Yellow leaves on tomatoes – causes and solutions

As a nightshade plant, the tomato plant is poisonous in all green parts due to its solanine content. Similar to potatoes, the fruits are only edible when they are fully ripe.

Unsuitable light and temperature conditions

In order for a tomato plant to thrive, it needs every ray of sunshine that is available in the local climate. In addition, warm temperatures of well over 15 degrees Celsius play an important role. If you assign the tropical plant a location that is too dark and cool, photosynthesis will stall. The result is yellow leaves on long horny shoots.

Solution: Sunny, warm location

Ideally, you should move tomato plants in the tub to a sun-drenched place that is warm and protected at the same time. Here the nightshade plant should recover within a short time. With tomatoes in the bed, you can take a closer look at the neighborhood. If there are specimens that cast shadows on the plant, they are either cut back or completely removed.

Sun shock

Given the minimum temperature of 15 degrees Celsius, young tomatoes are not allowed outside until mid-May at the earliest. As a result, hobby gardeners prefer the plants behind glass so that they can cope with the limited time in the bed due to a growth advantage. Moving out of the room or the greenhouse to the fresh air puts a lot of stress on the tomato plants, which in the worst case leads to yellow leaves.

Solution: hardening off

Begin preparing the young tomatoes for the field as early as the beginning of May. During the day, place the plants in a partially shaded, sheltered place on the balcony or in the garden. In the evening you carry the pots back into the house. In this way, your pupils gradually get used to the climatic conditions in the open air and are prepared when the sun shines on them from mid / end of May.

Nutritional deficiency

Tomatoes are among the heavy eaters. Even in nutritious, humus-rich soil, the plants are wasted without an additional supply of nutrients. If the young leaves turn yellow, there is a lack of iron, zinc and copper. If older leaves turn yellow, this indicates a deficit in the supply of potassium, magnesium and phosphorus. If you are confronted with the damage, although you always value balanced fertilization, the plant will lack lime.

Solution: fertilize organically in a balanced way

Since fertilizers with chemical components are frowned upon in private kitchen gardens, set the nutrient supply for yellow leaves to the following parameters:

• Fertilize with compost and horn shavings every 14 days until flowering begins
• Alternatively, fertilize with horse manure, guano or nettle manure
• In addition, if the leaves are yellow, give activated lime or rock flour every 2-3 weeks
• From the beginning of flowering until the fruit is ripe, fertilize organically every week
• Fertilize in the bucket with an organic liquid preparation for tomatoes

You can effectively prevent yellow leaves on tomatoes if you mulch with nettle and comfrey leaves from the start, mixed with the stripped foliage of the tomato plants.

Wetness

Tomatoes prefer a constantly moist substrate without extreme rashes in one direction or the other. If there is an oversupply of water, the plant reacts with yellow leaves, while drought turns into brown discoloration. In this regard, waterlogging represents the greatest danger in both the bed and the tub, since experience has shown that hobby gardeners tend to water too much rather than too little. As a result, the roots rot, so that the transport of nutrients comes to a standstill, which is expressed in yellow leaves.

Solution: Well-balanced water balance

So that a tomato plant does not suffer from an oversupply of moisture and the resulting yellow leaves, the following measures serve as an acute and preventive treatment:

• Cut off the yellow leaves at the first symptoms
• Do not water the plant until the thumb test shows that the substrate has dried on
• Protect against too much moisture from above with a rain canopy or a transparent hood
• Repot the tomatoes in the tub in fresh, dry substrate

A drainage acts as an effective precaution against waterlogging in beds and buckets. To do this, spread out some pottery shards or pebbles in the planting pit or over the water drain in the bucket before planting. If you also mulch with straw, nettle leaves or grass clippings, this will keep the soil moist and warm for longer.

Leaf spot disease (Septoria lycopersici)

If location problems and neglect of care can be ruled out as the cause, a widespread plant disease in tomatoes comes into focus. Especially during rainy, humid and warm summers, the sword of Damocles dangles over the apples of paradise. Yellow leaves are among the first symptoms of the dreaded fungal infection. In the further course, watery spots with a black border form on the yellow foliage. Ultimately, the leaves fall off and the supply of the fruit comes to a standstill.

Solution: Combat with natural means

Since using a chemical fungicide on tomatoes is out of the question, use natural remedies to combat leaf spot disease:

• Remove all yellowed leaves and dispose of with household waste
• Spray the plant itself and neighboring tomatoes with a mix of fresh milk and water (1: 2)
• Consistently exhaust the plant at least once a week

When planting, make sure that the tomatoes are well spaced from one another so that rainwater dries off faster. Ideally, you should protect the plants outdoors with a rain cover or remodel them with a tomato house.

Bakterienwelke (Corynebacterium michiganense)

If tomato leaves take on a yellow color from below, while at the same time brown-yellow vessels appear in the cut shoots, the bacterial wilt has struck. If the disease continues, the yellow leaves turn brown and fall off. In contrast to leaf spot disease, this infection spreads to the fruit. In the early infestation stage, there is still a chance that the plant will be saved.

Solution: intervene quickly or complete disposal

How to proceed:

• Rigorously cut and destroy all yellow leaves and infected shoots
• Clear away the mulch and loosen the soil thoroughly
• Sand compacted soil and optimize it with compost
• Do not use nitrogen-based mineral fertilizers
• Give tonic, such as Neudovital and liverwort extract

If more than 30 percent of the leaves have already turned yellow, there is a risk that the disease will spread further in the garden. In this case, dispose of the entire plant and maintain a crop rotation of 4 years before planting tomatoes again in this location.

Thripse (Thysanoptera)

Tomatoes in the greenhouse and in the bucket are part of the prey pattern of thrips. The tiny fringed wings, also known as thunderstorm flies, cause yellow leaves on tomatoes as part of their nefarious drift. The adult pests suck out the sap, while the larvae feast on the roots.

Solution: soft soap solution, primary rock flour and beneficial organisms

There is no need to use a chemical club to get rid of the tiny insects. Instead, you fight the plague with the classic soft soap solution. To do this, mix 15 ml of soft soap or curd soap in 1 liter of water to repeatedly spray the infected plant. Since moisture is not welcome on the leaves of tomato plants, primary rock flour serves as a practical alternative when in doubt. Using a powder syringe, apply the product every 2-3 days until there are no more thrips swarming around.

Thrips are particularly common on tomato plants in greenhouses. In this protected ambience, health-conscious and environmentally conscious hobby gardeners get animal support. The fringed-winged birds like to eat beneficial insects such as predatory mites and hoverflies. The specialized trade offers the helpful insects, which can be spread out very easily so that when the work is done, they finally wander out of the greenhouse.

Whiteflies (Trialeurodes vaporariorum)

The powdery white flies are actually pretty to look at, but tomato plants cause considerable damage in the greenhouse. In fact, they are whiteflies that cover their yellowish, 1-5 mm small body with white wings. Introduced from South America, the pests spread explosively in the warm climate of the greenhouse while they suck the life of the tomato plants. As a result, the leaves turn yellow as they increasingly lack nutrients. In addition, the foliage is covered with a sticky layer of honeydew, which attracts ants in hosts.

Solution: yellow boards, attractant traps, parasitic wasps

At the first sign of whitefly in the greenhouse, catch the adults with yellow panels. If the occurrence of white flies is still limited, put an end to the plague with attractant traps. These are sticky traps equipped with pheromones that attract and hold onto sexually mature insects. Close-meshed insect nets protect individual tomato plants from infestation. The release of parasitic wasps has proven to be an effective biological control measure. Specifically, the wasp Encarsia formosa targets the larvae in order to parasitize them. Two-point ladybugs and lacewings also take care of the pests with enthusiasm.

Conclusion
The cultivation of tomatoes is undoubtedly one of the most demanding challenges in the hobby garden. As a reward for all their efforts, the gardener can of course enjoy incomparable indulgence, which tomatoes on the shop shelf don’t even come close to. So don’t throw your gun in the grain when you see yellow leaves on tomatoes. Instead, embark on a dedicated root cause research. Often times, the problem can be remedied by means of simple measures, such as a change of location, a changed watering behavior or an adapted supply of nutrients. If an infestation with fungi, bacteria or pests turns out to be the trigger, there are various natural control agents to choose from. You can read about the most common causes and possible solutions here,