Black Spots on Basil leaves

7 Causes of Black Spots on Basil Leaves? (With Solutions)

It might be shocking to see Black spots on your Basil Leaves. It is natural that you would want to know the reasons and the likely solution to make your Basil lively and Healthy.

A basil plant might get black spots anywhere on the leaves: on the center, on the edges, or even on the underside of the leaves. This can happen due to varied reasons like pests, fungi, sunburn, overwatering, etc.

There are a lot of varieties of basil, and most of them are known to be prone to black spots on their leaves. The good news is that most of the time these black spots are curable.

The right solution for it would mean you have to find out the cause of these black spots on basil. Different types of problems would cause different types of black spots and would require different solutions.

Quick TipBlack spots on Basil leaves due to Fungi are generally dry whereas spots due to bacteria are typically waxy.

Let us look at the list of causes in detail and solutions that can help you removing the black spots and rejuvenating the basil plant.

1) Pests

A lot of the time, the reason for black spots on the basil leaves is pests who feed on its leaves.

If Pest is the cause of black spots on basil leaves, there will be lots of tiny, needle-sized black dots. The spots may also be greyish instead of being fully black.

To confirm that the spots are indeed the work of the leaf miners, look for yellow or white patches on the leaves around the spots.

If the spots are feces, then the leaf miners might have eaten your basil leaves. The white or yellow area will be dry to the touch and is probably a sign that the plant is invaded by leaf miners or pests.

But to be sure, you will need to actually find the insects residing on your plant as sometimes a pest infestation is mistaken for a fungal infection because it looks very similar.

Thrips, aphids, spider mites, whiteflies are some of the pests that can be found on the basil plant. They lay their eggs and reside in the leaf, eating away the leaves and destroying the plant.

Thrips

Thrips are minute (almost 1 mm long) insects with wings with which they can fly. They lay eggs that are around 0.2 mm so it would be difficult for you to identify.

Pic Source – Ian

Easy way to identify thrips on Basil

  1. Shake a few leaves over a white paper.
  2. Tiny specks will fall down on it.
  3. If the shape is a bit oval and long (like a small grain of rice), they are thrips.
  4. Look for small bumps on the underside of the leaves. They tend to lay eggs on the soft tissue.

Aphids

Aphids are soft-bodied and small about 7 mm long. They are often green and wingless. With their long thin mouthparts, they pierce the leaves and soft stems to suck out fluids.

Apart from sucking the vital fluids, they do additional damage by secreting a sticky, clear substance called “honeydew” that helps in forming a mold. This tends to limit the ability of the plant to photosynthesize.  

Aphids
Pic Source – Sean

Easy way to identify Aphids on Basil

  1. If you notice ants on your basil, it can be due to honeydew from aphids to which they are attracted.
  2. Another sign is the white or yellow sticky substance called “honeydew” on the leaves or stems.

Spider Mites

They are one of the smallest of the pests affecting basil and are less than 1 mm long. They are spiders and cannot fly.

Signs of a spider mite infestation on Basil are

  1. Silky webs woven around leaves and stems
  2. Put a white sheet of paper and Shake a few leaves above. If you see dust-like particles moving their own, you know that you have spider mite infestation.

White Flies

As the name suggests, they are white in color. They are small, somewhat triangular in shape. They have wings and can fly. Whiteflies love to feed on the underside of the leaves.

Quick Tip – White Flies do not like col weather and are not able to survive in USDA Zone 7 and colder.

Tips to identify White Flies on Basil

  1. Shake the plant and look for the flying insects closely.
  2. Check on the underside of the basil leaves where they lay eggs often in a circular pattern.

Solution for Pests

  • Remove infected leaves – Firstly, remove the parts of the plant that are infected by the pests. Used sterilized shears to prevent the spread of any harmful pathogens.
  • Remove eggs – It is necessary to physically remove the eggs that reside on the leaves. You can do this with the help of insecticides or wipe the leaves with a solution of soapy water for 2 to 3 days.
  • Use sticky traps – You can use sticky traps to catch the pests roaming around to find a place to grow their eggs.
  • Use row covers – Row covers are mostly made of a mesh net, which protects your plants from pests while also being breathable enough for the herbs to grow well. It is also a cost-effective and non-toxic way to keep pests away.
  • Neem oil – It is possible to get rid of many of the pests with the help of neem oil or insecticidal soap.

2) Bacteria

Pseudomonas Cichorii is the Bacteria that affects the Basil. If the black spot on basil is due to bacteria, then the spot area would be moist or waxy.

This is a bacterial disease that develops in high humidity and high levels of nitrogen in the soil. Therefore, the main causes for this bacteria to develop are overwatering and over-fertilization.

This bacteria may carry through spores and splashing water. If basil is impacted by this bacteria then the black spots are large. Eventually, the leaf material dies and the spots are replaced by holes.

Treatment for bacterial infections:

There isn’t a foolproof solution for bacterial infections.

  • You can try to change the environment and the soil. Try to use natural fertilizers.
  • When the plant is dry, remove the infected leaves or parts and destroy them.
  • Copper-based products to treat the bacterial infection can provide some protection.
  • Do not save the seeds of the infected plant for next year.

Protection against bacterial infection:

  • Keep adequate space between the plants for good airflow.
  • Water early in the morning so that the water has time to evaporate.

How to grow Basil from Cuttings? – Read here

3) Fungi

Fungal spores are spread through water splashes and can also be carried to the next season through infected seeds or plant debris.

If black spots on basil are dry, the cause can be fungal infection.

The two fungi mentioned below are often the main culprits behind the spots.

Cercospora Leaf Spot

This fungal infection is caused in sweet basil by Cescospora Ocimicola. This disease also thrives in a humid and moist environment.

Cercospora-Leaf-Spot-on-Basil
Source – https://plantvillage.psu.edu

The black spots are large and round with a yellowish ring around them. They soon develop a grey or white center in the middle of the black area, which soon gets replaced by a hole as the dead material falls off.

The main reasons for the spread of this fungus are watering the basil frequently from the top making leaves moist, and an environment that isn’t ventilated properly, creating humidity.

Downy Mildew

Downy Mildew is a new basil fungus that has been discovered plaguing basils only recently. It is highly contagious and can travel through the wind, water, or carried on in objects. Basil Downy Mildew also grows in moist conditions and humidity.

It is caused by a pathogen called Peronospora belbahrii and is transmitted through spores that are not visible to the naked eye. A greenhouse is a good place for this pathogen to develop.

The first sign of this fungus is seen directly underneath the leaf. Basil infected with Downy Mildew has a yellowish tinge, first appearing in major veins and then spreading through the leaves.

The leaves will have black spots and grey fuzz on the underside. The plant soon turns completely yellow and eventually dies.

Other Fungi

Fungal infections in basil can also be caused by Alternaria, Colletotrichum, or Septoria fungi.

  • Alternaria spots are dark brown or black in color with a yellow lining.
  • Septoria spots are small and grayish brown with a dark brown border.
  • Colletotrichum shows dark spots on the leaves which die and leave a hole.

Treatment for fungal infections

  • There are many fungicides available that can kill fungal spots. Minor fungal infections can be treated with a fungicide containing potassium bicarbonate.
  • Trim the infected part and treat the rest of the plant.
  • Remove and destroy severely infected plants and debris to prevent the infection from spreading.
  • The only solution to Downy Mildew is to remove the plant and destroy it completely. It is important to prevent the spores from spreading by covering them with plastic first. They can also contaminate the soil.
  • However, if you cannot destroy the whole plant, remove the infected parts, even the ones that have dropped, and destroy them completely. Then apply a fungicide containing phosphoric acid to the plant’s stems and to new growth that appears.

Protection against fungal infection

  • Keep the plants and soil well-aerated. Trim the leaves regularly and provide plenty of sunlight.
  • Water the plant in the morning at the base.
  • Water the soil, not the leaves, so that the plant doesn’t remain damp.
  • Applying mulch prevents the lower leaves from touching the soil. It also acts as a ground cover by preventing water splashes.

Why is my Basil bitter? How to fix it?

4) Poor nutrition

It is important for the soil and the plant to have proper nutrition. Poor nutrition can result in the leaves darkening or getting dark spots.

This happens mainly due to the overuse or underuse of nitrogen in the soil.

Solution

  • Organic, balanced compost can help to prevent the imbalance and make for healthy, rich soil.
  • Keeping the soil moist and well-drained will also help the basil plants to thrive.
  • Giving the soil kelp every 7 to 10 days will help provide a balance of micro and macronutrients like nitrogen, phosphorous, and potassium.
  • Space your plants to give all of them plenty of access to nutrients.
  • Drain and prepare the soil before planting, with a pH between 6.0 and 7.0.

5) Sunburn

Sunburn is a very common cause of black spots on basil, especially in regions where the summers are extreme.

Exposure to too much sunlight or for too many hours can cause the leaves to literally get burnt because of the sun. This results in yellow areas or black spots on the basil leaves.

The yellowing of the leaves due to the excess sun is a result of mild stress and there is a chance for revival.

The black areas, however, mean that those leaves are already dead.

Treatment and protection

  • It is important to keep the soil well-drained and moist so that the basil thrives.
  • Make sure to shift the plant to a cooler place if the sun is too harsh or if it has been exposed to the hot sun for many hours.

6) Frost

Basil is an expressive, delicate plant that requires plenty of sunlight to thrive. Even the slightest of cold can bring about a change in the plant. It is highly sensitive to frost and prolonged exposure to cold conditions will likely kill the plant off.

The optimal temperature for basil is at least 50°F (10°C) and around 70°F (21°C) for the best growth since it is a tropical plant.

If there is a drop in temperature and you notice your basil leaves are turning yellow and then black, it is possible that it is because of the cold.

Treatment

If the cold has damaged only a few of the leaves, then it is possible for the plant to survive.

In that case, just trim off the damaged leaves before it spreads to the whole plant. Then it is important to keep the plant at a temperature suitable for its growth.

Protection from the cold temperatures

  • Cover the plant with bell jars or hoops. The case will protect it from the cold weather.
  • You can harvest as many of the leaves as possible. It is recommended to preserve by drying or freezing them for future use.
  • Try acclimatizing the plants before shifting them outside. Keep them outside for a short period daily so that they get used to the outdoor temperature.
  • You can also move an outside plant to some covered location or inside if it is too cold so that it stays warm during the cold.
  • Keeping the soil moist keeps it warmer and retains more heat.

7) Overwatering

Overwatering is a very common reason in basil plants for developing black spots because the water requirements of basil are very specific.

If there is too much moisture in the soil, the roots of the basil get clogged and start rotting.

This results in the stems and leaves of the basil turning black, as the roots are not able to absorb the nutrients required by the plant to grow well.

Solution

Here is a post on how to water the Basil the right way.

If it is at an early stage, you can transfer the plant into another pot with well-drained soil where the roots can breathe.

Prevention is better than cure. It is always a better option to safeguard your plants so as to minimize the chances of any infections or infestations.

Popular post – Good and Bad Basil companion Plants

FAQ

Can you eat Basil leaves with black spots?

Even though, technically it is safe to eat the basil if you have gotten rid of the brown or black spots.

However, they can be slimy and bitter in taste. Further, everyone has different levels of immunity and you don’t want yourselves to get infected.

So we definitely donot recommend eating Basil leaves with black spots.