Black Spots on Basil: What is Causing These Spots?

Anyone who has ever grown, or tried to grow, basil knows the frustration of having a beautiful plant, only to have it run into unexpected problems a few weeks or months later. One common ailment for basil is black or discolored spots appearing on the leaves. It may be difficult to identify where these spots come from so they can be adequately treated.

Black spots on basil leaves can occur for several different reasons. These spots can occur due to a temperature change, a fungal infection, or poor nutrition. It is important to have preventative measures in place so that these black spots don’t kill the basil.

Knowing how to prevent these spots from occurring and knowing how to treat them once they show up is important for any basil plant owner.

How to Treat Black Spots

Basil plants with black spots should not be consumed. Because you probably grew this plant in order to enjoy some fresh herbs, it is important to stop these spots before they occur.

Treating black spots on the plant depends on the underlying cause of these spots. It is better to have preventative measures in place for these plants rather than try and treat the plants once the black spots are already taking hold.

These preventative measures include keeping the soil of the basil plants at a regulated pH, providing regular temperatures, frequently watering the plants, and using insecticide or natural oils to keep the bugs away.

Temperature Change

Basil leaves are very delicate and can be harshly affected by environmental factors.

Frost and Low Temperatures

Basil is especially sensitive to frost and cold temperatures. Even a mild frost can have detrimental effects on the basil plants, causing their leaves to develop brown or black spots and eventually shrivel and die.

To protect your basil plants from frost, you can cover the basil plants when there is a suspected frost or subfreezing temperatures looming in the forecast. Basil plants can be covered with mason jars or tents to help keep the heat in.

It can be a good idea to keep the soil of the plants fairly moist because wetter soil retains heat better and will stay warmer than dry soil.

The main thing you can do to help your plants and vegetables from big temperatures changes is to apply mulch.

Sunburn or Sunscald

On the other hand, extreme heat is also bad for basil.

Black spots on basil leaves could be an indicator of overexposure to sunlight. Black spots on basil leaves that are dry are typically on the edge of the leaves.

Basil plants love sunlight, but if you started growing basil seedlings inside, they need a little time to get used to this new environment. This period is called “hardening off” and essentially you start slowly leaving the basil outdoors more and more each day. At the end of your hardening off period (around 1-2 weeks) the plant should be fully acclimated to actual sunlight and ready to be transplanted outside.

Temperature regulation for basil plants can be kind of tricky. If you have an indoor plant, make sure that the plant is somewhere that it can be exposed to the sun but isn’t being fried 24/7. Outdoor plants are more tricky to manage, but if you notice that your basil is starting to get sunburnt, you should provide some kind of shade so they are not always directly exposed to the sun.

Fungal Infections & Disease

The most common explanation for black spots on basil leaves is due to a fungal infection. When it comes to fungal infection, the spots can occur anywhere on the leaves of the plant. Generally, these spots are round and slimy to the touch.

You can identify the fungal infection by looking underneath the leaves as the first signs of trouble (and pests) are underneath the leaves. These fungal infections can occur when the leaves of the plant stay wet for an extended period of time. Basil can turn yellow or brown as well when a fungal infection starts to take place.

A fungal infection can start with a gray spot on the underside of the infected leaves, but if it is allowed to get worse, it will develop into black spots, and eventually, the leaves will die.

To prevent a fungal infection, the following will help:

  • Regularly prune your plants – when plants get too bushy they can get overcrowded and not get enough airflow for the plant
  • Make sure they get plenty of sunlight – basil, like many other plants need at least 6-8 hours of direct sunlight.
  • Proper watering – watering your plants in the morning rather than evening helps give the leaves and top layer of soil time to dry. If you water them at night, they will stay wet and invite pests and fungal diseases a great home.

If you do happen to notice graying on a leaf or a black spot, you can still try to save the plant by cutting off the affected leaf or area. Once the infected area is removed, you should treat the rest of the plant with fungicides or with natural anti-fungal methods.

Common basil fungal infections:

  • Downy Mildew
  • Bacterial Leaf Spot infection – This is often caused by Pseudomonas cichorii which is a bacteria that can impact your plant.
  • Cercospora Leaf Spot

You can avoid these infections by using copper fungicides and keeping your plant in the proper conditions.

Poor Nutrition

Poor nutrition can also be a cause of black spots on basil plants. Basil plants need a decent amount of nutrition in their soil, but they aren’t considered “heavy feeders” like tomatoes are, so this is generally a less common issue. If they are not extracting the nutrients out, then they will be imbalanced and likely develop brown to black spots on their leaves.

You can manage the nutrition by enriching your soil or creating your own mix of soil with compost and other organic matter that promotes growth. You also want to make sure you water the basil plant enough so that the soil is always moist.

To make sure your basil plants are getting the proper nutrition that they need, they should be spaced out 12-18 inches apart. If they are placed too close together, the plants will be competing for resources and nutrients and will not get enough for them to thrive.

Pests & Insects

Many different pests like to eat basil leaves. Because they’re essentially sucking the sap out of the leaves and stems, this can cause dark spots on your basil plants.

Common pests that eat basil leaves are:

Aphids: Aphids come in all kinds of colors, but most commonly they are green, yellow, and white. Aphids are very small and can be hard to identify, and they multiply LIKE CRAZY. Plus they leave this gross sticky trail behind called honeydew. Here are some great ways to get rid of aphids.

Spider Mites: Spider mites are very small and super hard to identify. Their color can be brown, orange, red, yellow, or green. These spider mites eat many herbs like basil and they make webs on the plants that look like regular cobwebs but they can be very damaging for the basil plant.

Whiteflies: Whiteflies also prey on basil by sucking the juice out of the basil leaves and leaving a sticky trail called honeydew. These whiteflies are damaging to basil leaves and can also be repelled with neem oil. 

Can Basil Plants Survive Black Spots?

If you notice any of the signs of pest or fungal infection on your basil, it is important to act quickly in order to stop them from getting worse. If a leaf has been eaten away by pests and now looks blackened, there may be no hope for those leaves.

You should cut off the infected area if possible and treat the rest of the plant with fungicides or natural anti-fungal methods. Sometimes poor nutrition can cause brown spots on leaves as well so make sure your plants have plenty of organic matter in their soil like compost or mulch to get all the nutrients they need.

Basil plants are a great addition to any garden. They can be grown indoors or outdoors and offer many benefits for both your health and the environment. However, basil plants are susceptible to infection by pests and fungal diseases which can cause black spots on leaves as well as other types of damage like stunted growth.

Fortunately, there are ways you can prevent these infections from happening in the first place with proper nutrition, careful watering practices, regular pruning, and more!

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