Many gardeners enjoy growing their own tomatoes. For me, the taste of freshly picked, home grown tomatoes is one of the highlights of summer. They are also attractive plants. Their bright flowers and red fruit sit nicely against a backdrop of rich, green foliage. However, the plants can sometimes develop unsightly, yellow leaves. This can be frustrating, especially if you have invested a lot of time and effort into growing your plants.
There are a number of possible reasons why your tomato leaves are turning yellow. Finding out why and how to solve the issue can take time and is often a process of trial and error. However, it is well worth the effort. Most causes of leaves turning yellow can be easily rectified.
The condition of the foliage can tell you a lot about the health of a plant.
This article will take you through the most common causes of tomato leaves turning yellow. As well as explaining how to identify the problem we will also take you through various treatments and prevention methods.
Young Leaves Turning Yellow
The first leaves on the seedlings, known as cotyledons, commonly change color and fall away. This is a natural part of seed development. As long as the rest of the plant is healthy it isn’t a problem.
In the 2 weeks after transplanting foliage can fade and lighten. It is rarely a major issue, merely a sign that the plants are adjusting to their new position. Sometimes called transplant shock, tomato leaves turning yellow soon after transplanting into their final position is more common than you think. It can be particularly common if the nights are cool. New foliage, that is green and healthy will soon emerge. As soon as the new, green foliage has formed the discolored foliage can be cut away.
Foliage changing color soon after planting can be a sign of transplant shock.
Some varieties of tomatoes are more prone to transplant shock than others. To prevent the issue, take the time to properly harden off your plants and wait until nighttime temperatures consistently remain over 50 ℉ before transplanting. An Outdoor Thermometer is easy to use and helps you to accurately measure the temperature in your garden. While cool temperatures are unlikely to be an issue if you are growing indoors, you should still take care when transplanting.
Problems with Your Watering Routine
Both over and underwatering are common causes of leaves turning yellow. Knowing how frequently to water your plants can be difficult. As well as different plants and varieties having different watering requirements, the frequency with which you water can also be affected by the soil type and weather conditions.
In general you should aim to water the soil thoroughly once every 5 to 7 days. Don’t let the soil dry out. During wet spells you may not need to water the soil.
While you shouldn’t allow the soil to dry out, at the same time the plants should never be allowed to sit in wet or poorly draining soil. It can be difficult to know when to water plants. A soil moisture meter, such as the Gouevn Soil Moisture Meter, is a useful device that can help you to work out exactly when to water your plants.
When watering aim to water only the soil and the base of the plant. Keep the foliage as dry as possible. You may find that watering with a watering can gives you more control than watering with a garden hose. Watering early in the day gives the plants time to absorb moisture before the heat of the afternoon dries the soil.
A Sign of Nutritional Deficiencies
Some foliage turning a different color at the bottom of an otherwise healthy plant is nothing to worry about. It is particularly common on older, fruit bearing plants. In this case low down leaves turning yellow is an indication that they aren’t getting enough light. It can also be a sign that the soil is lacking in nutrients.
A soil test kit can quickly tell you which nutrients your soil is lacking in. A nitrogen deficiency is a common cause of tomato leaves turning yellow. However a lack of other nutrients can also cause foliage to become discolored and unsightly.
A lack of potassium in the soil can cause yellowing of the veins. They may also wilt. An application of potash can counteract this issue.
A calcium deficiency causes growing tips to yellow. Known as blossom end rot, apply a compound rich in calcium to the soil.
A change in the condition of foliage can be a sign that the plant is suffering from a nutrient deficiency.
A lack of magnesium stunts growth, causes yellowing around the edge of the leaf. It can also cause the color of the leaf to fade. Finally, the foliage may also develop a golden border. A magnesium deficiency can sometimes cause small, yellow spots to emerge on the foliage. Eventually the leaf fades completely and wilts. A quick way to amend a magnesium deficiency is to water a diluted solution of Epsom salts into the soil.
Sulphur deficiency can cause young leaves to turn yellow. Older foliage stays green and fresh. Similarly a zinc deficiency often turns the leaf veins yellow. This is particularly common on fresh, young leaf sets.
A nutrient deficiency is easily rectified. Fertilize the soil and your plants regularly. Begin once plants are established and continue at least once a month during the growing season. Check the information on your chosen fertilizer for exact dose size and frequency. Be careful not to over fertilize your plants.
Compacted or non aerated soil can result in leaves turning yellow. This is because a compact soil hampers root development. It also limits the amount of nutrients that a plant can easily absorb. To prevent this issue work the soil over well before planting. Working in organic matter such as compost can help to lighten the soil and prevent it from becoming compacted.
Untreated infestations are a common cause of tomato leaves turning yellow. Aphids, thrips, flea beetles, whitefly and spider mites can all cause foliage to change color. If left untreated, infestations can also cause problems such as wilting, stunted growth and fruit drop. Regularly check your plants foliage for signs of infestation. If you do notice any pests an application of homemade insecticidal soap or neem oil can treat most infestations.
Larger pests such as cutworm and hornworm can be picked off by hand. If you use chemical controls Bt, or bacillus thuringiensis, can also be used.
If left untreated infestations may cause foliage to fade and change color.
Fungal disease is one of the most common causes for tomato leaves turning yellow.
Early blight causes leaves to lose color and small spots or lesions to form. As the lesions develop they can take on a bulls-eye like appearance. While unsightly, the fruit is rarely affected unless the case is particularly severe.
Late blight is more problematic. Starting on upper foliage it causes oily lesions on foliage and stems.
Another common fungal disease is fusarium wilt. This usually occurs in warm weather and causes the older, lower leaves to begin turning yellow, usually on one side of the plant. Growth can also become stunted and, if left untreated, the plant may struggle to produce fruit.
Most fungal diseases can be prevented by correctly watering the plants. Spacing them out so that air is able to freely circulate around the plants also helps, as does regularly pruning your tomato plants. Should a fungal disease form, cut away and burn the affected foliage to prevent the issue from spreading through the plant. If left untreated the fungal disease may also spread to other, nearby plants. Fungal diseases can also be treated with fungicides containing chlorothalonil.
Fusarium wilt is a soil borne fungal disease. It is a common cause of leaves turning yellow and falling from the plant. Regularly check the pH level of your soil. Acidic and alkaline extremes away from the neutral 6.5 can encourage the fusarium wilt fungus to thrive. The disease can also enter the plants through damaged roots. Take care when transplanting and weeding not to damage the root system. There is no cure for fusarium wilt. Affected plants should be dug up and destroyed.
Septoria leaf spot is caused by the Septoria lycopersici fungus. Particularly common during wet and humid periods, this fungal disease affects the foliage, turning it yellow. This is not a soil based problem. Instead it survives on weeds or the foliage of the nightshade family such as peppers. Warm temperatures and high humidity awakens the fungus. As Septoria leaf spot develops, gray or brown circular spots appear on the already discolored foliage. Cut away affected foliage and destroy it to prevent the disease from spreading.
Infected soil splashing onto stems when watering helps the disease to spread. Mulching around the plants helps to prevent soil from splashing onto stems when watering. You should also ensure that the leaves and branches of the plant don’t touch the ground. Finally regularly weed around the plants, ensuring that you properly destroy any nightshade weeds.
Often spread by pests such as thrips and whitefly, viral diseases are a common cause of tomato leaves turning yellow. Viral diseases can also be transmitted by dirty tools.
Some of the most common viral diseases include:
- tomato mosaic virus,
- tobacco mosaic virus,
- cucumber mosaic virus,
- single streak virus,
- tomato yellow leaf curl
As well as causing leaves to lose color, viral diseases can also stunt growth and, in some cases cause a mosaic like pattern to form.
Viral diseases are often devastating to tomato plants. Once affected they are almost impossible to save. There are currently no approved, effective chemical controls. Instead affected plants should be dug up and destroyed. Correctly planting and watering your plants, as well as cleaning your tools regularly, helps to prevent viral diseases. A number of varieties are now also disease resistant.
Fusarium Root Rot
Fusarium root rot causes the lower leaves to fade and the stem to begin turning brown and soften. Fusarium root rot can be caused by planting in poorly draining soil, low temperatures or a low soil pH. Dig up and destroy affected plants as quickly as possible. Sterilize the soil before planting again.
If you are growing in a pot and catch the issue early enough you can try to save the plant by digging it up. Gently wash the roots in cold water to clean away any remaining soil. Decayed sections of root may fall away. Cut away any mushy or black root sections and destroy. Wash the remaining root system again before applying a fungicide. Replant in a clean pot filled with fresh potting soil.
The End of the Growing Cycle
At the end of the season as temperatures fall, leaves often yellow and begin to die back. This is perfectly natural and a visible indication of the end of the growing season. When this begins to happen, cut away new blossoms. This encourages the plant to put its energy into ripening the green fruit that is already on the vine. Reduce watering and fertilizing. You can also cut away discolored or dieing stems and foliage.
Foliage changing color or fading may simply be an indication that the plant is coming to the end of its lifespan.
Tomato leaves turning yellow can have a number of causes, from serious issues to easily treated problems. It can even be a natural stage of the plant’s life cycle. Knowing how to identify why tomato leaves turning yellow means that you can easily treat the issue, helping your plants to thrive.
Jen is a master gardener, interior designer and home improvement expert. She has completed many home improvement, decor and remodeling projects with her family over the past 10 years on their 4,500 sf Victorian house. She is also a passionate farmer who keeps goats, chickens, turkeys cows and pigs on her farm, and an instructor for her community’s Organic and Sustainable Farming project.