Organic Gardening

My Tomato Plant Leaves Look Gray/Purple: What is the cause?

Identifying issues with your plants can be quite disconcerting and you may not immediately know the reason why your tomato plant’s leaves are changing color. It is important to troubleshoot the problem quickly.

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Introduction

While gardening, things may not always go as planned. One minute your tomato plant is looking green and beautiful, but a couple of days later, you may notice a purplish/grey tinge to the leaves. Before we look at ways to remedy the purple leaves, we must first examine why the leaves are turning purple in the first place. 

A plant can communicate to us through a variety of ways. It may begin to wilt, signaling a need for water. The plant could fail to thrive and appear stunted in comparison to other plants. Its leaves change a different color depending on what it is experiencing. For example the leaves can turn yellow or brown. It may be growing strong and fruit is abundant, showing that it is happy with its environment. These are signs that the plants are trying to communicate to us whether it is doing well or not. As a gardener, it’s important to watch for these changes in our plants to ensure they thrive.

Tomato Plant and Basil plant in wine barrel
Tomato plant with phosphorus deficiency

In this picture are tomato and basil plants in a wine barrel planter. The basil plant is the forefront of the photo, with the tomato plant in the back. If you look closely at the tomato plant leaves, you’ll notice a grey-ish/purple tinge to them. This is a sign of phosphorus deficiency. The tomato plant was purchased from a nursery nearby. The soil conditions in the planter were not identified as the issue and may have been from its grown conditions or exposure to full sunlight too quickly and for long periods of time. At the nursery, it was in a greenhouse and not in full sunlight.

Let’s take a look at the causes and remedies for phosphorus deficiency.

What is causing my tomato plant’s leaves to turn purple?

There are a few reasons why your tomato plant may have purple leaves that will help determine the cause of the discoloration:

  • Phosphorus Deficiency
  • Potassium Deficiency
  • Light Intensity
  • Pests
  • Diseases

An article published in Plant Physiology, “What’s behind Purple Tomatoes? Insight into the Mechanisms of Anthocyanin Synthesis in Tomato Fruits” sheds light on the effect of environmental stresses on plants and phosphorus uptake.

“When plants are exposed to different types of stresses, such as high light, heavy metals, salty soils, drought, high/low temperatures, as well as pathogen or insect attacks, there is typically an increase in reactive oxygen species (ROS) production, which may then be counterbalanced through the generation of antioxidant molecules (Sharma et al., 2019). Anthocyanins, which primarily accumulate in the subepidermal cell layers of vegetative tissues, act as a photoprotective light screen, by absorbing potentially damaging UV-B radiation (Gould, 2004). They can also act as antioxidant compounds through different chemical mechanisms, such as releasing hydrogen atoms from the hydroxyl groups or as metal-chelating agents, thus efficiently scavenging free radicals and ROS (Gould, 2004), (Colanero, 2020).” 

Colanero, S., Perata, P., & Gonzali, S. (2020). What’s behind Purple Tomatoes? Insight into the Mechanisms of Anthocyanin Synthesis in Tomato Fruits. Plant physiology, 182(4), 1841–1853. https://doi.org/10.1104/pp.19.01530 

Let’s examine each of these causes, along with the solutions to remedy them.

How to Determine the Cause is Phosphorus Deficiency

Phosphorus is an essential macronutrient for plant growth, production, and development. To determine whether your plant(s) is/are suffering from phosphorus deficiency, you’ll notice discoloration of the leaves. Phosphorus deficiency can cause a plant to have stunted growth and/or prevent any new growth. The leaves will turn a dark, dull, blue-green color, and they may even turn pale if there is a severe deficiency. The leaves can turn various shades of purple or red. Leaves will appear small and their root systems may be shallow. This is known as anthocyanin discoloration.

“Discoloration and color-changing phenomena have been observed in plant tissues during development (Oren-Shamir, 2009). Anthocyanin discoloration might be due to either anthocyanin reduction in plant tissues or to structural changes of the anthocyanin molecule that leads to a loss of color. (Liu et al., 2018)”. 

Liu, Y., Tikunov, Y., Schouten, R. E., Marcelis, L., Visser, R., & Bovy, A. (2018). Anthocyanin Biosynthesis and Degradation Mechanisms in Solanaceous Vegetables: A Review. Frontiers in chemistry, 6, 52. https://doi.org/10.3389/fchem.2018.00052 

How To Treat A Lack Of Phosphorus In Soil

Having healthy soil is the best course of action. It is important to make sure we’re testing soil as a preventative measure. By testing soil, it allows us to see the composition of our soil – what minerals exist in the soil currently and what it is lacking. A good soil testing company can provide you with steps as to exactly to amend the soil.

Phosphorus is normally abundant in soil. It might not be available for plants to absorb because of cold soil temperatures. Avoid planting when it is too cold. Cold and wet soil conditions reduce the growth rate of roots which thereby prevents the plant from accessing the phosphorus available in the soil.

For example tomatoes do not like temperatures lower than 60 degrees Fahrenheit. Colder temperatures can stunt the plant, slow growth, or even kill the plant. By waiting for the appropriate time of year to put your plant in the ground outside, the issue can be avoided. If you must plant tomatoes whether you’re growing your own seedlings from seeds or purchased seedlings, keep them indoors, use row covers, or use a greenhouse.

pH levels in the soil can be too low or too high. Check the pH levels of your soil to make sure it’s an environment that encourages your plants to grow. pH levels can be affected by the following:

  • Microorganisms
  • Pesticides
  • Heavy metals
  • Soil corrosivity
  • Mineralogy
  • Climate
  • weather

Most plants enjoy a pH level of 6 to 7. Liu et al. found that even changes in pH levels, from either acidic or neutral, can lead to a complete change in the color of the plant. By adjusting the pH of the soil, the color change can be reversed. If levels are low, it can indicate that there is a lack of calcium, magnesium and phosphorus. Solubility of aluminum, iron, and boron are high. If pH levels are high, calcium and magnesium are higher. There may be low levels of iron, manganese, copper, zinc, phosphorus, and boron.

I unfortunately have recently had seedlings that I purchased that had phosphorus deficiency. Initially, I did not know what was causing the discoloration, but noticed that they were also stunted in growth. I was not able to remedy the deficiency and took the seedlings as a loss.

Further Remedies

For healthy plants, the implement the following:

  • Add compost to your soil
  • Use amendments, such as lime to adjust pH levels
    • Liming is a common way to increase pH levels
    • For soil with a high pH level, applications of phosphorus, iron, copper, and zinc are the most efficient ways to amend it
  • Plant during appropriate times of the year and avoid temperatures that the plants can’t tolerate.
  • Apply sources of phosphorus like manure, bone meal, or fertilizer (there are organic fertilizer sources for those that are growing organically).
Tomato seedling in a tomato cage in a garden
Healthy Homegrown Tomato Seedling

Potassium Deficiency

Potassium is an important nutrient for plants and helps to regulate growth, as well as promote photosynthesis. Just as with phosphorus deficiency young tomato plants are unable to absorb potassium from cold soil. The leaves may turn purple or yellow if planted in cold soil. When the outside temperature and soil become warm, the leaves may become green. However, the plant will remain stunted and produce little to no fruit.

Treatment for potassium deficiency

To amend potassium deficiency, give the plant fertilizer that has a high amount of potassium. Plant during appropriate times of the year and avoid temperatures that the plants can’t tolerate.

Plant

Light Intensity

Lighting can be too intense (both artificial and sunlight) for a plant. When lighting is too intense it can cause a discoloring of the leaves, which is a result of damage. The plant then produces anthocyanins to hopefully absorb damaging UV-B radiation. Intense light also inhibits root formation.

How to Adjust for Light Intensity

Artificial Lighting

  • The artificial lighting may be too close to the plant. Adjust the height of the lightning to reduce damage.
  • Use light bulbs that are less intense.

Planting outdoors and Intense Lighting

  • Before transplanting seedlings outside, it is important to ‘harden off’ the plants. ‘Hardening off’ refers to providing limited exposure to sunshine, and slowly increasing the plant’s exposure over a given period of time, usually 7 to 14 days. The process of ‘hardening off’ also exposes the seedling to environmental variations, such as temperature, rain, and wind, that it was not previously exposed to before. Remember, when seeds are started and grow into seedlings in your house, they are grown in a controlled environment. Exposing a plant too quickly to sunlight for extended periods of time can cause sunburn. Sunburn to the plant can be so severe that it can cause the plant to die. So just like a human can become sunburnt, so too can your plant.

Use row covers or a greenhouse

  • Row covers and greenhouses can help reduce the amount of light a plant is exposed to.

Pests

It is very important to check and inspect plants often and pay close attention to any changes. Bugs are very sneaky and like to hide. They can usually be found underneath the plant’s leaves and even lay their eggs. There are a number of ways to deter pests without having to use chemical pesticides and herbicides.

Preventative Measures

Use row coverings

Row coverings are not just for maintaining temperature or protecting a plant from intense light. They can be used to protect plants from pests as well. A tightly woven fabric can keep out all types of insects.

Hand pick insects

  • Hand picking pests is a great preventative to damage to the plant. By physically removing the bug, you can possibly prevent it from laying eggs.

Attract predatory insects

  • There are some insects that naturally prey on others that are deemed pests. They are actually a friend to the gardener as they help keep unwanted foes’ numbers down. For instance, wasps prey on mosquitoes. Ladybugs eat aphids. Praying mantises eat cockroaches, crickets, spiders, beetles, and other insects.

Use mixture of essential oils

  • Essential oils are a great tool in the garden. There are many oils that deter insects. Caution should be used when applying oils. It is best to treat plants with essential oils in the morning or in the evening. If oils are applied in direct sunlight, they will cause the plant to become sunburnt and may kill the plant.

Plant Certain Herbs and Flowers

  • Certain herbs and flowers deter pests from the garden.

Here are some examples of herbs and plants that help:

Plants:

  • Citronella grass
  • Lemongrass
  • Marigolds
  • Chrysanthemums 
  • Petunias

Herbs:

  • Mint
  • Basil
  • Lavender
  • Chives
  • Rosemary
Healthy tomato plant in tomato cage
Healthy Tomato Plant with cherry tomatoes

Diseases

There are diseases that can cause the plant to develop purple/gray leaves. A few examples are:

  • Curly Top Virus
  • Tomato Spotted Wilt Virus/impatiens necrotic spot tospoviruses
  • Tomato Purple Leaf Disorder

Curly Top Virus

There are more than 300 plant species (tomatoes, peppers, potatoes, etc) that are susceptible to curly top virus besides tomato plants. It is an extremely destructive plant disease that is spread by the beet leafhopper. The virus is picked up by the insect quickly, but its incubation period to be transmitted from the insect to the plant depends on the temperature. 

Characteristics of infection are:

  • Curling leaves with a grey/purple color and purple veining
  • Stunted growth
  • Fruit ripens prematurely and is deformed, dull in color, or wrinkled. The fruit has an odd taste.
  • Death of the plant

Tomato Spotted Wilt Virus/impatiens necrotic spot tospoviruses

The virus was first identified and an issue in commercial farming, but is now found in home gardens. It’s transmitted by the western flower thrip. 

Characteristics of the infection:

  • It causes dark brown or purple spots 
  • Stiff leaf tissue
  • Yellow rings or spots on the fruit
  • Fruit may be deformed

Tomato Purple Leaf Disorder

This disorder presents itself in plants when it has been exposed to too much sunlight. The tops of the leaves will turn purple, but the veins will not. After some time, the bottom of the leaves will turn purple.

Treatments for these viruses and disorders

Once a plant has a virus, many times the plant will not be able to get rid of it. The best treatment for the three mentioned diseases is to introduce row coverings. 

What other types of diseases can tomato plants have?

Here is a general list of other diseases to look out for with your tomato plants:

  • Early Blight
  • Late Blight
  • Bacterial Wilt
  • Septoria Leaf Spot
  • Leaf Mold
  • Bacterial Spot
  • Tomato Pith Necrosis
  • Buckeye Rot
  • Anthracnose
  • Fusarium Wilt
  • Southern Blight
  • Seedling Disease (Damping-off)
  • Tomato Spotted Wilt
  • Tomato Yellow Leaf Curl
  • Other Viruses
  • Root-knot Nematodes

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