Why Are My Tomato Seedlings Turning Yellow?

Tomatoes are one of the most popular vegetables to grow at home. Millions of people try every year. And chances are, you’re not the only one having problems and wondering why are my tomato seedlings turning yellow. 

Why Are My Tomato Seedlings Turning Yellow

Don’t get me wrong, one of the reasons why tomatoes are such a gardener’s favorite is because of how easy they are to plant and grow. But the yellowing of tomato seedlings is more common than you think. 

Generally, for tomatoes to grow healthy, it needs the following: (1) at least 6 hours of full sun per day, (2) fertilized moist soil, (3) steady supply of water, and (4) daily warm temperatures ranging from 78 and 92 degrees Fahrenheit. 

Pretty basic right? Definitely perfect for beginners. 

However, tomatoes are also susceptible to a number of diseases and pests that can cause the tomato leaves to turn yellow so you really need to keep an eye on your plant. These diseases and pests are the trickier part of growing tomatoes at home. 

And—just to be clear—if you didn’t plant them correctly, that will be another probable cause of the yellowing of your tomato seedlings. 

But I’m here to share some of the top reasons why there are yellow leaves on your young tomato plant. This includes the general tomato growing problems, pests to look out for and diseases you should prevent. 

So don’t worry! I will be giving you all the details you need to know for a thriving plant in the end.

Tomato Growing Problems: Seedlings Turning Yellow

  1. Water Stress due to Excessive Watering


    What to look out for:

    Waterlogged soil, yellowing of leaves and over-moist soil between waterings. 

    What to do:

    Check your soil if it’s draining well. Chances are, the yellowing of seedlings is due to overwatering of the young plant. If you used peat pots, make sure you water them gently during the germination to seedling emergence. 

    Peat pots retain more water than your regular seeding tray so even if they appear dry if you actually check the soil it may be damp—damp up to the roots. So avoid over watering the seedlings. 

    To avoid this problem, try using this way of planting tomatoes, so you can assure your seedlings will grow properly:

    1. First, fill half of the container with a potting mix. Make sure to use a garden fork that is clean to avoid contamination or transfusion of diseases. Tap the mix but do not overdo it so it won’t compact completely. 

    2. Then, put 2 -3 seeds into one container. Cover it with ¼ potting mix which leaves another ¼ empty space in the container. 

    3. Water the mix, but don’t overwater it. Just enough to keep it moist. Also, make sure not to have a soaking potting mix. Cover the pots with black garden blankets to stimulate heat in the soil. Or you could also use a light mat that could help in speeding the germination. 

    4. Place the pots in a warm spot. Tomato seeds will germinate faster in room temperature and with warmer soil. During this stage, the plant does not need light but enough warmth.

  2. Incorrect Amount of Light 


    What to look out for:

    Mature leaves turning yellow during the seedling stage. 

    What to do:

    During the germination stage, your tomato plants should be in a warm spot so it will germinate faster. They should be placed at room temperature with warmer soil. 

    The germination stage will occur in 5 to 12 days after your tomato seeds are sowed. And tomato seeds will turn into seedlings in around six to eight weeks.

    During the seedling stage, your plants are going to be needing more light than warmth so it’s time to adjust them again. 

    Ideally, seedlings need about 12 hours of light daily and about 8 hours of darkness too. 

    You can use a grow light and a timer to set your seedlings with the best possible light condition. 

  3. Overfeeding of Tomato Seedlings

    Tomato over fertilizer

    What to look out for:

    White deposits around the drainage holes. 

    What to do:

    Generally, the initial potting mix you’ve used during the soil preparation will go a long way for your tomato plants. This will give them a much-needed boost of nutrition during germination. 

    During the seedling stage, true leaves will start to appear in 2 to 3 days. Once these leaves have appeared, feed with a diluted all-purpose fertilizer.

    Do follow the instructions of your chosen fertilizer, seedlings don’t need that much fertilizer during the earlier stage of their lives. So, if you’ve been feeding them nearly every day, you’re doing it wrong. 

    Check the drainage holes if there are white deposits around them. If there are, the minerals from your fertilizer have already built up in your seedlings’ small container. This strangles your plant and eventually turns the tomato seedling leaves yellow. 

    To save your plant, immediately stop feeding your seedlings and gently pour water on the container to flush out the built-up minerals. 

Tomato Plant Pests: Seedlings Turning Yellow

  1. Whiteflies

    Tomato Pests Whiteflies

    What to look out for:

    Check the underside of the tomato leaves for tiny white-winged insects. When disturbed they fly up immediately. Whiteflies transmit the Tomato Yellow Leaf Curl Disease which causes the leaves to turn yellow. 

    What to do: 

    To both control and prevent the infestation of whiteflies, you can use yellow sticky traps to catch them. Once the insect infestation proves to be severe, choose a suitable insecticide and apply it on the infected plant. 

  2. Spider mites

    Tomato Pests Spider mites

    What to look out for:

    Spider mites or Two-spotted spider mites can’t be exactly seen by the naked eye as they are too small. But their activity can be detected by the formation of webs on the underside of the leaves. 

    Damage appears as yellowing of leaves and eventually dropping from the plant. 

    What to do:

    The easiest way to control the spider mite population in your garden is by spraying a strong jet of water on your tomato plants. When the spider mites are too severe or the infestation is too high, apply insecticidal soap to your plants. 

    Make sure to only use the insecticidal soap if the infestation is too severe, applying insecticides when unwarranted will only kill off the natural enemies of the spider mites. Doing so will only help the pest and not your plants. 
  3. Aphids

    Tomato Pests Aphids

    What to look out for:

    Aphids are soft-bodied insects that often appear in clumps. Always be on the lookout for these critters. They can come in black, brown, red or green colors. They will suck your plant’s juices and can transmit viral diseases. 

    Expect slight curling of the leaves and eventual discoloration. The appearance of shiny specks is also possible. 

    What to do:

    Tomato plants can generally tolerate light to medium infestation, but once aphid infestation is too severe, you may use insecticidal soaps, neem oil or canola oil to control the population.

    If aphids are seen, remove them by spraying a strong jet of water to your plants. 

    To prevent, plant and grow tolerant varieties 

Tomato Plant Diseases: Seedlings Turning Yellow

  1. Powdery Mildew

    Tomato disease Powdery Mildew

    What to look out for:

    You can spot this disease when you observe white powder on your plants. White spots may form on the leaves and stem of the tomato plant. The fungi, if left alone, will then turn the leaves yellow. 

    What to do:

    The main cause of powdery mildew is the lack of air flow around your plants. Adding high humidity to that will assure you this problem. 

    To control this problem, I suggest using a preventive spray (formulated with sulfur) on both the clean and affected areas. 
  2. Tomato Yellow Leaf Curl Disease

    Tomato Yellow Leaf Curl Disease

    What to look out for:

    Leaves curling upward. They will be crumpled-looking and both the veins and leaf margins will turn yellow. Overall, the whole plants will look bushy and stunted. Eventually, the flowers may not even develop or just drop off without maturing. 

    What to do:

    As mentioned before, whiteflies are the ones that transmit this virus. If tomato seedlings or the overall plant is infected during the early stage, you may expect a complete yield loss. 

    If infected, immediately remove from the rest of the plants. Since whiteflies carry this virus, you can use yellow sticky traps to control the pests. If you think that the insect infestation is severe, then you may use a proper insecticide for your plant. 

    To avoid this problem, make use of the resistant varieties available for growing. And importantly, you should only transplant seedlings that are disease and pest free. As always, keep your garden free from weeds and plant debris. 

  3. Early Blight

    Tomato disease Early Blight

    What to look out for:

    Check tomato leaves from brown spots. These spots will start to develop rings and around these spots, leaves will turn yellow. After the yellowing of leaves surrounding the brown spot, the entire leaf will turn brown and eventually fall off. 

    What to do:

    Once you see the signs of Early Blight, immediately apply the proper amount of fungicide for your tomato plants. 

    To avoid this problem, practice crop rotation. This will aid in preventing new plants from catching this disease.

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