasons why the leaves are yellow on tomato plants, including nutritional deficiencies, environmental factors, diseases, and pests.
Below, we’ll go over the most common potential causes for the yellowing of leaves on tomato plants, the symptoms, and the treatments.
Nutritional Deficiencies and Environmental Factors
Nutritional deficiencies, compounded by environmental factors, are the likely causes of yellowing leaves on tomato plants.
Tomatoes are heavy feeders and need a host of nutrients and minerals to produce ripe fruit and healthy leaves.
Environmental factors (e.g., sandy soils, low or high pHs, drought, heat, overwatering, etc.) can also affect nutrient uptake, further compounding these deficiencies.
Below are the common nutritional and environmental factors that cause yellowing tomato leaves:
Nitrogen or Sulfur Deficiencies
Nitrogen (N) is essential for making amino acids, enzymes, and chlorophyll. A shortage of nitrogen is the reason why many tomato leaves turn yellow.
- Symptoms: On tomatoes, a lack of nitrogen results in small, pale green to yellowish leaves, usually on older, lower leaves. Yellowing is typically uniform, affecting the entire leaf, including veins.
Sulfur deficiencies (rarer) have similar symptoms to nitrogen deficiencies, except chlorosis (or yellowing) will start with the upper, younger leaves and work its way down. The undersides of leaves, veins, and stems may also show a reddish color.
- Treatment: If a soil test shows low nitrogen, you can boost nitrogen by adjusting the NPK ratio on fertilizers to a higher nitrogen mix or adding organic matter such as blood meal or urea, which both provide nitrogen.
Apply gypsum, magnesium sulfate, or other sulfur-rich fertilizers for sulfur deficiencies.
Potassium (K) is essential for many plant processes, especially in the mid to late season, as plants develop fruits. A lack of potassium in tomatoes will affect yields and result in yellow leaves.
- Symptoms: Lower, older leaves will start to yellow at the leaf edges before progressing up the plant. Leaves furthest from the main stem will usually show yellowing first. Brown edges may also appear as leaves die. Yellowing shoulders on tomato fruit is another common sign of K deficiency.
- Treatment: Treat K deficiencies with an application of potash (only K), potassium chloride (K and other salts), or kelp (not as concentrated, but organic). The exact amount and mix will depend on the results of a soil test.
Sandier soils with high pH, poorly drained soils, and soils that are too cold for planting often cause iron deficiencies. Affected plants will show poor growth, small flowers, and less fruit.
- Symptoms: Younger, uppermost leaves will turn a pale green or slightly yellow color. This yellowing usually occurs between the leaf veins and at the base of new leaves before spreading to the whole shoot tips.
- Treatment: Spraying leaves with liquid fertilizers such as ferrous sulfate helps correct the deficiency. Amending the soil with blood meal and iron powder may help as well. Excessive phosphorus or calcium can block iron uptake, so use other fertilizers with caution.
Symptoms usually appear mid-season on plants with heavy fruit loads. Mg deficiency usually doesn’t affect fruit quality but can cause reduced yields.
Excessive application of ammonium nitrate, phosphates, and potassium fertilizers can contribute to magnesium deficiencies.
- Symptoms:Mg-deficient leaves will curl, and yellow areas will appear between leaf veins resembling a mottling pattern. It usually affects older, lower leaves. Leaves may also die and turn brown at the edges.
- Treatment: To treat, add two tablespoons of magnesium sulfate (Epsom salt) to water in a spray bottle and target the affected areas. Leaves should green up in a few days.
Tomatoes require daily watering during the first week of planting but should be weaned down to about 1-1.5 inches (2.5-4 cm) of water a week.
Overwatered tomatoes have difficulty absorbing oxygen and nutrients, and their leaves will turn yellow.
- Symptoms: Overwatering causes soggy soils, drowns out roots, and prevents nutrient uptake – all of which lead to yellowing leaves. Roots may also darken or appear rotten. Fruits may also crack, and plants wilt in extreme cases of overwatering. White blisters (edema) may also appear on the leaves.
- Treatment: Reduce watering to once a week. Keep soils moist by drip irrigation or hand watering at the soil level. Avoid getting leaves wet, as this invites diseases.
Water early in the day to ensure your tomatoes absorb the water in the morning. Earlier watering also allows wet leaves to dry in the afternoon sun.
Mulching helps keep soils evenly moist (without the need to overwater). It also prevents splashback, which carries diseases from the ground onto leaves. A raised garden bed also helps with drainage.
- Underwatering can also cause tomato plants to wilt and older leaves to turn yellow. If underwatered, leaves will start to yellow at the edges before browning and dying. Make sure your soils are evenly moist. Tomatoes have deep roots, so water deeply 1-2 times a week.
- Symptoms: These key nutrients are essential to plants, and a deficiency in N, P, K, S, Fe, and Mg will result in yellowing leaves. Other symptoms will develop depending on the specific nutrient deficiency.
- Treatment: Make sure you do a soil pH test to determine your soil’s pH before amending.
- Symptoms: Compacted soils will cause water to pool or puddle. Water will also run off the soil in high areas. Affected plants appear stunted. Compacted soils will also have bare areas where even weeds cannot grow. It will be hard to shovel through compacted soil.
- Treatment: Aerate the soil by loosening it with your hands or shovel (avoid root damage). Prevention, however, is the best cure. Make sure you give your soil lots of organic matter when preparing, and consider growing tomatoes in raised beds to avoid soil compaction.
- Symptoms: In extreme heat, leaves can become sun-scorched and yellow, starting first from the edges before turning brown. Sunscalds also occur on fruits, causing them to develop a brown or yellow spot on the sun-exposed sides.
- Treatment: To prevent this, use a shade cloth in high heat or space tomatoes closer between rows to provide natural shading.
- Symptoms: Exposure to herbicides may cause curling or twisting leaves, wilting, deformed fruit, stunted growth, and yellowing or browning leaves.
- Treatment: Avoid using manure or common weed killers anywhere near your tomatoes. Check the labels on commercial mulches for signs of herbicides used.
- Symptoms: Small water-soaked, black, and scabby spots will develop on fruit surfaces. On leaves, the spots may have a yellow halo. As the disease progresses, it will cause leaves to defoliate, turn brown, and curl up.
- Treatment: A copper fungicide is relatively good at controlling bacterial leaf spot. Improve air flow, water from the bottom, mulch to prevent splash, and rotate crops to avoid.
- Symptoms: Large brown circular lesions will start small and develop on older leaves, fruits, and stems. Spots develop into larger concentric circles that look like a bull’s eye. Surrounding leaf areas will turn yellow.
- Treatment: Resistant cultivars exist. To prevent, practice crop rotation, weed around tomatoes, mulch, water from below, and prune any infected branches and leaves. Fungicides such as mancozeb and copper fungicides work well in fighting early blight. 
- Symptoms: Symptoms first appear in the lower leaves after fruits have set. Small circular leaf spots will develop and become light gray or tan with dark borders. A yellow halo may also surround spots. Leaves may also turn slightly yellow, then brown, before withering.
- Treatment: Prune diseased leaves. Applying fungicide sprays will prevent new leaves from being infected.
Low or High Soil pH
Tomatoes do best in slightly acidic soil pH between 6.0 – 6.5. pH. Soil pH affects the solubility and, thus, availability of critical nutrients and minerals.
Highly acidic soils result in lower nitrogen, phosphorus, potassium, sulfur, magnesium, and calcium – all essential plant nutrients.
Highly alkaline soils will cause phosphates to interact with calcium and magnesium, making all three less available. 
If soils are too acidic, apply some form of lime (calcium carbonate), such as limestone or wood ashes, to raise the pH.
If soils are too alkaline, lower pH by adding organic matter, aluminum sulfate, or sulfur. Aluminum sulfate dissolves immediately in the soil but may be highly toxic to plants in high amounts.
Soil compaction prevents oxygen, water, and other nutrients from reaching the roots, which causes leaves to yellow.
Compaction is caused by using poor soil (e.g., clay or sandy soils) without much organic matter. Too many people passing by your garden can also cause soil compaction.
Sunburn and Sunscald
Tomato plants need lots of sunlight to thrive, up to 6-8 hours per day. However, too much sun will cause leaves and fruits to develop sunscalds.
Do not prune the scorched leaves, as they can still provide shade and photosynthesis. Wait for new leaves to emerge or for the weather to cool off to prune sun-scorched leaves.
Herbicides will usually affect the more sensitive newer growth of tomato leaves first.
Even if you don’t use herbicides, they can still be found in manure, carried in the wind, or used in commercial mulches such as grass or hay.
Besides nutritional and environmental issues, diseases are another major cause of yellowing tomato plants.
Bacterial Leaf Spot
Bacterial leaf spot appears during wet rainy seasons, and several species of bacteria are responsible for the disease.
Infected plants have reduced yields, defoliated leaves, and unpleasant-looking fruit.
Soil fungi cause early blight, a disease that also plagues potatoes.
Early blight usually develops in moderate to warm temperatures and high relative humidities.
Septoria Leaf Spot
Septoria leaf spot is caused by a fungus and occurs in areas where wet, humid weather has persisted for an extended period.
Improve air circulation around the plant to encourage faster drying and mulch around the base of the plant to prevent splashing. Avoid watering from the top and getting leaves wet.
Rotate different crops next year where your tomatoes were to prevent future outbreaks.
A soil-borne fungus causes Fusarium wilt. The disease causes tomatoes to wilt and develop yellow, then brown leaves. Fusarium wilt occurs during the summer when air and soil temperatures are high.
- Symptoms: Symptoms first appear when fruits begin to mature. The lower, older leaves will turn yellow first, then brown, usually only on one side of the plant (a signature of this disease).
Yellowing will then spread up the plant. Plant growth is also stunted, and few to no fruits will develop. Infected stems will have browned tissue when split in half.
- Treatment: There is no cure, so destroying affected plants is best. To prevent Fusarium wilt, select resistant varieties, rotate crops every 3-5 years, and use sterile materials when potting and transplanting.
Use lime to raise pH (if too acidic), as more alkaline soils suppress the fungi. Avoid using too much nitrogen, as excess foliage can invite diseases such as Fusarium wilt.
Another soil-borne fungal disease, Verticillium wilt, develops during cooler periods of late spring.
- Symptoms: Similar symptoms to Fusarium wilt will develop. Older leaves are infected first, turning yellow, wilting, and eventually browning and dropping. Verticillium wilt affects the lower leaves uniformly (not one-sided in the case of Fusarium).
- Treatment: See Fusarium wilt treatment.
Powdery mildew is another fungal disease that thrives in high-nitrogen environments and stunts tomato growth and yields. This disease also thrives in high-humidity environments and wet leaves.
- Symptoms: Light green or yellow blotches develop on affected leaves, followed by white mildew spores on the upper leaf surfaces.
- Treatment: Sulfur dust and sprays are excellent and effective preventative treatments. Apply in the morning or evening to avoid burning plants due to the reaction of sulfur in direct sunlight.
Horticultural oils such as neem oil or fungicides are also effective at treating powdery mildew on plants before it gets too severe. Remove or destroy infected plants.
Tobacco/Tomato Mosaic Virus
Hundreds of tomato viruses may affect the appearance and size of ripening tomato fruits. Some viruses will also cause tomatoes to not turn red.
Tomato mosaic virus and tobacco mosaic virus are the two common tomato viruses. They cause stunted growth, leaf distortions, reduced yields, and marbling on fruit.
- Symptoms: Leaves will appear mottled with yellow, white, or light or dark green spots and streaks. The patterns will look like blisters.
Infected plants appear stunted, and infected leaves will turn crinkly or wavy. Fruits may also be distorted, with yellow blotches and dead spots appearing on the skin.
- Treatment: There is no chemical control, so remove infected plants immediately. When it comes to viruses, prevention is essential. Plant disease-resistant cultivars and control weeds to keep the virus at bay. Aphids and leafhoppers are virus spreaders.
Tomato Yellow Leaf Curl Virus
Spread by the Silverleaf whiteflies, this disease is quite severe, destroying fruit production in plants affected at an early age.
- Symptoms: Leaves in infected plants will curl upward or downward in a cup pattern. Leaf edges will also turn yellow, usually in the newest leaves. The presence of both leaf cupping and yellow edges in combination will help you diagnose this disease.
- Treatment: Destroy or burn infected plants. Do not compost. Avoid planting tomatoes with cucumber, squash, okra, eggplant, or peanuts, as whiteflies also love these plants. If you notice whiteflies or their larvae, spray with insecticidal soap or neem oil. 
Tomato Pests that cause Yellowing of Leaves
These tiny insects have yellow bodies and white wings. They feed on the underside of tomato leaves, sucking out sap and weakening the plants.
Whiteflies also carry and transmit the tomato yellow leaf curl virus.
- Symptoms: Infected plant leaves will wilt and turn pale or yellow. Plants will also appear stunted. A heavy infestation will cause the curling and crumpling of leaves.
- Treatment: Insecticidal soap, hot pepper spray, or horticultural oils such as neem oil work well in warding off whiteflies. Insecticides like permethrin or cyfluthrin work on whiteflies but are a bit harsher. Insect yellow sticky traps also help catch whiteflies.
Spider mites look like tiny red, brown, or black, moving dots to the naked eye.
They feed on tomato leaves, especially the undersides near leaf veins. Infestations usually occur during hot and dry conditions.
- Symptoms: An infestation of spider mites will cause numerous yellow or white tiny granulated spots on tomato leaves to appear. Yellow blotching may also turn brown and dry off later. In severe cases, webbing may be visible.
- Treatment: You can spray the undersides of leaves with a garden hose to get rid of spider mites. You can also wipe leaves with mild dish soap and water. Horticultural oils such as rosemary or neem oil also help prevent spider mites. Miticides or insecticides are also effective in removing spider mites.
Frequently Asked Questions
There are numerous reasons why your tomato leaves may be turning yellow. For treatment, you must isolate the exact causes. Yellowing leaves are commonly a sign of nutritional deficiencies (e.g., nitrogen, sulfur, iron, potassium, magnesium, etc.) or other factors which compound these deficiencies, such as overwatering, pH imbalances, soil compaction, sunburn, and herbicides, among others. Diseases and pests can also cause tomato leaves to turn yellow.
In general, diseased and pest-infested leaves should be pruned or destroyed to prevent further spread. However, sun-scorched leaves can be left alone as they still biologically function and can help shade other parts of the tomato plant. When removing yellow leaves, sanitize your shears and gardening tools to prevent diseases from spreading.
Overwatering drowns out roots, preventing nutrient uptakes, which leads to yellowing tomato leaves. Tomatoes require daily watering the first week of planting but should be weaned down to about 1-1.5 inches (2.5-4 cm) of water a week.
 Kluepfel, M., Lippert, B., & Williamson, J. (2022, June 24). Changing the pH of Your Soil | Home & Garden Information Center. Home & Garden Information Center | Clemson University, South Carolina. https://hgic.clemson.edu/factsheet/changing-the-ph-of-your-soil/.
 Gugino, B. K. (2012, January 4). Early Blight. Penn State Extension. https://extension.psu.edu/early-blight.
 Simone, G. (2019). Tomato Yellow Leaf Curl Virus Management for Homeowners. Integrated Pest Management Florida | University of Florida Extension. https://ipm.ifas.ufl.edu/agricultural_ ipm/tylcv_home_mgmt.shtml.