There are many reasons why leaves on tomato plants could turn yellow. One cause is a lack of light, so be sure to put tomatoes in full sun. Another cause could be too much or too little water, which can occur in compacted soil and can be solved by working to aerate the soil. Many of the causes expanded on below include virus, fungus, or bacteria as well as nutritional imbalances in nitrogen, minerals, and alkalinity, Yellowing of older leaves may show a lack of nitrogen (N), yellowing of new growth a lack of sulfur (S), yellowing between veins a lack of potassium (K), and yellow leaf edges a lack of magnesium (Mg). Read on to learn about many of these diagnoses and solutions.
Chlorosis can be caused by many diseases, nutrient deficiencies, insect or mite damage, or environmental issues like too much water. Imbalanced soil pH may be another cause of iron chlorosis. The ideal pH level for tomatoes is between 6.2 and 6.8, but plants may grow in more acidic soils with a pH as low as 5.5.
Tobacco/Tomato Mosaic Virus
Tobacco mosaic virus may lessen the number of ripe fruits produced, but it will rarely kill plants. Symptoms include curling leaves or green or yellow mottling on leaves. This virus affects many garden plants including ornamentals and can be spread by insects, plant debris and contaminated tools. If it becomes prevalent in your tomato crop, remove affected plants and move your crop to a different location the following year.
Fusarium wilt and Verticillium wilt
Yellow leaves may be a sign of fusarium wilt and verticillium wilt. Fusarium wilt can cause a seedling disease, but verticillium wilt, which is less common, does not affect seedlings and usually occurs later in the season in cooler soils. These diseases are caused by fungi, which can survive for many years in the soil even if tomatoes are not planted in the same spot every year.
Fusarium wilt is a warm-weather disease caused by the fungus Fusarium oxysporum. First, young plants may droop and the lower leaves may wilt and turn yellow. If left unchecked, the plant may eventually succumb. Symptoms of fusarium wilt often occur with only one side of the stem or leaf turning yellow. The stem will show no soft decay, but if cut lengthwise, the lower stem will likely be dark brown inside. Like verticillium wilt, the soil-born bacteria enters through the roots, and is carried into the stem's water-conducting system which blocks the plant's nutrient carrying vessels.
Unfortunately, it is not possible to chemically control these fungal wilt diseases. Remove and discard diseased plants (but not in a compost pile where disease could also spread). Nursery grown tomato cultivars and seed packets may have the resistance letters VF on the plant tag. Most heirloom varieties have little to no resistance to either fusarium wilt and verticillium wilt. Look for Celebrity, Mountain Pride, and QuickPik tomatoes among others that may be resistant to both forms of wilt.
The fungi Alternaria linariae (formally known as A. solani) causes Early Blight. The first sign is the emergence of small, brown lesions on older foliage. Spots get bigger and create concentric rings usually in a bulls-eye pattern in the center of the diseased area. Around the spots, the tissue may become yellow. High temperatures and humidity, can cause the the foliage to die off completely. The infection typically progresses from the base of the plant upward. On fruits, lesions can be very large, sometimes even the size of the entire fruit, which, when infected, often drops to the ground. Early blight can cause much if not all of the foliage to die off, which exposes fruit to sunscald and results in less production. The fungus can survive in infected debris on the soil, on the seed, on volunteer tomato plants that may come up the following spring, or on other hosts like Irish potato, eggplant, and black nightshade.
Trim and dispose of lower branches and leaves that have become infected. If the disease becomes severe, use mancozeb fungicide or chlorothalonil or copper fungicides.
Tomato Yellow Leaf Curl Virus
Whiteflies may transmit this disease into the garden from nearby infected weeds such as nightshades and jimsonweed. After initial infection, plants may not show symptoms for as long as 2 or 3 weeks. Symptoms may include yellow leaf margins, upward curling of leaves, and stunted growth of leaves. Flowers may drop and the plant may become stunted as well. Tomato Yellow Leaf Curl Virus (TYLCV) dramatically lessens fruit yield. If infection occurs early in the season, the plant may not form any fruit at all.
Remove plants and bag them for disposal. To repel whiteflies and reduce feeding, a mix of 0.25 to 0.5% oil spray (2 to 4 teaspoons horticultural or canola oil with a few drops of dish soap per gallon of water) is recommended weekly.
Black walnut trees produce juglone, which is a toxic material that can injure and kill tomatoes among other vegetable crops. Symptoms of walnut toxicity include yellowing and wilting of leaves as well as stunted growth. Juglone is present in all parts of the black walnut tree with the greatest concentration in the roots and within the drip line of the tree's canopy.
Unfortunately, nothing can be done to save tomato plants that are damaged by juglone. Remove and destroy dead plants. Moving forward, avoid planting tomatoes within the dripline of walnut trees. Keep them at least 75 to 100 feet away.
Septoria Leaf Spot
Infecting tomato foliage, petioles, and stems, septoria leaf spot rarely affects the fruit. Caused by the fungus Septoria lycopersici, infection typically occurs on the lower leaves near the ground after the plant starts to bear fruit. Many small circles, one-sixteenth to one-fourth inch in diameter, with dark borders surrounding a beige center appear on the older leaves—small black specks (spore-producing bodies) appear in the center of those spots. Spreading upwards from oldest to newest growth, severely infected leaves turn yellow, die, and drop. While the fungus is not borne in the soil, it can survive winter on crop residue and some weeds. It is most active when temperatures are between 68 and 77 degrees Fahrenheit. It thrives in high humidity and is spread by rainfall splashing up from the ground onto the vines and leaves.
Keep the disease from spreading by repeatedly applying chlorothalonil or copper fungicide, or mancozeb.
Caused by the fungus Passalora fulva, older leaves are first affected near the soil where the air may not circulate as well and humidity levels are high. First symptoms include pale green or yellowish spots on the upper leaves, turning to yellow and, under especially humid conditions, covered with gray, velvety spore growth. Spots may come together, producing a more severe infection and killing the foliage off completely. Sometimes the fungus attacks stems, flowers, and fruits. Fruit that is green or mature may have a black, leathery rot on the end of the stem. This fungus survives on crop residue and in the soil. Spores are spread by rain, wind, and/or contaminated tools and can potentially spread to the fruits' seeds.
Remove crop residue. Stake and prune to improve air circulation.
Several species of the bacterium Xanthomonas (typically Xanthomonas perforans) cause bacterial spot on green, but not red, tomatoes. More common during wet seasons, many small water-soaked spots on the leaves, angular to irregular in shape, appear on the leaves and become slightly raised to scabby spots on the tomatoes themselves. Leaf spots can have a yellow halo and the centers dry out and often tear. This bacteria survives winter on volunteer tomato plants and on plant debris.
Any movement of water from one leaf or plant to another can cause this bacterial disease to spread very easily—making it difficult to control. Even so, it can be treated with a copper fungicide.
Tomato Pith Necrosis
Multiple species of soil-borne Pseudomonas bacteria, including Pseudomonas corrugata, as well as Pectobacterium carotovorum (a trigger for bacterial soft rot) cause Tomato Pith Necrosis. Such bacteria is a relatively weak pathogen. This early-season disease happens in greenhouse and high tunnel tomato production, but during cool, cloudy, rainy spring weather, it can infect tomatoes in home gardens too. Symptoms include blackened (necrotic) spots on the stems. These spots may be connected to the leaf petioles at first, the blackened areas coming together and growing as a band on the stem. Then, bacteria may grow on the inside of the stems and cause the stems to split, shrink, and crack. The stem interior (the pith) can become "segmented" or "laddered." If the disease continues to spread, stems become hollow and as a result, the water supply is cut off to the rest of the plant, causing upper leaves to yellow and shoots to wilt.
If plants do become completely diseased, remove and dispose of the plants, including the roots. Avoid fertilizing with too much nitrogen, as this can cause the disease to escalate. Fortunately, since tomato pith necrosis thrives in cooler weather, it rarely lasts the entire growing season. Plants may recover as temperatures rise in late spring and early summer. Increase watering to help the soil and the plant recover from a lack of moisture. Water at the base of the plant with drip irrigation or a soaker hose or by hand.
The element magnesium is required in the chlorophyll molecule, which enables the plant to synthesize sunlight into starches and sugars for food. Because chlorophyll gives leaves their green color, yellowing leaves can be a sign of deficiency in magnesium, iron, or nitrogen.
High-potash tomato fertilizer may lock up the soil or compost, keeping the plant from receiving the nutrients it needs. To solve the problem, feed the plant with common or garden Epsom salts. Dissolve ½ oz in a pint of warm or hot water or 20g per liter of warm or hot water. Allow the water to cool to room temperature before using. Apply in the morning before the temperatures rise and the plant's pores close to conserve moisture. Spray generously from top to bottom, wetting the leaves so that the undersides absorb the feed.
How to Prevent Yellowing
Buy green, healthy seedlings in spring after the last frost date. Avoid wilted plants or ones with already yellow leaves or brown spots. Use resistant or tolerant tomato cultivars. If starting from seed, use ones that are pathogen-free. Install a good staking system and space plants 2 1/2 to 3 feet apart to improve air circulation. Fertilize well, weed around them, don't wet foliage overhead, and practice a 3-year crop rotation.
Establish tomatoes in slightly acidic soil with a pH between 6.2 and 6.8. Plants can tolerate a wider range of pH as long as the soil drains well and has organic matter. For vigorous growth, feed plants monthly with calcium nitrate.
To prevent fusarium wilt and magnesium deficiency, avoid very sandy soils. To reduce the whitefly feeding that causes tomato yellow leaf curl virus, use reflective mulches (aluminum or silver colored) in rows.
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