Why Are My Tomato Leaves Turning Yellow?

Help! My tomato plants are turning yellow! There are several possible reasons for leaves on tomato plants and tomato seedlings to turn yellow, and getting to the right answer requires careful consideration and sometimes a bit of trial and error.

Read on to learn about the most common reasons for those yellow tomato leaves, and what you can do about it.

What Tomatoes Need

If you have tomato leaves turning yellow, it first helps to know what these plants need to grow and thrive:

• Well-drained soil
• Slightly acidic soil
• Fertile soil or appropriate fertilizer
• Full sun
• Warm temperatures
• Adequate spacing
• Deep, regular watering

Why Tomato Plant Leaves Turn Yellow

Yellow leaves on tomato plants can be frustrating, and there are a number of possible causes. Luckily, a lot of them are easily fixable. Identify the problem and apply the solution to grow healthy, productive tomato plants.

Watering Problems

Improper watering is one of the most common causes of yellow tomato leaves. Unfortunately, too much water or too little water can both be the problem, so it's best to just try to do it right. If it’s not raining about an inch (2.5 cm) per week, soak tomato plants thoroughly once every five to seven days, depending on weather and soil type. Let the soil dry between watering and never allow the soil to remain soggy.

Water tomato plants carefully at the base of the plant and keep the leaves as dry as possible. Watering early in the day is best.

Transplant Shock

If you've just transplanted your young tomatoes outdoors and notice some yellowing leaves, transplant shock is likely the cause. When the plants move from a warm, protected greenhouse to a cooler garden, there's a good chance they'll show their displeasure with yellow leaves. There's no need to worry, though -- they just need some time to adjust.

Shading

Any leaves on the plant might begin to yellow if it is not getting enough sun. Tomato plants require at least 6 hours of direct sunlight to thrive.

One of the ways that leaves may yellow on a tomato plant that is not a major problem is shading by upper leaves. If there are no other issues and only the bottom leaves have yellowed, simply snip them off as they are no longer productive.

Fungal and Bacterial Diseases

If all the conditions are right for your tomato plant, yellowing might be a sign of disease. These are some fungal and bacterial infections that cause leaf yellowing:

• Early blight is caused by both Alternaria tomatophila and Alternaria solani. This fungal disease shows up as brown spots, mostly on older leaves. The spots often develop yellow edges.
• Late blight is a rare but destructive disease. You can recognize late blight by the large, oily-looking lesions on both leaves and stems.
• Septoria leaf spot is caused by the fungus Septoria lycopersici. Signs usually appear first on lower leaves and include small spots with dark borders. The leaves eventually turn yellow and fall.
• Leaf mold (Passalora fulva) manifests as yellow spots on older leaves. The spots grow larger with time, turning leaves mostly yellow.
• Fusarium wilt (Fusarium oxysporum) causes wilting and yellowing of lower leaves. Often, the yellowing begins first on one side of the plant.
• Verticillium wilt appears on older leaves as a yellow v-shaped pattern that eventually spreads, turns brown, and causes the leaf to drop.
• Bacterial leaf spot is mostly characterized by water-soaked spots that might develop a yellow margin.

Viral Diseases

Viral diseases are much less common than fungal or bacterial infections, but there are still several that could be the cause of your yellow leaves. One is tomato yellow leaf curl virus. You might see random plants affected. It’s best to pull them out at the first sign of the disease.

Tomato yellow leaf curl is caused by a virus spread by whiteflies. A characteristic symptom is curling leaves with yellow margins. The leaves are typically smaller than normal, and the plant’s overall growth is stunted.

Other viral diseases include tomato mosaic virus, tobacco mosaic virus, single streak virus, and cucumber mosaic virus.

Although symptoms vary, tomato viruses are generally recognized by stunted growth and a mosaic pattern on the leaves. Some types may cause malformations such as fernleaf, broccoli-like growth, brown streaks, or severe curling. Viral diseases are often spread by pests such as whitefly, thrips, or aphids, and are also transmitted by tools or hands.

Viral diseases are devastating, and plants may not survive. Unfortunately, there are no chemical controls. Often, the best recourse is to discard the infected tomato plant and start over by planting disease-resistant varieties in a new section of your garden. Water properly and maintain proper pest control.

Preventing Disease

Diseases as a source of yellowing leaves are more difficult to fix than growing conditions. If you suspect an infection or disease, remove affected foliage or entire plants as soon as possible to prevent spread.

To prevent disease in the first place, start with plants or seeds from reputable growers. Look for varieties that are resistant to common infections.

Fungal infections readily develop and spread when plants are crowded, too wet, or have poor airflow. Allow for plenty of spacing between plants. Use mulch under plants to prevent soil and water from splashing onto leaves. Water at the soil level, not from above.

Pests

A number of pests can wreak havoc on plants, frequently causing yellow tomato leaves. Insecticidal soap or horticultural oil is good for treating smaller pests such as:

• Aphids
• Thrips
• Spider mites
• Flea beetles
• Whiteflies

Larger tomato pests like hornworms and cutworms can be picked off by hand, or controlled with applications of Bt (Bacillus thuringiensis).

Nutritional Deficiencies

Tomato plants are heavy feeders. They need rich, fertile soil with compost and also benefit from applications of fertilizer. There are a few specific types of deficiencies that can cause yellow leaves on tomatoes:

• Nitrogen. Too little nitrogen can show up as yellowing older leaves.
• Magnesium. Magnesium deficiency also manifests on older leaves, but produces a more speckled yellow pattern.
• Potassium. If you see yellow margins on green leaves, you could have a potassium deficiency.
• Iron. If your plant is getting too little iron, the younger leaves will turn yellow.
• Zinc. Very young tomato plants have yellow between the veins of their leaves, and may be stunted.

Nutrient deficiency occurs most often on older plants that are bearing fruit. Fertilize tomatoes at planting time and monthly throughout the season, as tomatoes have hearty appetites. Follow the directions carefully and beware of overfeeding, which can cause lush plants at the expense of fruit.

With the right growing conditions and preventative measures, you should be able to avoid tomato plant leaves turning yellow and get a good harvest.

Looking for additional tips on growing perfect tomatoes? Download our FREE Tomato Growing Guide and learn how to grow delicious tomatoes.