If the leaves on your tomato plants are turning purple, then you are probably wondering what is wrong, and how to fix the problem.
So, why are the leaves on your tomato plant turning purple? A phosphorus deficiency will cause the leaves on a tomato plant to turn purple, especially if the plant is young or if temperatures are low (below 60 degrees Fahrenheit). Exposure to intense light can also cause purple leaves on tomato plants. There are also pests and diseases that can cause 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. Let’s take a closer look at the causes, signs, and remedies of purple leaves on tomato plants.
A phosphorus deficiency is one common cause of purple leaves on a tomato plant. Phosphorus is an important nutrient for the growth of roots and flowers in a tomato plant, so it is important to identify the cause of the phosphorus deficiency and address it quickly.
Lack of Phosphorus in Soil
If there is not enough phosphorus in your garden soil, then your plants will not have enough to absorb for proper growth.
If you plant the same crop in the same location every year, you can end up with any number of nutrient deficiencies, including 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.
You can also end up with nutrient deficiencies in your soil if you fail to treat your garden with compost, fertilizer, or other supplements. Think about it: every year, your plants are absorbing nutrients from the soil, so you should be replacing those nutrients every year.
One good way to replace nutrients in your soil, including phosphorus, is by adding compost to your garden. You can make compost by putting grass clippings, leaves, and fruit or vegetable waste in a pile, and allowing it to decompose until it looks like black soil.
Adding compost to your garden has the added benefit of replacing organic material, which 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.
You can also use chemical fertilizers in your garden to replace nutrients. However, remember that fertilizer will not replace organic material. Also remember that it is possible to over fertilize your plants – for more information, check out my article on over fertilizing plants.
You can also add bone meal to your soil to act as a phosphorus supplement. Bone meal is available online or at garden centers.
Before you go adding supplements to your soil, it is a good idea to do a soil test. You can either buy one online or at a garden center, or send a soil sample to your local agricultural extension office for testing.
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…
Improper Soil pH
Every plant has an ideal pH range for optimal growth. Most plants like soil that is slightly acidic, which means a pH from 5.5 to 6.5.
If your soil pH is too high or too low, then it can interfere with a tomato plant’s ability to absorb nutrients from the soil, even if there is plenty of that nutrient available in the soil.
This can lead to nutrient deficiencies, including phosphorus deficiency and the resulting purple leaves. For more information, check out this chart from Research Gate, which shows the connection between pH and nutrient availability.
As mentioned above, you can do a soil test yourself 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. However, this will add calcium to your soil, and too much calcium can 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 can prevent your tomato plant’s roots from absorbing phosphorus, even if there is plenty of phosphorus in the soil. If soil temperatures are below 60 degrees Fahrenheit, then you may want to wait on planting or transplanting your tomatoes.
One alternative is to start seeds indoors and keep the soil and seedlings warm enough to avoid phosphorus deficiency. If the seedlings are mature enough, but the soil is still too cold, you can transplant them into a cold frame or greenhouse.
That way, they can stay warm as they get used to more sunlight. 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. This will retain heat from the sun in the 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.
Exposure to Intense Light
If your plants are exposed to intense sunlight or artificial light when they are young, they can develop purple leaves. This is due to anthocyanin, which is a flavonoid that can cause purple leaves in tomato plants.
Often, young plants will grow out of this condition. One way to distinguish excessive light from phosphorus deficiency is that plants with a phosphorus deficiency will have stunted growth and poor flower production, in addition to purple leaves.
Many pests can stress your plants and cause problems. 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.
Psyllids, on the other hand, are a pest that directly causes yellow leaves with purple veins on tomato plants. These insects feed on the sap from tomato plants, while injecting their saliva into the plants.
This saliva contains a toxin that causes veins to turn purple. If you suspect 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.
To learn more about psyllids, check out this information from the Colorado State University Extension.
There are a few different diseases that can cause tomato plant leaves to appear purple. Let’s review a few of the common ones.
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.
This virus is transmitted by the beet leafhopper, which can migrate over a distance. There are no effective treatments, so prevention and containment is the key.
If you find plants infected with curly top virus, remove them from the garden and destroy them. Do not place the discarded plants in your compost pile.
The virus can survive a season (or more) in your compost pile, and then 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, and any fruit that forms may have yellow spots or rings. Western flower thrips are responsible for spreading the virus.
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 purple leaf disorder. Often, this will only happen to leaves exposed to sunlight, with shaded leaves left unaffected.
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, and how to treat the problem, and how to prevent it in the future.
I hope this article was helpful. If you have any questions or ideas of your own, please leave a comment below.