Last Updated on April 19, 2023 by Tony Manhart
Why are my tomato leaves turning yellow? Tomatoes are quite sensitive plants, and several mistakes can cause the leaves to turn yellow. Some of these problems are easier than others to deal with. In this article, I will outline 9 common reasons for yellow tomato leaves and how to fix these.
Quick Summary: Reasons Why My Tomato Leaves Are Turning Yellow
If you don’t fertilize enough with nitrogen or a lack of nutrients in the soil, such as potassium and magnesium you can get yellow leaves. Too much water is a common problem that can lead to root rot, inconsistent watering, and under-watering can also stress plants. Also, pale leaves are a sign of nitrogen deficiency.
A Quick Overview of Tomatoes
Tomatoes are a very diverse fruit. The tomato, Solanum lycopersicum, can be bred into new cultivars with great ease. The result is that there are thousands of cultivars of this species. Each one of these is different and unique, ranging from little yellow pear tomatoes to giant beefsteak and oxheart tomatoes.
Tomatoes have two broad growth patterns. Determinate tomatoes grow to a specific size, flower, and fruit and produce a crop that matures at the same time. These often die after fruiting and are well suited to the industrial farmer.
Indeterminate tomatoes just ramble and sprawl and produce flowers and fruit and will take over your garden, and your life. They are boundaryless, excessive, and delightful plants for the home grower. If things go completely crazy with these tomatoes you can end up with buckets and buckets of tomatoes – so many that they will make you sick of tomatoes. Some of the fruits will fall on the ground, and a seed bank of tomatoes builds up in your garden such that the dominant weed in your garden will be the tomato. I am still in two minds as to whether this is a good or a bad thing.
Tomatoes have different growth requirements at different stages in their development. In this regard, seedlings are pretty bulletproof, whereas as the plants get bigger, their nutritional, water, and light requirements increase – as these increase, the stress that the plant is placed under also increases, and the likelihood of disease, nutrient deficiency, and water stress increase.
Why Are My Tomato Leaves Turning Yellow?
I am going to list these in order of commonness based on my experience:
By far the most common mistake we make as gardeners is to think that water is a good thing and give plants too much. Much like us as humans, we need to drink 8 cups a day to be healthy, but if we drink 800 cups we will drown. Tomatoes that are overwatered will suffer from waterlogged soil that becomes anoxic (no oxygen) causing the roots to rot, and the plant to not be able to absorb nutrients -this leads to yellow leaves and a sad tomato plant.
You will see that plants that have been overwatered have wilted leaves, yet the soil is wet. If this happens, ensure no more water gets on your soil, and allow the soil to dry out. You may save your tomato plant. Plant another tomato nearby just in case, so you don’t lose your season.
If you are worried about measuring soil moisture, you can buy a meter such as this and use it to make sure your soil is not dry or wet. It needs to be in the zone in between.
If you take a handful of soil and squeeze it in your hand, wet soil will yield a bit of water you can squeeze out of the soil. I have got to the point where I can just prod the soil with a finger and I know where it is moisture-wise.
2) Nitrogen deficiency
Tomatoes are heavy feeders. They grow rapidly and suck nutrients out of your soil. If you have poor, depleted soil, it will deplete nutrients more rapidly than the soil can regenerate. In this case, the plants will start to show signs of Nitrogen deficiency, which is evident in a general overall yellowing of the plant. A tomato should have healthy dark green leaves – nitrogen deficiency starts as a general conversion to light green, and eventually yellow leaves.
A remarkably effective short-term solution to a nitrogen-deficient tomato plant is to pee on the root zone. I recommend this often, and some people find the suggestion distasteful. But if you think about it, every day, your body releases urine which is rich in nitrogen. This nitrogen is a by-product of your body processing protein, the most expensive part of your diet that you paid for.
It is an ideal fertilizer, and you will actually pay money to get rid of the stuff by flushing it down the toilet – this is bizarre behavior. Little boys and men in general derive great pleasure from peeing on things – peeing on a tomato vine is a useful way of saving money and the environment and producing healthy tomatoes. Naturally, you need to grow the tomatoes on a trellis of sorts so you don’t end up with pee on the fruits.
When you build soil for tomatoes, it is important to include a slow-release nitrogen fertilizer in your soil mix. This will ensure that the soil remains fertile throughout your growth cycle.
3) Potassium deficiency
Potassium is an essential nutrient for tomatoes. It is also a very water-soluble nutrient and washes out of the soil easily. Generally, when we make tomato soil, we make light, fluffy compost-rich well-drained soil. These soils are susceptible to developing potassium deficiency.
A Potassium deficiency is similar looking to a Nitrogen deficiency, except that the yellowing starts on older leaves, and moves to younger leaves as the deficiency gets worse. When you formulate tomato soil, it is best to include a slow-release fertilizer in the mix to avoid these problems in the first place.
I keep my wood ash from our heating system and wood stove and mix this into the soil together with a lot of compost. I have never had a potassium deficiency since I have been doing this. Wood ash is a great source of potassium.
4) Borax deficiency
This is one of those things that you can really learn the hard way. I remember reading that if you find your radishes and turnips have holes inside them, this is a good early warning sign of a borax deficiency. I ignored these signs, and then I had a year where my tomatoes, and other plants all just underperformed. They had weird symptoms – the growth tips curled and sometimes died, the leaves had spots on, went a bit purple, and the few fruits that formed were misshapen. I had a near-perfect crop failure.
I purchase a bit of borax powder every year. When you are correcting borax in your soil you can easily add too much of this which causes many problems too. To get your dosing right get a big jar. I dose my garden in approximately 100 square foot patches. I take a tablespoon of borax and place it in the jar, and then I add a half gallon of sand. I mix this up well by shaking it and rolling it around and shaking it some more. Then I throw this powder over the 100-square-foot zone. This makes it much easier to distribute a tablespoon of borax evenly over the area as it has been diluted into a far larger volume of sand.
I do this again until I have dosed the whole garden. I have read suggestions about mixing borax into the water and spraying it around and so on, but as a scientist that mixes doses of things for a living, I suggest you use my river sand and borax method. It has worked for me for ten years, and I have not killed my plants.
This is really just neglect. In my experience, more often than not we overwater tomatoes, but, if you find that your tomatoes are looking a bit yellow and half dead and wilted, and the soil is bone dry, you have a relatively easy fix ahead!
I have found that if a plant gets very dry and wilted, it responds well to a foliar sprinkle and gentle soil watering. Do not flood the plant – much like a dehydrated person, giving them water can kill them from shock! Moisten the soil, sprinkle the leaves, let the plant hydrate for a few hours, and then water it again, so that it has more water, naturally being careful not to go too far overboard! You would hate to overwater an underwatered plant.
Once you have saved the plant, set up a regular schedule – if necessary put reminders on your phone. Tomatoes generally enjoy a good watering every three days.
I have an automated system that is the ancestor of this system.
With this, I can turn the irrigation off for rainy weather (it does this automatically) and I can ask it to water a bit more if the weather is very hot. It connects to the weather forecast and is remarkably clever with ensuring that your plants are just given the right amount of water. It also has a seasonal adjustment, so it waters a bit less in spring and autumn when the evaporation rate is lower and increases in the middle of summer.
6) Spider mites – Yellow spots on tomato leaves
Spider mites are a pain in the neck. They tend to thrive in hot dry weather and produce a terrible mess of little spider threads under the leaves. You will see little red dots running around under the leaves in this mess of web.
The first thing you will notice with spider mites is if you look at the top of the leaves, you see little yellow dots. As the infestation progresses, these yellow dots merge and eventually, your plants just look terrible.
I went and scratched around in a hedge in my garden and found this wild cherry tomato that has a bad infestation of red spider mites. On the top you can see the yellow dots on the leaves and on the bottom you can see the red dots which are the spider mites, circled in green, on the underside of the leaves.
When you have a problem with spider mites the good news is they hate water. Just spray the leaves with water for a few days and this can help control them. If you can get Persimilis predatory mites buy these and introduce them in your garden. The little joys run around and suck the juices out of red spider mites. I derive great joy from watching these evil little predators destroying red spider mites. They are just brutal!!
You can also use neem oil to bring the situation under control. This takes a few days and repeated applications – you have to spray the underside of the leaves because that is where the mites are. Once you have the situation under control you can get persimilis mites to stop the problem from recurring. Naturally spraying your persimilis biocontrol with neem would defeat the purpose.
7) Fungus issues
There is a range of horrible fungi that can just ruin your tomato crop. I tend to try and grow fungus-resistant strains. For me, the two that I have the worst problems with are early and late blight – both of these can ruin a plant quickly. Early blight causes leaf yellow, late blight makes leaves go black, so we will cover early blight here.
This starts off typically close to the ground – the spores lurk in the ground and get splashed up onto the leaves by rain. You will see little black dots on the lower leaves, and then the leaves go yellow, and from here onwards you are basically doomed if it gets further up into the higher leaves of the plant.
To get around early blight the best thing is to get fungal-resistant seeds such as Roma tomato seeds. I always start my seedlings out indoors in fresh topsoil soil that is not from my garden (I get it from a Sand mine) and get them to be at least 10″ tall. When you plant the seedlings out, put a mulch layer below the seedlings and remove all the lower leaves. Try and train your plants up a trellis to get them as far away from the ground as fast as possible. Early blight thrives in humid wet environments. If possible, irrigate your tomatoes with drip irrigation, or by pouring water on the root zone without splashing. This avoids the spores from splashing up onto the plant.
If you follow these steps you should be fine.
This is an irritating fungus that lives in the soil. It infects the roots and then spreads up the stem blocking the flow of nutrients to the leaves. You will see lower leaves going yellow, and brown streaks on the stem, and eventually the entire plant dies.
The common wisdom is to take any infected plants and dispose of them by putting them in the trash. I did the opposite. I took my infected plants, composted them, and then I used that compost to grow tomato seedlings by dumping any damaged fruit I had on my plants in the fusarium heap. Over time, I have now developed a cherry tomato that is completely everything resistant. It seems to not get diseases, as it grew up in the disease compost heap.
For other tomato strains, such as my Crimean Black, I have found that using a bit of Trichoderma fungal root inoculant in the tomato soil mix really helps, as this fungus is beneficial and blocks the sites by which fusarium fungus can enter the tomato roots.
Rust disease – Yellow leaves with brown spots
I have had this cause yellowing on tomato leaves in the past. It is a minor nuisance caused by the fungus Puccinia pittieriana. The basidiospores of these rust fungi lurk in the soil and certain host plants and will get onto your tomato leaves under humid wet conditions and germinate causing spots and yellow patches on the leaves. I used neem oil and it seemed to control the problem. I also stopped some sprinklers from getting the tomato leaves wet – I think the combination of reducing leaf wetness, and the neem works well.
Some writers seem to have a big problem with rust – in my experience, it is a minor nuisance that flares up every year or two and is easily controlled with neem oil.
8) Virus issues – Yellow spots on tomato leaves
Viruses are a pain in the neck as our recent pandemic event has demonstrated. There are a plethora of viruses that infect tomato plants, with the mosaic viruses causing a yellowing mosaic pattern on leaves. These viruses typically need a vector and a source of infection to be a problem. Hence if you buy an infected plant from a nursery, and then you have aphids, thrips, spider mites, white flies, or some other creature that moves around making holes in one leaf, sucking juice, and making holes in another plants leaf doing the same thing, it will move the virus around.
I tend to view virus problems as more often than not a symptom of not controlling other pest problems. Make sure that your pest management plan is in place, and that it includes pre-emptive treatments with neem oil to control thrips, aphids, whitefly, and so on before they get out of hand and carry every virus on earth all over your garden.
I also try to grow virus-resistant tomato genetics – you can see if the seeds you buy are TMV, CMV, etc resistant. Generally, a resistant genetic is a good idea, as this places less stress on you.
9) Magnesium – Yellow leaves with brown spots
I have found this to be relatively uncommon, as normally we formulate tomato soil to be pretty rich in nutrients. However, in the unlikely event that your soil does get leached and become Magnesium deficient (in which case it will also probably be Potassium deficient), you will notice that on older leaves you get round to diamond-shaped yellow spots developing between veins, and these can then start to die in the middle.
You can easily treat this by putting one or two teaspoons of Epsom salts in a 1-gallon watering can and sprinkling this on the leaves. I regularly have Epsom salt baths and keep the bath water sprinkled around the garden. If you do this, you get to derive the benefit of having a nice healthy Epsom salt bath, and then you can use that water to water your garden and correct Magnesium levels in your soil.
A few additional notes
Coming up with an exhaustive list of all the things that could go wrong with tomatoes is really difficult. If you mix good soil, with compost, pearlite, and a healthy potting mix it should be able to support a healthy crop. All sorts of little wobbly micro-nutrient deficiencies can creep in here and there – Iron, Zinc, etc. The more interesting things you feed your tomatoes, the healthier they tend to be.
It is also possible to overfertilize a tomato with Nitrogen. In this case, it will also go yellow. Generally, using organic slow-release fertilizers should avoid any risk of overfertilizing, but if you don’t follow label instructions and just go a bit crazy applying even these relatively harmless substances, you can end up burning a plant. I have done this with kelp extract when I was a teenager when I just mixed it like a cowboy and didn’t read the instructions. My tomato turned bright yellow and died shortly afterward.
Tomato Seedlings Turning Yellow
Often when we buy tomato seedlings they come from industrial plant factories. These factories use the cheapest way to mass-produce seedlings which is to mechanically insert seeds into little coir blocks in seed trays and then hydrate these with a hydroponic solution. The seedlings germinate and absorb nutrients from this solution. The coir cannot provide anything other than a place for the roots to grow.
When the seedlings are ready to ship, the company sends them to the nursery, where they are invariably watered, but not fed nutrients. The result is that the seedlings become nutrient deficient in about a week, and start turning yellow. Buying such seedlings is a waste of time as you are buying a plant that has been stunted. Do not buy yellow tomato seedlings. Even if you get a good discount. I would not even take them for free. The seedlings will be weak, susceptible to disease, and just invite failure into your life.
In Conclusion: Why Are My Tomato Leaves Turning Yellow?
If you don’t fertilize enough with nitrogen or a lack of nutrients in the soil, such as potassium and magnesium you can get yellow leaves. Too much water is a common problem that can lead to root rot, inconsistent watering, and under-watering can also stress plants. Pale leaves are a sign of nitrogen deficiency, and by running through the remaining diagnostics listed above you can work out if you have incorrect watering, or fungal, viral, or micronutrient problems.
All in all, growing tomatoes is easy if you build good healthy organic soil, with slow-release balanced nutrient sources that will nourish the plant throughout its growth cycle. Do not be shy to mulch, and add manure and compost as you go along – tomatoes enjoy being fed well. And last but not least, remember that pee is a natural, sustainable source of nitrogen that is excellent for tomatoes – and you currently waste good water and money paying to flush it away!
Dr. Garth A. Cambray is a Canadian/South African entrepreneur and beekeeper with 28 years of experience in apiculture and specializes in adding value to honey. His Ph.D. research developed a new advanced continuous fermentation method for making mead that has resulted in a number of companies globally being able to access markets for mead. His company, Makana Meadery, exports honey mead to the USA where it is available to discerning connoisseurs. He has also developed technologies to commercially manufacture organic honey vinegar in Zambia for export globally. He holds a few patents globally in the ethanol industry and believes in technology and knowledge transfer for human development and environmental sustainability. One of his proudest achievements is the fact that the wind farm he started at one of his old apiary sites has essentially made his hometown carbon neutral.