Yellow leaves on tomato plants are a sign that there's a problem. The causes are varied—including disease, pests, environment, and plant care. But there is good news. Yellow leaves on tomato plants can often be fixed (or at least managed). Use this guide to figure out what's happening with your plants and address the issue so you can still harvest plenty of sun-ripened tomatoes.

1. Transplant Shock

A few yellow leaves on tomato plants within a week or two of transplanting the seedlings is not uncommon. After spending weeks growing in a predictable and consistent greenhouse environment, the young plant is acclimating to the temperature swings, wind, and fluctuating moisture levels. Some slight yellowing is a common response.    

What to do: Provide adequate water. Seedlings thrive in soil that is moist, but not wet. They will establish an extensive root system within 4 weeks or so. At that time, transition to watering plants deeply once or twice a week during dry periods.  Also, limit seedling stress by protecting young plants from cold temperatures. If nighttime lows drop below 45°F, cover plants with a bucket or box. Uncover in the morning. 

2. Nutrient Deficiency

Tomato plants grow fast and that growth requires plentiful nutrients. Yellow leaves on tomato plants often happen when the plant isn't getting the necessary nutrients to facilitate rapid growth. The most common nutrient deficiency is nitrogen. A lack of nitrogen causes older leaves to yellow as the tomato plant saps the nitrogen away to fuel the growth of new, young leaves. Iron deficiency or low magnesium levels are also common issues. Yellowing young leaves on an otherwise healthy plant often indicates iron deficiency while magnesium-deficient tomato plants have yellow spots on old leaves.

What to do: Simply fertilize tomato plants with an all-purpose vegetable plant fertilizer. Follow package directions exactly.  

3. Early Blight

This frustrating fungal disease first shows up on the lower leaves of a plant. Small brown lesions develop and soon the surrounding tissue turns yellow. Early blight survives in the soil and splashes up onto the leaves during rain or when plants are watered.

What to do: Remove diseased tomato leaves and stems and dispose of them. Minimize the spread of disease by cleaning your pruning shears between cuts. Prevention is key to avoiding early blight. Rotate crops in the tomato family—pepper, eggplant, and potato—on a three-year rotation to prevent early blight buildup in the soil. Mulch plants to avoid splashing soil on the leaves and deliver water directly to the base of the plant with a water wand or drip hose. Do not water overhead. 

4. Late Blight

Especially common during cool, wet weather, late blight causes young leaves at the crown of a tomato plant to turn yellow. Late blight is characterized by yellowing leaves with irregularly shaped brown lesions. If this occurs, the diseased leaves will turn brown and quickly fall off the plant. Spread by the wind and rain, late blight can move through a garden or neighborhood quickly. When late blight infects the stem, everything above the infection is unsalvageable.  

What to do: Remove and destroy diseased plants as soon as they are noticed. Discourage late blight by planting seedlings 3 to 4 feet apart to encourage good air circulation. This helps prevent late blight from taking hold by ensuring that foliage dries more quickly. Avoid overhead watering; instead, use a water wand or drip hose to direct water to the root zone of your tomato plants.

5. Powdery Mildew

Powdery mildew is a leaf disease that causes yellow spots on leaves throughout the plant canopy at any time of development, but it is most common when tomatoes are ripening. Inspect the leaves closely and you’ll see white powdery growth on the top and undersides of leaves.

What to do: Fungicides containing sulfur or copper can be effective. Biological products are effective too. Look for biological products containing sesame, rosemary, or thyme botanical oil or a potassium bicarbonate as the active ingredient. Fungicides must be applied weekly to maintain control.

6. Fusarium Wilt

Fusarium wilt, which causes leaves on the bottom half of the tomato plant to turn yellow, is triggered by a fungus in the soil. It most commonly occurs when tomato fruit begins to mature and may be visible on just one side of the plant. Once fusarium wilt takes hold, the diseased leaves will wilt and the entire plant will eventually die.

What to do: Remove and destroy infected plants. The best defense against the disease is to plant fusarium wilt-resistant tomato varieties like ‘Solar Fire,’ ‘Top Gun,’ and ‘Mountain Merit’.

7. Salt Damage

Tomatoes growing in pots are especially susceptible to excess salt and other mineral buildup in the soil. Excess salts and minerals from water collect in the soil, depriving the plant of valuable nutrients and causing leaves to turn yellow.

What to do: Once a week, water the container until water runs out of the bottom of the container. This will flush out excess salts and minerals. The rinse will also wash away valuable nutrients so be sure to fertilize tomatoes growing in containers regularly. 

8. Herbicide Damage

Tomatoes are susceptible to weed killers and can be damaged from even small amounts that might drift over from a nearby application. Leaves impacted by herbicides will quickly turn pale yellow, often mottled with white. Herbicides can also cause damage (like leaf curling and twisting, wilting, and stunted growth) that mimics symptoms of other plant ailments.  

What to do: There is no remedy for a tomato hit by herbicide. The best tactic is prevention. Do not use weed killers near tomatoes; remember the volatile compounds can travel long distances on breezes and impact plants far from where the spray is being applied.

9. Too Much or Too Little Water

Tomatoes generally need about 1 to 2 inches of water per week. Plants growing in fast-draining, sandy soil need more weekly water, while plants growing in loam soil or slow-draining clay grow best with 1 inch of water. Too much water or too little water can cause leaves to turn yellow.

What to do: Avoid overwatering your tomato plant by checking the soil moisture before turning on the hose. Sink your finger into the soil at the base of the plant. If the soil feels moist 2 inches below the surface, hold off on watering and check your soil the following day. Water when the soil feels dry to the touch 2 inches below the surface. When watering tomatoes, a slow trickle over a longer period is better than a rapid splash. This will encourage your plants to develop a deep root system that is more drought-resistant.