When you see purple leaves on tomato plants, it is natural to worry about their health. Luckily, there are ways to treat the problem if you see it in your garden.
So, why are the leaves on your tomato plant turning purple? Tomato plant leaves will turn purple due to a phosphorus deficiency. This can happen if the soil lacks phosphorus or if the plant cannot absorb enough phosphorus from the soil due to a pH imbalance. Purple leaves on tomato plants due to phosphorus deficiency is more common in cold soil (below 60 degrees Fahrenheit). Exposure to intense light can also cause purple leaves on young tomato plants. In addition, certain pests (such as psyllids) and diseases (such as tomato purple leaf disorder) cause tomato leaves to turn purple.
Once you figure out what is causing purple leaves on your tomato plants, you will need to take steps to fix the problem.
In this article we’ll talk about the causes of purple leaves on tomato plants. We’ll also discuss how to treat each of these causes to solve the problem.
Causes of Purple Leaves On Tomato Plants
There are several possible reasons that a tomato plant’s leaves will turn purple, including:
- Phosphorus Deficiency – this can happen in a few different ways.
- Intense Light – this can happen indoors or outdoors.
- Pests – they can cause stress and transmit diseases.
- Diseases – some can cause the leaves to turn purple.
Let’s start with one that you can easily rule out with testing: phosphorus deficiency.
Purple Tomato Leaves Due To Phosphorus Deficiency
A phosphorus deficiency is one common cause of purple leaves on a tomato plant. According to the Colorado State University Extension, a phosphorus deficiency causes the veins on tomato leaves to turn purple.
The rest of the leaf (not just the veins) may also turn somewhat purple, more so on the undersides of the leaf. According to the University of Maryland Extension, phosphorus deficiency makes the older leaves (on the bottom of the plant) turn purple first.
A phosphorus deficiency is more likely early in the season. At this stage, the soil is still cool, and a tomato plant’s roots are not well developed.
Phosphorus is an important nutrient for the growth of roots and flowers in a tomato plant. For this reason, is important to quickly identify the cause of the phosphorus deficiency and address it.
There are three main reasons that a tomato plant will suffer from a phosphorus deficiency:
- Lack of phosphorus in soil – there is not enough phosphorus for the plant’s root to absorb.
- Soil pH imbalance – there is phosphorus in the soil, but extreme pH prevents the plant’s root from absorbing it.
- Cold soil – low temperatures prevent the plant from absorbing phosphorus.
All of these can be checked with a soil test (first two) or a thermometer (last one).
Phosphorus Deficiency Due To Lack Of Phosphorus In Soil
If there is not enough phosphorus in your soil, your tomato plants will not absorb what they need for proper growth. There are two main reasons that you may have a phosphorus deficiency in your soil:
- Lack of crop rotation
- Failure to restore nutrients and organic material
If you plant the same crop in the same location every year, you can end up with a phosphorus deficiency. To prevent this, use crop rotation.
Crop rotation simply means that you plant a crop in a different part of your garden each year. For example, you could plant beans this year in the spot where you planted tomatoes last year.
A failure to restore nutrients can also lead to a phosphorus deficiency in your soil.
How To Treat A Lack Of Phosphorus In Soil
Think about it: every year, your plants are absorbing nutrients (including phosphorus) from the soil for their growth. In response, you should replace those nutrients every year by adding the following:
- Compost – this is your first line of defense against nutrient deficiencies. Compost adds both nutrients and organic matter to your soil.
- Aged Manure – this can give your plants a nice boost of nutrients and organic matter, just like compost.
- Fertilizers and Supplements – this is what you would use if there is still a nutrient deficiency after using compost and manure.
You can make compost by putting grass clippings, leaves, and fruit or vegetable waste in a pile. Then, leave the pile to decompose until it looks like black soil.
Adding compost to your garden has the added benefit of replacing organic material. This is important if your soil is dry or does not drain well.
Manure is another good way to add both nutrients and organic material to your garden. Manure from chickens, cows, and horses are all good sources of nutrients for a garden.
Just be sure to allow the manure to age before using it. Otherwise, the salts in the manure could burn your plants.
Some good natural sources of phosphorus include:
- Hair – avoid stuff that is treated with product!
- Rock phosphate – this is what is often used in large-scale agriculture.
- Bone or fish meal – making sure nothing goes to waste! These can be found online or at garden centers.
You can also use artificial fertilizers in your garden to replace nutrients. According to the University of Maryland, applying a liquid fertilizer to the root zone will work to treat a phosphorus deficiency.
You can also spray the leaves of a tomato plant with a liquid solution, known as foliar feeding.
Just remember that artificial fertilizers will not replace organic material in the soil.
Also, keep in mind that it is possible to over fertilize your plants. For more information, check out my article on over fertilizing.
Before you go adding supplements to your soil, it is a good idea to do a soil test. One option is to buy a soil test online or at a garden center.
Another option is to send a soil sample to your local agricultural extension office for testing. You can find a list of local Extension offices by state on this page from the USDA.
A soil test will tell you if your soil has a phosphorus deficiency. A soil test can also tell you if the pH in your soil is too high or too low.
Speaking of which … let’s address soil pH next.
Phosphorus Deficiency Due To Improper Soil pH
Every plant has an ideal pH range for optimal growth. According to the USDA, most plants like soil that is slightly acidic, which means a pH from 6.0 to 7.0.
Tomato plants fall into this category. According to the University of New Hampshire Extension, tomato plants prefer a soil pH of 6.2 to 6.8.
If your soil pH is too high or too low, it can interfere with a tomato plant’s ability to absorb nutrients from the soil. This is true even if there is plenty of that nutrient available in the soil!
This can lead to nutrient deficiencies in plants, including phosphorus deficiency and the resulting purple leaves.
How To Correct A Soil pH Imbalance
As mentioned above, you can do a soil test on your own with a kit, or you can send your soil away for a test to determine the pH.
To learn more, check out my article on soil testing.
If your soil pH is too low (acidic), you can raise it by adding lime (calcium carbonate) to the soil. Remember that lime will add calcium to your soil.
Too much calcium will prevent your plant from absorbing magnesium or potassium. So, take it easy when adding lime!
For more information, check out my article on raising your soil pH.
If your soil pH is too high (basic or alkaline), you can raise it by adding sulfur to the soil.
For more information, check out my article on making your soil more acidic.
Remember that adding certain fertilizers and nutrients to your soil can also change the pH, so keep this in mind when making adjustments.
Cold soil will prevent your tomato plant’s roots from absorbing phosphorus. This can happen even if there is plenty of phosphorus in the soil!
“Cool soil temperatures inhibit the activity of soil microorganisms and processes that cause nutrients to be released and made available to plant roots.”https://extension.umd.edu/hgic/topics/phosphorus-deficiency-vegetables
In other words, if the soil is too cold, bacteria won’t convert phosphorus from the soil into a form that plants can use.
How To Address Cold Soil
If soil temperatures are below 60 degrees Fahrenheit, then you may want to wait on transplanting your tomatoes. Of course, this can be a challenge in areas with a short growing season.
One alternative is to start tomato seeds indoors. That way, you can keep the soil warm enough to ensure fast germination and prevent phosphorus deficiency.
If the tomato plants are already mature enough, but the soil is still too cold, you have a few options:
- transplant into a cold frame or greenhouse
- use cloches
- use row covers
When you transplant tomato plants into a cold frame or greenhouse, they can stay warm as they get used to more sunlight. This helps with the process of hardening off.
Once the soil warms up, you can transplant the tomato plants into full sunlight in your garden.
What if you have already planted tomatoes in the garden and are facing a stretch of unseasonably cold weather?
For smaller plants, you can use a cloche (bell-shaped cover made of glass or plastic) for each one. A cloche will retain heat from the sun in the air and soil to keep the plants warm at night.
You can also use row covers for larger tomato plants, to prevent the cold from affecting them. Even a few degrees can make a big difference to your plants.
For more information, check out my article on how to protect tomato plants from cold and frost.
Purple Tomato Leaves Due To Intense Light Exposure
If your tomato plants are exposed to intense light when they are young, they can develop purple leaves. This can happen with both sunlight (outdoors) and artificial light (indoors).
The purple leaves are caused by the presence of the flavonoid known as anthocyanin. According to this article from NCBI:
“Anthocyanins … act as a photoprotective light screen, by absorbing potentially damaging UV-B radiation (Gould, 2004).”https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7140959/
In other words, when exposed to intense light, a tomato plant protects itself by producing anthocynanins that give their leaves a purple color.
How To Address Intense Light Exposure
If you are starting tomato plants indoors, then reduce the intensity of the light that they are receiving. There are two ways to do this:
- move the lights higher above the plants
- use light bulbs that are less intense than the ones you were using
If you planted or transplanted tomato plants outdoors, then there are two ways to reduce the light intensity:
- row covers – row covers allow 50% to 95% of light through. They also protect plants against cold and pests. You can learn more about row covers in my article here.
- shade cloth – shade cloths vary quite a bit in the shade they provide. Some allow 5% of light through (heavy shade), while others allow 95 % of light through (light shade). You can learn more about shade cloth in my article here.
Remember to remove the row covers or shade cloth later in the season so that your plants can get enough sunlight.
Often, young plants will grow out of this condition. Their growth is unlikely to be stunted.
On the other hand, tomato plants with a phosphorus deficiency will have stunted growth and poor flower production, in addition to purple leaves.
Purple Tomato Leaves Due To Pests
There are many pests can stress your plants and cause problems, including purple leaves.
For example, pests that attack the roots can disrupt a plant’s ability to absorb nutrients, including phosphorus. This can indirectly cause purple leaves to develop on your tomato plants.
On the other hand, the psyllid is a pest that directly causes yellow leaves with purple veins on tomato plants.
These insects feed on the sap from tomato plants, injecting their saliva into the plants as they feed. This saliva contains a toxin that causes the veins on tomato plants to turn purple.
How To Address Psyllids On Tomato Plants
If you suspect that you have this problem, you can confirm by checking the undersides of the leaves on your plant. If you see insects that are about the size of an aphid, then you likely have psyllids.
You can treat your plants by dusting the leaves with sulfur. Be sure to get the undersides, where the psyllid nymphs live.
To learn more about psyllids, check out this information from the Colorado State University Extension.
Purple Tomato Leaves Due To Diseases
There are a few different diseases that can cause tomato plant leaves to appear purple, including:
- Curly Top Virus
- Tomato Spotted Wilt Virus
- Tomato Purple Leaf Disorder
Let’s take a look at each one in turn.
Curly Top Virus
This disease is also known as beet curly top virus. If your tomato plant leaves are rolling up (towards the sun) and becoming purple in the veins, then it might be infected with curly top virus.
According to the University of Colorado Extension, this virus is transmitted by the beet leafhopper. There are no effective treatments for curly top virus, so prevention and containment is the key.
If you find plants infected with curly top virus, remove them from the garden and destroy them. This will prevent the disease from spreading to other plants in your garden.
Do not place the discarded plants in your compost pile, since the virus can survive a season (or more) in your compost pile. After that, it can return to infect next year’s tomato plants when you put your compost in the garden!
Tomato Spotted Wilt Virus
If you see dark brown or purple spots on your leaves, then your tomato plant may have this virus. These dark areas will eventually make their way to the stems of the plant.
As the disease progresses, leaves will wilt, although the leaf tissue will still be stiff. Any fruit that forms may have yellow spots or rings.
As mentioned above, any infected plants should be removed from your garden and destroyed, not placed in your compost pile.
Tomato Purple Leaf Disorder
If the top of your leaves turn purple, without the veins turning purple, then your plant may have tomato purple leaf disorder. Eventually, the bottom of the leaves will also turn purple.
This disorder can happen even when a plant has plenty of phosphorus. The severity of the problem seems to depend on the variety of tomato grown.
There is no known treatment or cure for this mysterious tomato plant disease.
Purple Tomato Leaves Due To Stress On Tomato Plants
Finally, stress on your tomato plant can cause the leaves to turn purple. For instance, stress could come from a lack of water.
If you suspect a lack of water, add organic material to your soil to help with moisture retention. If you have trouble with dry soil, check out my article on how to treat dry soil.
Also, water slowly in the morning, when temperatures are cooler and the sun is lower. This will allow the water to soak into the soil before evaporating.
Competition between plants can also cause stress. If you plant tomatoes too close together, or allow weeds to thrive, then they may face competition for nutrients.
This will lead some of your plants to exhibit nutrient deficiencies, including phosphorus deficiency and the resulting purple leaves.
To avoid this, weed thoroughly or put down a layer of mulch or grass clippings in your garden to prevent the growth of weeds.
Also, make sure to space your tomato plants appropriately at the beginning of the season: they should be 2 to 3 feet apart.
Hopefully, you have a better idea of what is causing purple leaves to appear on your tomato plants. You should also have some ideas on how to treat the problem and how to prevent it in the future.
If you are still stumped, check out this article on tomato plants from Penn State University. They suggest that nitrogen or magnesium deficiency can cause slight purple color of tomato leaves.
The following table provides a summary of the causes of purple leaves on tomato plants. It should be helpful in distinguishing the cause based on appearance.
|Purple veins on leaves. |
Rest of leaf may turn
leaves affected first.
More likely in cold
soil and young plants.
|Purple leaves. More |
likely in young plants
and with bright
indoor lights too
close to plants.
|Psyllids||Yellow leaves with |
purple veins. Psyllid
nymphs on underside
|Purple veins on |
leaves that curl
or roll upward.
|Dark purple or brown |
spots on leaves. Dark
areas later reach
stems; leaves wilt.
|Top of leaves turn |
purple, but not veins.
Underside of leaves
later turn purple.
purple leaves on tomato plants and
the appearance in each case.
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